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Gloucester · Cheltenham · Stroud · the Cotswolds

VELUX Window Installation

VELUX is the brand most Gloucestershire homeowners have in mind when they ask for a skylight, and it is a system rather than a single product: a sash and frame in one of a handful of ranges, a size code that fixes the structural opening to the centimetre, a pane code that fixes the thermal performance, and a flashing kit chosen for the exact covering on your roof.

Get a quote Read the technical detail

  • 15 years in Gloucestershire
  • 10-year workmanship guarantee
  • Building Control notification handled
  • The roof is never left open overnight

Tell us about the roof

We will come and look at it, and tell you honestly what it needs.

What the job covers

The decisions that get made before anything is ordered.

Why installations fail

The unit is the small half of the job.

A roof window sits in the plane of the roof, so nothing about it is watertight until the covering goes back around it correctly. Four details decide that, and every one is buried under the tiles by the time the scaffold comes down.

  1. The flashing kit. Matched to the covering and its profile depth, not ordered generically.
  2. The underlay. Cut, dressed and lapped over the head so water reaches the gutter rather than the ceiling.
  3. Soakers and the head flashing. Set under the course above, never surface sealed on top of it.
  4. The trimmed opening. Rafters cut and doubled trimmers fixed, so the load goes somewhere designed.

That is what the ten-year workmanship guarantee is written against.

A roof slope at 15 degrees carrying a roof window, with a shallower dashed slope below it marked too shallow. 15° Roof window Too shallow Measured off the rafter, not the ceiling
Pitch decides which units are permitted at all. Most roof windows want 15 degrees as a minimum, and below that the manufacturer will not warrant the weathering, whatever the installer says. On the shallow rear extensions common in Gloucester it is worth measuring before choosing anything.
How it runs

Two figures, and no surprises between them.

  1. Send photographs A few pictures of the roof and the room, and rough measurements. An indicative range comes back by email.
  2. The survey Forty minutes on site. Rafter centres, pitch off a digital level, the deck, the fall, and how a scaffold reaches the elevation.
  3. A fixed price Written against what was measured, not against the drawing. It does not move unless you change what you asked for.
  4. The installation Scaffold, opening, kerb, unit, weathering, reveal and making good. The roof is never left open overnight.
  5. Notification and guarantee The Building Control notification is submitted and closed out as part of the job, and the workmanship carries ten years.
Asked before every job

The questions that come up first.

What is a roof window?

A roof window is a glazed unit that sits in the plane of a pitched roof, in line with the covering around it, and opens on a hinge. It arrives from the factory as a complete sealed object: an outer frame, a glazed sash, gaskets on every closing face, and a matched metal flashing kit that ties it into the…

The full answer

How much does a roof window cost to fit?

Fitting a roof window is priced from the roof, not from the window. Two houses a mile apart in Gloucestershire can take the identical unit off the identical shelf and produce jobs that share almost nothing in labour, access, materials or days on site. That is why no figure appears on this page: any number printed here would be wrong…

The full answer

What size roof window should I get?

Start with the room and let the roof edit the answer. The useful method is to work out how much glass the space below actually wants, then check what the rafters, the run of slope and the obstructions in the loft will physically allow, then pick the nearest manufactured size that satisfies both. Almost everybody does it the other way…

The full answer

Can a roof window be an escape window?

Yes, and on most loft conversions in Gloucestershire a roof window is the escape opening. It only works if three things line up: the sash has to open wide enough to leave a clear hole of the required size, the hinge arrangement has to let the sash swing right out of the way rather than sit in the middle of…

The full answer

How do you clean the outside of a roof window?

On a centre-pivot roof window you clean the outside from inside the room. The sash rotates a full 180 degrees in its frame, a small bolt holds it in the inverted position, and the outer face of the glass then presents itself to you at chest height with your feet on the bedroom floor. That single feature is the reason…

The full answer

Do roof windows come with blinds?

No. A roof window arrives as a frame, a glazed sash and a flashing kit, and the blind is a separate item with its own order code, ordered either at the same time or at any point afterwards. Every major manufacturer makes a full blind range built specifically for their own frames, and those factory blinds are made to clip…

The full answer

Can you replace a roof window with a bigger one?

Yes, and the direction you grow in decides almost everything about the work. Going taller up the slope stays inside the same rafter bay, needs no new structural design, and adds daylight where daylight does the most good. Going wider means cutting at least one rafter and carrying it on doubled trimmers, which is routine on a traditional cut roof…

The full answer

How long does a roof window last?

A well installed modern roof window has a service life of roughly twenty to thirty years, and the parts of it age at different rates. The sealed glazing unit is usually the first thing to reach the end of its span, commonly at fifteen to twenty-five years on a roof slope. A timber frame kept in its factory finish outlives…

The full answer

The technical detail

The whole job, from the batten out.

VELUX is the brand most Gloucestershire homeowners have in mind when they ask for a skylight, and it is a system rather than a single product: a sash and frame in one of a handful of ranges, a size code that fixes the structural opening to the centimetre, a pane code that fixes the thermal performance, and a flashing kit chosen for the exact covering on your roof. Get the four right and the window disappears into the slope as if the roof had been built around it. Get the flashing wrong for the covering and the unit will still open and close perfectly while water tracks down the inside of the frame every time the wind comes from the south west. This page is the ordering and fitting detail: how to read the codes, what each range is rated for, which kit goes with Cotswold stone and which with a Quedgeley concrete tile, and what the roofs around Gloucester allow. Severn fits VELUX, Fakro, Keylite and Roto units as part of its wider rooflight and skylight services, and the reasoning below applies to all four, though the codes quoted are VELUX’s own.

28 sections 33 minute read

The four letters at the front of a VELUX code, and what they commit you to

Every VELUX roof window carries a three-letter type code, and it is the first thing a surveyor writes down. GGL is the lacquered pine centre pivot window, the one in more British lofts than any other single product. GGU is the same window with a white polyurethane skin moulded over a timber core, which is what goes into bathrooms, shower rooms, utility rooms and kitchens because there is no exposed timber for steam to reach. GPL and GPU are the top hung versions of each, pine and polyurethane, where the sash swings out from a hinge at the head. GXL and GXU are the escape variants, which matter enormously if the room below is a bedroom formed in the roof.

There are also fixed units with no opening mechanism, vertical elements that sit below a window to carry the glazing down to floor level where the room has a knee wall, and the GDL Cabrio, which is a two-part unit where the upper sash swings up as an awning and the lower part drops forward to become a balustrade. The Cabrio is a genuinely different piece of work: it needs a taller structural opening, two rafters trimmed in most cases, and the internal reveal set out so the balcony floor lands flush with the finished floor of the room.

The letters are not interchangeable and they are not a preference. A centre pivot and a top hung window of the same size code use different flashing geometry at the head, are rated for different pitch ranges, and behave completely differently in a room with a low knee wall.

Reading a size code: two letters and two digits that fix the opening

A VELUX size code such as MK04 or SK08 is not a marketing label. The first letter is the width, the third character pair is the height, and both refer to the outside of the frame in centimetres. Get the code from the survey and the structural opening is decided before anybody touches a rafter, because the rough opening is the frame size plus a small tolerance at each side for packing and fixing.

The widths run B at 47cm, C at 55cm, F at 66cm, M at 78cm, P at 94cm, S at 114cm and U at 134cm. The heights run 01 at 70cm, 02 at 78cm, 04 at 98cm, 06 at 118cm, 08 at 140cm, 10 at 160cm and 12 at 180cm. Not every combination is made, and the ones that are not made are usually the ones nobody could carry up a scaffold anyway.

Size code Frame size Rafter bays needed at 400mm centres Where it earns its place in this county
CK02 55 x 78cm Fits between two trusses at 600mm centres Landings, box rooms, and any trussed estate roof at Quedgeley or Ashchurch where the webs stay whole
CK04 55 x 98cm Fits between two trusses at 600mm centres The default trussed-roof window. Two of them in adjacent bays beat one wide unit
CK06 55 x 118cm Between trusses, but needs a longer run of slope Steeper Forest of Dean slates where there is height to spare above the gutter
FK06 66 x 118cm One rafter cut, trimmers head and cill Victorian terrace rear additions in Gloucester where 78cm is too wide for the bay
MK04 78 x 98cm One rafter cut Interwar hipped rear slopes at Longlevens and Hucclecote, where the run of slope is short
MK06 78 x 118cm One rafter cut The most common loft conversion size in the vale. Good balance of glass to slope
MK08 78 x 140cm One rafter cut, deeper trimmers Full loft conversions on 35 to 45 degree slopes. Too long for most lean-to additions
PK06 94 x 118cm Two rafters cut in most 400mm roofs Living space in a converted attic on a generous rear slope at Charlton Kings
SK06 114 x 118cm Two rafters cut, designed trimmer likely Wide rear elevations. A two-person lift and a real consideration on a Stroud valley site
UK08 134 x 140cm Three bays, engineer’s calculation expected Large openings in a substantial roof. Access dictates whether it is even deliverable

Why 55cm wide is the most useful number on a modern estate

A roof slope at 15 degrees carrying a roof window, with a shallower dashed slope below it marked too shallow. 15° Roof window Too shallow Measured off the rafter, not the ceiling
Pitch decides which units are permitted at all. Most roof windows want 15 degrees as a minimum, and below that the manufacturer will not warrant the weathering, whatever the installer says. On the shallow rear extensions common in Gloucester it is worth measuring before choosing anything.

Trussed rafter roofs across Quedgeley, Abbeydale, Brockworth, Bishops Cleeve and Yate are built at 600mm centres, which leaves roughly 550 to 570mm of clear width between two trusses once the timber is deducted. VELUX makes the C width, 55cm, precisely for that gap. A CK02 or CK04 slides into the bay, needs a head trimmer and a cill trimmer between the two trusses, and asks nothing of the truss webs.

That constraint is the whole reason two narrow windows are so often the right answer on those roofs instead of one wide one. Two CK04s in adjacent bays deliver more glass than a single MK06, light the room from two points rather than one, and avoid a structural submission entirely. The 47cm B width exists for the tighter cases: older prefabricated roofs, and bays partly obstructed by a services run or a water tank that cannot move.

Widen beyond the bay and the job changes character. Part of a truss has to be removed and carried on a beam spanning to the trusses either side, which means a structural calculation, a deeper timber or a small steel, and Building Control approval before the tiles come off. That is regular work and often exactly right for a loft room. It is simply a different job from a same-day window between trusses, and it gets described that way at survey rather than discovered on the morning.

Centre pivot against top hung, decided by the pitch and the knee wall

A GGL or GGU centre pivot is rated for pitches from about 15 to 90 degrees, which covers every pitched roof in Gloucestershire except the shallow upper slopes of a mansard. The sash rotates about its middle, so half of it swings into the room as the other half swings out. On a 45 degree Painswick or Coleford roof that is fine, because the sash clears your head. On a 25 degree vale roof with the window set low in the slope, the descending half of the sash arrives at eye level.

A GPL or GPU top hung window pivots at the head, so the whole sash goes outwards and nothing enters the room. It gives an unobstructed view out and it is the right choice where the cill sits low over a knee wall, over a desk, or over a bed. The trade is the pitch range: top hung units are rated to roughly 15 to 55 degrees, because above 55 degrees the sash weight works against the mechanism. On the steep terraces inside the Forest of Dean, 40 to 45 degrees is comfortable, but the steepest Cotswold slopes at 55 degrees and beyond are at the edge of it. Modern GPL units include a centre pivot function for cleaning, so you get the outward-opening sash in use and the rotating sash when the glass needs washing.

Range Opening Rated pitch Finish Best placed
GGL Centre pivot About 15 to 90 degrees Lacquered pine Bedrooms and living space where the sash swings clear of head height
GGU Centre pivot About 15 to 90 degrees White polyurethane over timber Bathrooms, shower rooms, utility rooms, kitchens, anywhere with steam
GPL Top hung with pivot for cleaning About 15 to 55 degrees Lacquered pine Low cills, knee walls, and where an uninterrupted view matters
GPU Top hung with pivot for cleaning About 15 to 55 degrees White polyurethane over timber Wet rooms with a low cill, and rear additions over a sink or worktop
GXL and GXU Side and top hung, opens wide Pitched slopes within range Pine or polyurethane The escape opening from a bedroom formed in the roof
Fixed elements None Within the range of the flashing kit Pine or polyurethane Daylight only, in a run alongside an opening unit

The 15 degree floor, and what happens below it

No VELUX roof window is rated to sit in the plane of a roof shallower than about 15 degrees. That is not caution, it is drainage: below 15 degrees water does not shed reliably off the sash and away over the flashing lap, and the head detail begins to rely on sealant rather than gravity. Sealant is a maintenance item. Gravity is not.

Gloucestershire produces this problem constantly. The shallow upper slope behind a Cheltenham parapet is frequently between 5 and 15 degrees. Lead and zinc mansard flats sit lower still. A 1970s rear addition with a shallow tiled lean-to can measure 12 degrees when the homeowner was sure it was twenty. In all of those the answer is a unit sitting on a built-up kerb rather than an in-plane roof window, which is the flat roof rooflight job with its own upstand, falls and membrane detailing. VELUX makes kerb-mounted flat roof windows for exactly this, on a purpose-made insulated upstand or on a timber kerb built on site, and the survey measures the pitch off a digital level before anything is ordered rather than trusting the look of it from the garden.

A pitch reading is taken off a digital level at survey, because a slope that looks like twenty degrees from the lawn is often twelve.
  • 15°
  • 12°

The flashing kit is half the product, and the covering chooses it

A VELUX window arrives as two parcels: the window, and a flashing kit with its own code. The kit is not an accessory and it is not generic. It is a set of pressed aluminium sections and, in some kits, a malleable lead apron, shaped to terminate a specific type of roof covering against the frame at the head, the sides and the cill. The wrong kit does not fail immediately. It fails in the third winter, from one direction of wind, and by then the covering has to come back off to correct it.

Kit Suits Gloucestershire coverings What it does at the perimeter
EDW Profiled coverings up to about 120mm profile height Concrete interlocking tile at Longlevens and Quedgeley, clay pantile through the vale Deep pressed side sections and a formed apron that dresses down over the tile roll
EDL Flat and irregular coverings, with a lead apron Welsh slate in the Forest of Dean, thick or uneven stone, handmade plain tile Lead skirt dressed by hand into an uneven surface where a pressed apron would bridge
EDJ Recessed installation into profiled coverings Concrete tile and pantile where a flush appearance is wanted Sets the frame deeper into the roof plane so the sash sits closer to the tile line
EDN Recessed installation into flat coverings Slate roofs on rear elevations, and conservation area slopes where projection is queried The lowest profile a standard roof window achieves in slate
EDS Flat interlocking tiles and slates of moderate thickness Flat profile concrete tile and thicker slate formats A middle option between the flat and deep profile kits
Combination kits Two or more windows side by side or in a vertical run Twin units on a rear slope, common on interwar hips and loft conversions A single continuous weathering across both units with a fixed gap between frames

Two further details decide how the kit performs. The first is the batten line. VELUX designs each kit around a specific relationship between the frame and the tile batten above the opening, and the frame carries a marked installation line for it. Set the frame too high in the slope and the head apron has nothing to lap under. Set it too low and the cill apron sits proud of the course below. The second is the drainage gutter above the head, formed by the underfelt collar, which takes any water that has crossed the covering and runs it around the frame rather than onto it.

Recessed installation, and the 30mm that changes how a window reads

A standard installation sets the frame so the sash sits a little above the plane of the covering. A recessed installation, using the EDJ or EDN kit with the frame fixed lower in its brackets, drops the whole unit deeper into the roof by roughly 30mm. Externally the difference is obvious once you know to look for it: the window reads as part of the roof rather than as an object on it, and the shadow line around the frame is much shallower.

That 30mm matters for three practical reasons in this county. In a conservation area, a planning officer looking at whether an alteration projects beyond the roof plane treats a recessed unit far more sympathetically than a standard one. On slate, where the covering is thin and the frame would otherwise stand well clear, the recessed detail is simply the better-looking answer. And internally, a recessed window brings the sash closer to the plasterboard line, which shortens the reveal box and puts slightly more glass in view from the middle of the room.

The cost of it is depth. Recessing takes 30mm out of the space between the top of the rafter and the underside of the sash, so the insulation zone around the frame is tighter and needs setting out properly rather than stuffed. On a roof with only 100mm rafters and a full-fill insulation specification, that gets checked at survey before the kit is ordered.

  • 30mm
  • 100mm

The three collars that go in before the window does

Fitting a VELUX properly involves three collars, none of which is visible when the job is finished, and all of which are the difference between a dry installation and a damp reveal.

The insulation collar is a moulded jacket that wraps the outside of the frame and fills the gap between frame and rafter with continuous insulation. Without it, that gap is either an air void or a handful of loft wool pushed in with a scraper, and either way it is a cold ring around the warmest object in the ceiling. Black spotting in the top corners of a reveal in February is almost always this.

The underfelt collar is a pre-formed membrane apron with a gutter moulded into the head. It connects the existing roof underlay to the frame, dresses into the frame’s own drainage channel at the sides, and directs anything crossing the covering out over the tiles below the window. Cutting a straight slit in old underlay and tucking it behind the frame is the single most common defect found when an old unit comes out.

The vapour barrier collar works from the warm side. It seals the room’s vapour control layer to the frame so humid air from the room is not driven into the insulation zone, where it would condense against the cold aluminium. On a bathroom GGU this one is not optional in any meaningful sense.

All three are fitted in sequence with the covering off and the underlay dressed, which is why the outside work on a single window takes a day rather than an afternoon.

Two windows side by side: the gap is a fixed dimension

A roof window sitting in the plane of a tiled slope

Where two or more units go into one slope, a combination flashing kit weathers them as a group with a single continuous set of sections between the frames. The gap between frames is not a free choice. VELUX supplies combination kits for specific dimensions, commonly a narrow 18mm gap where the two units read almost as one large opening, and a wider 100mm gap where a visible band of covering runs between them. Wider spacings exist, and beyond a certain distance each unit simply takes its own individual kit.

The choice has consequences beyond appearance. An 18mm gap needs the structural timber between the two openings to be a single trimmed member carrying both frames, which usually means a designed timber. A 100mm gap on a 400mm rafter roof can sometimes land either side of an existing rafter with no cutting at all, which is the cheapest good result available on a rear slope. On trussed roofs at 600mm centres, two windows in adjacent bays land naturally at a wider spacing and take individual kits, and that is why twin CK04s are such a common recommendation at Bishops Cleeve and Ashchurch.

Vertical combinations, one unit above another up the slope, are also possible and give a tall band of glass on a steep roof. They ask more of the structure, because the trimmers now sit between the two openings and carry cut rafters top and bottom.

  • 18mm
  • 100mm
  • 400mm
  • 600mm

Sizing a VELUX against the run of slope on a vale rear addition

Gloucester’s Victorian and Edwardian terraces, along with Tewkesbury, Longford and the vale villages, share a two-storey rear addition one room wide with a lean-to roof over it at 22 to 30 degrees. The kitchen underneath is the darkest room in the house and the obvious place for daylight from above. The obstacle is arithmetic.

Measure gutter to abutment on one of those roofs and you commonly get 1.8 to 2.4 metres of slope. A VELUX needs a run of covering above the head for the flashing to lap under, and a run below the cill for the apron to sit on. Take those away and an MK08 at 140cm long stops fitting on most of them. The realistic options are an MK04 or FK06 set high in the slope, twin narrow units, or a made-to-size unit on the flat area where the addition meets the back of the house. Forcing a long window into a short slope is how you end up with 200mm of tile above the head and a head flashing with nothing to lap into.

There is a related rule worth knowing before you choose a size: the shallower the pitch, the longer the window needs to be to give the same view and the same depth of daylight into the room. A 78 by 118cm window on a 45 degree roof shows you the garden. The same window on a 22 degree lean-to shows you a great deal of sky and very little else, because the glass is lying closer to horizontal. On shallow roofs the useful move is length, not width.

  • 30°
  • 2.4m
  • 200mm
  • 45°

Interwar hips at Longlevens and Warden Hill, and the window that has to get shorter

The hipped concrete tile roofs across Longlevens, Hucclecote, Tuffley, Warden Hill and Charlton Kings sit at 30 to 35 degrees and are cut roofs with purlins and struts, which makes them among the friendlier roofs to trim. The shape is the problem, not the structure.

On a hip, the slope narrows towards the corner, so the rectangle actually available is a good deal smaller than the roof looks from the lawn. Add a purlin crossing at about mid-height and a ceiling binder below it, and the clear zone on a typical interwar rear hip works out around 1.2 metres of vertical run across two rafter bays. That is an MK04 at 98cm, comfortably. It is not an MK08.

People arrive at these houses having seen a photograph of a big window in a big roof, and the honest answer is that the roof will take a shorter one, or two shorter ones. Twin CK04s or twin MK04s in adjacent bays either side of a rafter give more total glass than a single MK06, need no structural design, and light the room evenly instead of throwing one bright rectangle onto the floor. Where the purlin is genuinely in the way, moving the opening below it rather than through it keeps the job to a day outside.

Cotswold stone slate, the lead apron kit, and when the conservation unit wins instead

Painswick, Bibury, Northleach, Chipping Campden and the wolds villages between them are roofed in Cotswold stone slate laid in diminishing courses on riven battens, commonly with a lime mortar torching underneath and often no underlay at all. The slates are thick, frequently 20 to 30mm, and the roof loading runs well over 100 kilograms per square metre.

A standard VELUX flashing kit is designed to terminate a covering a fraction of that thickness. Fitted to a stone roof it sits proud, the apron bridges over the uneven surface instead of dressing into it, and the finished window reads as a modern object screwed onto a three-hundred-year-old roof. Where a VELUX is the right product on a stone slope, and it sometimes is on a rear elevation nobody sees, the kit is the lead-apron version so the skirt can be dressed by hand into the irregular surface, and the courses above the opening are stripped back up the slope and re-laid to re-establish the diminishing gauge with reclaimed slates matched for thickness and weathering colour.

In a conservation area or on a listed building, the more likely outcome is a steel conservation rooflight instead, with its slim cast frame and single central glazing bar sitting nearly flush in the stone. That product exists because of roofs like these, and it has its own page at conservation rooflight installation. Deciding between the two is a conversation with the conservation officer, held with photographs before anything is ordered, not a decision taken on the scaffold.

A roof window seen from inside a room in the roof
A roof window seen from inside a room in the roof

Forest of Dean slate: headlap over the minimum, and a handle out of reach

Coleford, Cinderford, Mitcheldean and Lydney are sandstone terraces with Welsh slate at 40 to 45 degrees, sometimes laid over sarking boards rather than open battens. The weather arrives from the south west with force behind it, and the Forest is wetter and windier than the vale.

Two adjustments follow. Head laps go beyond the manufacturer’s minimum and side flashings are taken wider than the kit’s default, because a lap sized for a sheltered vale roof is not sized for driving rain on an exposed slope above Cinderford. Where the covering is on sarking boards, the underlay and drainage detail changes: the boards have to be cut back cleanly and the membrane collar dressed onto the board face rather than onto open battens.

The steep pitch also moves the window up the wall. On a 45 degree roof the top of an MK08 frame sits high enough that reaching a control bar from standing height stops being realistic, particularly in a room with a raised floor over the old ceiling joists. A rod-operated opener handles it, or an electric unit with a rain sensor where the window is above a stair or a tall void. Electric operation, control pads and the wiring involved are covered at electric rooflight installation.

Stroud, Chalford and Nailsworth: why the size code is an access question

A modern trussed roof on an estate house

The valley towns are built into the hillside, so a cottage is one storey at the front and three at the back, reached by a shared lane, a footpath or a flight of steps, with retaining walls hard against the elevation and nowhere flat to stand a tower.

On sites like those the size code stops being about daylight and becomes about carrying. A CK04 is a one-person lift up a scaffold. An SK06 or a UK08 is a two-person lift with the sash removed from the frame first, and on a stepped path with a low wall on one side it is a genuinely awkward one. Scaffold frequently has to be hand-carried in from wherever the lorry can stop, sometimes a long way, and that takes longer and costs more. It is priced from the survey with the access properly looked at, so the figure holds when the crew arrives instead of moving.

The compensation is that these are the houses where a roof window does the most work. A cottage cut into a bank, blank retaining wall behind, neighbours tight on both sides, has no elevation left to put a window in. Daylight from above is the only daylight there is.

The pane code, and what the last two digits are telling you

A full VELUX product code looks like GGL MK04 3070. The three letters are the range, the four characters in the middle are the size, and the last four digits are the variant, of which the final two identify the glazing. A pane ending 70 is the standard double-glazed unit with a toughened outer pane and a laminated inner pane. A pane ending 66 is triple glazed. Other endings cover solar control coatings, acoustic laminates and obscured glass, and the older codes on a unit fitted in the 1990s tell you the same thing about a product two generations back.

The performance difference across those generations is large. A double-glazed roof window from the early 1990s sits somewhere near 2.8 to 3.0 W/m2K for the whole window. A current standard pane brings that to around 1.3, and a triple-glazed option to near 1.0. Building Regulations set a limiting standard of 1.4 for a replacement roof window, which any current unit clears without trying.

The coating choice matters more than the pane count in one specific situation: a south or west facing loft room. A solar control coating on the outer pane cuts the solar heat coming through, bringing the g-value down from around 0.5 to roughly 0.25 to 0.3. On a west-facing conversion in Gloucester, that is the difference between a bedroom that is usable in July and one that is not. The energy efficient rooflights page works through the numbers properly.

Laminated inner panes, rain noise, and the rooms that require them

Safety glazing in a roof is not a specification choice. The outer pane is toughened so it breaks into blunt fragments rather than shards, and the inner pane is laminated wherever there is habitable space, a stairwell, a bath or a shower beneath the glass, so that a broken inner pane holds together in its interlayer instead of falling into the room.

Laminated glass brings a second benefit that surprises people, which is sound. The plastic interlayer damps the drumming that heavy rain makes on a single sheet of toughened glass. It is most noticeable on shallow pitches, where the rain strikes closer to perpendicular, so a vale lean-to at 22 degrees benefits far more than a Cotswold slope at 50. Where a bedroom sits directly under the glass and the household is sensitive to noise, an acoustic laminate goes a step further.

Self-cleaning outer coatings are worth specifying in one particular case: a unit high above a stairwell or a double-height space where cleaning the outside of the glass realistically means a ladder up the outside of the house. The coating does not remove the need to clean, but it changes the interval from months to years.

Escape windows: which VELUX sizes actually clear the requirement

Where a loft room is a bedroom, the roof window frequently has to serve as the means of escape, and the regulation is dimensional. The clear openable area must be at least 0.33 square metres with a minimum of 450mm in both height and width, and the bottom of that opening must sit between 800 and 1100mm above the floor.

Two consequences follow that catch people out regularly. First, a centre pivot window cannot satisfy it, because the rotating sash sits across the middle of the opening and halves it. The escape unit is a GXL or GXU, which opens side hung and swings clear to give the whole opening. Second, the 450mm minimum clear width rules out the narrow sizes: deduct the frame and sash sections from a 55cm wide unit and there is not 450mm left. In practice an escape window starts at the 78cm widths, which is why an MK06 or MK08 GXL turns up in loft conversion drawings so often.

The cill height is the other half of it. Setting the frame to land between 800 and 1100mm above the finished floor has to be resolved before the opening is cut, and the finished floor in a conversion is often 100mm or more above the old ceiling joists once insulation and boarding are in. Getting that wrong is expensive, because the correction is a new opening.

A roof window sitting in the plane of a tiled slope
A roof window sitting in the plane of a tiled slope
  • 450mm
  • 1100mm
  • 100mm

The ventilation flap, the filter, and the vent nobody opens

Every current VELUX opening window has a ventilation flap built into the control bar at the top of the sash, with a replaceable air filter behind it. The flap gives continuous background ventilation with the window closed and locked, which is the point: it is secure, it is weathertight, and it can be left open through the winter.

It is also the feature most often left shut for a decade. A closed flap in a loft bedroom, with two people breathing overnight and no other background ventilation in the room, produces condensation on the coldest surface available, which is the glass directly above them. That water is not water getting in. It is the room’s own moisture landing on the coldest thing in it, and it clears when the flap is opened and the room gets a proper purge each morning.

Purge ventilation is the regulation term for opening the window properly, and for a habitable room the openable area needs to be at least a twentieth of the floor area. A single MK06 satisfies that in most loft rooms of reasonable size. Two smaller units satisfy it more comfortably, and they also give cross ventilation if they face different directions, which a single window never can.

External awning blinds, and the honest answer to a hot loft

Internal blackout blinds control light and privacy. They do very little about heat, because by the time the blind intercepts the sunlight the energy is already inside the room. External awning blinds sit on the outside of the glass, in the same frame grooves the flashing kit terminates against, and they stop the heat before it reaches the pane.

The difference is not marginal. An external awning on a south or west facing unit cuts solar heat gain dramatically while still letting diffuse light through the mesh, so the room stays usable and does not go dark. Combine one with a solar control pane and a west-facing loft bedroom in Gloucester behaves entirely differently in a heatwave. Manual awnings operate on a cord from inside the room. Electric and solar-powered versions retract automatically in wind, which matters on an exposed Forest slope where a deployed awning in a gale is a liability.

The fixing detail is worth planning at installation rather than later. Awning brackets clip into the outer frame grooves, and if the flashing kit and the surrounding covering are set tight against those grooves it becomes fiddly to retrofit. Saying at survey that an awning may follow costs nothing and saves an afternoon.

Cleaning a centre pivot: the 180 degree rotation and the barrel bolt

A roof window seen from inside a room in the roof

A genuine advantage of the centre pivot design, and one many owners never discover, is that the sash rotates fully through 180 degrees so the outer face of the glass presents itself to the room. A barrel bolt on the frame drops into the sash to hold it in that position while you clean it, and releasing the bolt returns the window to normal operation.

Top hung units achieve the same thing through their secondary pivot function, which lets the sash be rotated for cleaning even though it opens outwards in daily use. Fixed units have no such option, which is one reason a fixed rooflight over a stairwell wants a self-cleaning coating or a realistic plan for reaching it.

While the sash is rotated, the data plate is visible at the top edge. It carries the range, the size code and the year of manufacture, and on a replacement job it is the single most useful thing on the whole window: it turns a guess about what will fit into an exact order. Photographing that plate before enquiring about a replacement saves a visit. The skylight replacement page covers how a like-for-like swap runs.

Old VELUX units, changed frame profiles, and the fixing line

A VELUX from the 1980s or 1990s in an MK04 opening does not guarantee that a current MK04 lands on the same fixing line. Frame profiles, bracket positions and flashing geometry have all moved through several generations, and the nominal size code has stayed the same while the details around it changed. The frame will fit the structural opening. Whether the internal reveal survives untouched depends on where the new frame sits within it.

That gets checked at survey against the data plate and by measuring the existing structural opening from inside the roof void, not by reading the code and ordering. Where the reveal will need making good, it is said in advance so the decoration is planned rather than discovered. Where the opening is being enlarged at the same time, going taller within the same rafter bay is usually easier than going wider across a rafter, though it may run into a purlin at the head.

The other thing that turns up when an old unit comes out is the perimeter behind it: underlay slit rather than dressed, insulation missing for 100mm all round the frame, lead face-nailed both sides and cracked along the nail line, battens split where the original fixings went in. All of that is brought up to current detail as part of setting the new unit, which is why the work takes as long as a new installation.

What the two days on site actually involve

Scaffold or a tower goes up first. Inside, floors and furniture are sheeted before anything is cut, and the loft hatch is closed and sealed while cutting happens so dust stays out of the house.

The opening is set out from inside so the reveal lands where the room wants it, then transferred outside by drilling through at the corners. The covering is stripped back and stacked for re-use, the underlay is cut and dressed rather than slit, the rafter is cut and the head and cill trimmers fixed. The frame goes in, is packed and fixed square and to the marked installation line, and the sash is refitted and checked for even operation before anything is weathered. Then the collars: insulation, underfelt with its gutter, vapour barrier. Then the flashing kit, head last so every lap runs in the direction of flow. The covering is re-laid into the kit and the roof is whole.

That is the first day, and it ends with a closed roof. The roof is never left open overnight, so an opening is not started unless it can be closed and made watertight the same day, and if the forecast turns during the morning the outside work stops at a point where the roof is complete. Day two is inside: the reveal box built with a vertical lining above the window and a horizontal lining below it, insulation continuous and tight to the frame, vapour layer taped rather than trimmed short, then plasterboard, skim and clean-up.

The reveal box on a VELUX, and the two linings that do the work

The plastered box between the frame and the room is not trim. Built properly it roughly doubles the daylight reaching the floor from the same piece of glass, and it is where a good installation and an average one part company.

The convention is a vertical lining at the head and a horizontal lining at the cill. Vertical above throws light deeper into the room instead of bouncing it back at the ceiling. Horizontal below lets warm air from a radiator underneath rise straight up the face of the glass, which is the cheapest condensation control available and costs nothing but geometry. Splaying the two sides widens the beam. On a north-facing room in a Cheltenham terrace that half day of extra carpentry and plastering is the difference between a room that is bright and a room that is merely lit.

On a VELUX specifically there is a dimension to respect: the sash needs clearance to rotate, so the reveal cannot be brought tight around the frame at the sides where the sash swings through. Getting that clearance wrong produces a window that catches on the plaster at 80 degrees of rotation and will not go on round to the cleaning position.

A modern trussed roof on an estate house
A modern trussed roof on an estate house

Building Control on a VELUX installation, and who submits it

Putting a new opening into a roof is notifiable work, and so is replacing a unit in most circumstances. Several parts of the regulations apply at once.

  • Structure. Trimming a rafter, and certainly carrying part of a truss on a beam, is structural work. Where the span or the load needs it, an engineer’s calculation goes in with the application.
  • Conservation of fuel and power. The limiting standard for a replacement roof window is 1.4 W/m2K, and continuity of insulation around the reveal is inspected under the same requirement.
  • Fire safety. Where the window forms the escape opening from a room in the roof, the clear opening and cill height dimensions set out above have to be met by the unit as installed.
  • Ventilation. Purge ventilation of at least a twentieth of the floor area, with the sash’s background vent contributing to the background rate.
  • Safety glazing and electrical work. Toughened outer and laminated inner panes in the positions described, and any mains wiring for an electric opener certified under the electrical part.

The notification is submitted as part of the job, the inspections are booked, and the completion certificate comes back to the homeowner in their own name. Keep it somewhere findable: it is the document a buyer’s solicitor asks for, and discovering it was never obtained at the point of sale is a far worse problem than dealing with it now.

Permitted development, and the 150mm projection that decides it

Most roof windows in a house go in under permitted development with no planning application. The limit people trip over is the projection: an alteration to the roof of a house must not project more than 150mm beyond the plane of the existing roof slope, and it must not be higher than the highest part of the existing roof.

An in-plane VELUX is comfortably inside that figure, which is precisely why it is the default product on the majority of Gloucestershire roofs, and why a recessed installation makes the point beyond argument. A bulky kerb-mounted unit on a pitched slope is where the 150mm becomes a live question.

Side-facing units in a room formed in the roof carry an extra condition: obscure glazing, and non-opening unless the openable part is more than 1.7 metres above the floor of that room. Flats and maisonettes have no permitted development rights for roof alterations at all, so an application is needed whatever the size. Article 4 directions remove rights street by street, and both Gloucester and Cheltenham have areas covered by them, so a written confirmation from the local planning authority before work starts is worth the modest fee, particularly with a sale anywhere on the horizon.

Two guarantees, and which one covers what

The workmanship on a Severn installation carries a ten-year guarantee, and it is written against the four details that decide whether the perimeter stays dry: the height and position of the frame relative to the batten line, the laps and fixing pattern of the flashing kit, how the underlay and its gutter are dressed around the frame, and the continuity of the insulation and vapour layer. None of those can be inspected once the reveal is plastered, which is exactly why they are the ones that get skipped by people working quickly.

The unit itself carries VELUX’s own warranty on the glass, the frame and the mechanism, which is a separate document with its own terms and its own duration by component. The survey notes set out which document covers which part, and the registration details for the unit are handed over with the completion certificate rather than left in a bag of packaging.

Where a sealed unit fails inside its warranty the manufacturer’s process applies to the glass, and the labour to change it is a separate matter. Being clear about that boundary in writing at the start is more useful than a broad assurance that everything is covered.

What to send for a figure, and how a VELUX job starts

No figures are published on this site, because a lead-apron kit into diminishing courses of stone slate on a Painswick cottage with hand-carried scaffold and a CK04 between two trusses on a Quedgeley semi with a driveway to park on are not the same job at any price. What moves the number is the covering, whether any structural work is needed, the size wanted against the rafter or truss spacing, how many units and whether one scaffold covers them, the access, the glazing and operation specified, how far the internal finish is taken, and any conservation requirements. The costs page sets out the process.

Then two figures, in this order. An indicative range by email first, from photographs and a description, free and back within a day or two, so nobody spends an afternoon on a survey for a project that was never going to work financially. Then a fixed price after a survey, itemised so it is clear what is unit, what is labour, what is scaffold and what is Building Control. That figure holds unless the scope changes at the customer’s request, or unless something structural comes to light that was flagged as a risk in the survey notes.

Four things make the first reply genuinely useful: a photograph of the roof from the garden or the street, a photograph of the room from inside, what the covering is if you know it, and roughly where you want the daylight to land. If there is an existing VELUX going out, add a photograph of the data plate with the sash rotated. Email info@severnskylightsgloucester.co.uk to start. Severn works within thirty miles by road of Gloucester, covering the city and its suburbs, Cheltenham, Stroud and the valleys, Tewkesbury and the vale, the Cotswold villages east to Cirencester and Northleach, the Forest of Dean, and south through Yate and Thornbury: town by town detail is on the areas pages. Related products sit at roof window installation, roof lantern installation, walk-on rooflight installation and sun tunnel installation, the written guides answer individual questions in more depth, and about explains how the company works.

A roof window sitting in the plane of a tiled slope
A roof window sitting in the plane of a tiled slope
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