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 tiles or slates. It comes in a fixed list of manufactured sizes rather than being made to your measurements, and it is rated to work between roughly 15 and 90 degrees of pitch. VELUX, Fakro, Keylite and Roto all build them to the same broad pattern. That is the whole definition, and everything else on this page is what follows from it. The installation itself is set out on the roof window installation page.
In the plane of the slope, not standing on top of it
The defining feature is the plane. A roof window’s outer frame sits down between the battens with the covering re-laid up against it, so the glass finishes roughly level with the tiles and the whole thing reads as part of the roof rather than an object bolted to it. Look at a Gloucester terrace with three of them on the rear slope and the silhouette of the roof is unchanged.
That matters for more than appearance. Permitted development for a house allows an alteration to a roof slope that projects no more than 150mm beyond the plane of the existing covering, and an in-plane roof window comfortably sits inside that figure. A unit that stands on a kerb above the slope may not. It also matters for water: because the frame is down in the covering, the flashings can lap over and under the tiles in the same direction water is already travelling, with nothing relying on a bead of sealant.
What the unit is made of, from outer frame to drainage channel
A roof window is five things stacked together, and knowing the names makes every later conversation easier.
- The outer frame, fixed to the trimmed opening with brackets, carrying the whole unit and the flashing kit.
- The sash, the moving part, hinged to the frame and holding the glass.
- The sealed glazing unit, two or three panes with a spacer and desiccant between them, toughened on the outside and usually laminated on the inside.
- The gaskets, a set of soft profiles that close the sash against the frame on all four sides.
- The drainage channel, moulded into the outer frame, which collects the small amount of water that always gets past a gasket in driving rain and carries it back out onto the covering below the cill.
Timber is the traditional frame material, laminated pine with a factory lacquer or a white paint finish. There is also a polyurethane-coated version built around a timber core, which is one moulded piece with no joints to open up, and it is the better answer in a bathroom or a kitchen. Every unit also carries a ventilation flap in the head, which is a separate thing from opening the sash.
The pitch band a roof window is designed for
Most manufactured roof windows are rated from 15 degrees at the shallow end to 90 degrees at the steep. That band is not a preference, it is where the flashing kit works. Below 15 degrees rain no longer sheds down the glass with enough energy to clear the frame, water travels sideways under the head flashing, and dirt sits on the pane instead of washing off. The correct answer at that point is a kerb-mounted unit on an upstand, which is a different product and a different job.
Gloucestershire supplies plenty of roofs at both ends. Cotswold stone slate above Painswick and Bibury runs at 45 to 55 degrees, Forest of Dean slate around Coleford and Mitcheldean similar, and interwar concrete tile at Longlevens sits comfortably at 30 to 35. The shallow upper slopes of a Cheltenham mansard, hidden behind a parapet at 5 to 15 degrees, fall below the band, and that is where a kerb-mounted rooflight takes over.
Fixed sizes, ordered as a code rather than a measurement
This is the part that surprises people who have bought vertical windows. A roof window is not made to your opening. You choose from a grid of manufactured sizes, each with a short code stamped on the sash, and the roof opening is then formed to suit the code. The widths cluster around the rafter spacings that exist in real houses, which is why 55cm and 78cm wide units are so common: they drop between rafters at 600mm centres without cutting anything.
The codes themselves, how the letters and digits work, and how they translate into a structural opening are covered properly on roof window sizes and how they are coded. What matters here is simply that the code is the thing that gets ordered, and every accessory afterwards, the flashing kit, the blind, the insect screen, orders against the same code.
Fixed, manual, rod operated or electric
How the unit opens is a separate decision from its size, and it is decided by how far away the handle is.
A fixed unit does not open at all and is used purely for daylight. A manual unit has a bar handle across the top of the sash, which suits any window you can reach standing on the floor of the room. Where the head of the window is higher than that, typically on a steep Forest of Dean slate roof or above a stairwell, a control rod hooks into the handle from below. Above that again is an electrically operated unit with a wall switch or remote and a rain sensor that closes the sash the moment it starts, which is the practical answer for any opening nobody can physically reach. That is covered at electric rooflight installation.
The flashing kit, which is half the product
A roof window is never bought on its own. It comes with, or is ordered alongside, a flashing kit chosen for the profile of the covering it is going into: an apron at the cill that sits over the tiles below, side soakers or a continuous side flashing, and a head flashing dressed under the course above.
The kit for a flat plain tile is not the kit for a pantile, which is not the kit for a slate, and the kit for a thick stone slate is different again. Getting this wrong is one of the more expensive ordering mistakes, because the covering has to come back off. There is a proper breakdown at roof window flashing kits by roof covering.
What sits behind the plasterboard once it is in
From inside the room you see a rectangle of sky and a plastered box around it. Behind that box is the part the ten-year workmanship guarantee is written against. The opening is trimmed with doubled timbers at head and cill so the load from any cut rafter transfers safely into the rafters either side. Insulation runs continuous and tight to the outer frame with no voids, and the vapour control layer is taped to the frame rather than trimmed short of it.
The reveal itself is built with a vertical lining above the window and a horizontal lining below it. Vertical at the head throws daylight deeper into the room. Horizontal at the cill lets warm air from a radiator rise straight up the face of the glass, which is the cheapest condensation control available. Splaying the two sides widens the beam of light considerably, and on a north-facing slope it is the difference between a bright room and a lit one.
Which Gloucestershire roofs take one well
Concrete interlocking tile on the interwar semis at Hucclecote, Warden Hill and Charlton Kings is the most straightforward covering there is: the tiles unclip, a standard kit fits the profile, and the covering goes back the same afternoon. Welsh slate in the Forest of Dean and clay tile on the Severn Vale terraces are both routine, with the caveat that a narrow rear addition often gives only 1.8 to 2.4 metres of slope from gutter to abutment, and the flashing laps above and below eat into that.
Cotswold stone slate is the exception that shaped this county’s answer. The slates are thick, laid in diminishing courses on riven battens with a mortar torching underneath, and a standard roof window frame is built for a covering a fraction of that thickness, so it sits proud and reads as a modern object on an old roof. That is the reason the conservation rooflight exists, with its slim cast frame and single central glazing bar, described at conservation rooflight installation.
How a roof window job starts here
Severn Skylights Gloucester fits new roof windows into roofs that have never had one and replaces units that have reached the end of their service life, across Gloucester and thirty miles by road around it. The brand-specific detail, including the four-letter product codes and the pane numbering, sits on the VELUX window installation page.
To get a useful first answer, email info@severnskylightsgloucester.co.uk with a photograph of the roof from the garden or the street, a photograph of the room from inside, the covering if you know it, and the address. The postcode alone usually tells you the likely covering, the pitch and the conservation position in this county. That produces an indicative range within a day or two, and a fixed price follows a survey. The process is on the costs page.
Next on this subject.
- Question 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…
- Question 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…
- Question 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…
- Question 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…