Roof Window Installation
Roof windows trimmed into slate, clay and concrete tile without proud flashing.
Gloucester localities
Rooflight and skylight installation in Innsworth, planned before anything is ordered.
Around Innsworth you are mostly dealing with former airfield housing, uniform roofs, and that decides more about a rooflight than the rooflight does.
- Covered from Gloucester
- 15 years on Gloucestershire roofs
- 10 year workmanship guarantee
- From Gloucester by road
- 1 to 6 milesApproximate, and inside the 30 mile radius
- Dominant roof stock
- Interwar hipped tile and Severn Vale red brickWhat the covering is decides the detail
- The usual constraint
- Tight rear access on terraced streetsSettled at survey, before anything is ordered
- Local character
- former airfield housing, uniform roofsRecorded for Innsworth specifically
In Innsworth
The work we do here, and where to read about it.
Walk-On Rooflight Installation
Structural glass rated to stand on, over rooms below a terrace.
Sun Tunnel Installation
Daylight into landings and bathrooms with no roofline to work with.
Conservation Rooflight Installation
Slim cast frames set flush, for stone slate, listed and conservation roofs.
Roof Lantern Installation
Flat roof glazing sized to the room and detailed into the kerb it stands on.
Flat Roof Rooflight Installation
Flush and kerb mounted units into felt, single ply and GRP flat roofs.
The local detail
Seventeen things worth knowing about Innsworth roofs, worth reading before the survey.
Innsworth is modern housing on flat, open, former airfield ground north of Gloucester, and that produces a very consistent set of roofs. Trussed rafters at 600mm centres, breather membrane underlay, large-format concrete tiles laid at a modest pitch, dry verges and dry ridges, mechanical fixing throughout, and insulation at ceiling level in a loft that was never designed as a room. Almost nothing about a rooflight job here is decided by the covering. It is decided by the trusses, by the bracing that holds them together, and in some cases by paperwork that never comes up on an older house. Severn Skylights Gloucester travels out to Innsworth from the city. This page is about what is genuinely different on a recent estate roof.
Houses on former airfield land, and what an Innsworth roof is made of
Airfield land is flat by definition, and it was cleared of almost everything before the houses arrived. That gives Innsworth wide roads, level plots, generous driveways and, from a working point of view, some of the easiest access in the Gloucester area. Scaffold stands on level ground, a lorry turns without difficulty, and side access to a rear elevation is available on most plots.
The same flatness and openness means very little shelter. There are no mature trees or older, taller buildings breaking the wind across this ground, so a roof here sits in a more exposed wind category than its position in the vale might suggest. That feeds directly into how tiles are fixed around a new opening and how far the flashing laps are taken beyond the manufacturer’s minimum, which is dealt with further down.
Structural warranties and estate covenants on an Innsworth house
This is the item most likely to be missed, so it comes early. A newly built house carries a ten-year structural warranty, and while that warranty is running the provider expects to be told about alterations affecting the structure or the weathertightness of the building. Forming an opening in a roof is both. The conversation is short, but having it before the work rather than at the point of sale is the difference between a clean file and an awkward question from a buyer’s solicitor.
The second item is in the deeds. Transfers on modern estates sometimes carry restrictive covenants about external alterations, occasionally administered by a management company that also looks after the open space and the unadopted roads. A covenant is a private matter and entirely separate from planning, so a house can have permitted development rights and still owe somebody a consent. It takes a look at the transfer to know, and it is worth doing before a unit is ordered. Neither of these is a barrier. They are just two boxes that older housing stock does not have.
Uniform house types, and why the survey still happens
Estate housing repeats. A run of Innsworth houses will be two or three types built to the same drawings, and once one has been surveyed the neighbours are largely predictable, which makes an indicative range from photographs unusually reliable here.
What still varies, and what still has to be looked at, is everything the builder did on the day and everything the owners have done since. Truss layouts change where a house type has a different garage arrangement or a hip instead of a gable. Bracing gets moved to suit a service run. Loft hatches sit in different places. Extract ducting from a bathroom, a soil vent pipe, a flue, a solar array and its cabling all cross the void or the slope in positions no drawing shows. And plot orientation flips down a road, so the sunny slope on one house is the shaded slope on the one opposite. The forty minutes a survey takes is spent on those things rather than on the tile.
The covering
Interwar hipped roofs: shallow pitches, and a hip that decides where an opening can sensibly go.
Interwar hipped tile and Severn Vale red brick, which is what the survey is looking at before it looks at the glass.
Trussed rafter roofs, and the timbers in an Innsworth loft that stay whole
A trussed rafter is a factory-made triangulated frame, cut from relatively slim timber and joined at every node with punched metal nail plates. It works as a designed whole, in tension and compression, and it is efficient precisely because nothing in it is oversized. Cut a web or a chord to make room for glass and the load path changes, the truss deflects, and the evidence turns up months later as a line across the ceiling or a door that stops closing.
There is a second family of timbers people miss. A trussed roof is stabilised by diagonal rafter bracing running from eaves to ridge across the underside of the top chords, by longitudinal binders tying the webs together along the length of the roof, and by gable restraint straps tying the roof to the masonry. Those are not offcuts left behind by the carpenter. They stop the whole set of trusses folding sideways, and removing a length of binder to slot a frame in is a genuine structural error. Where bracing crosses the position of a new opening, it is rerouted and reinstated to an equivalent, not simply taken out. What can and cannot be touched is set out on rooflights in a trussed roof and what cannot be cut.
The diagonal bracing and longitudinal binders in an Innsworth loft are structure, and they are reinstated rather than removed.
The 600mm module, and the roof window sizes made for it
Trusses in these houses sit at 600mm centres, which after the timber thickness leaves roughly 550 to 570mm of clear width between them. That single dimension governs what can be fitted without any structural design at all.
The manufacturers know it, and every major brand makes a narrow unit for the module: nominally 550mm wide, in lengths of 780, 980 or 1180mm. A unit in that range needs a head trimmer and a cill trimmer fixed between the two trusses, the covering back, the frame set and flashed, and nothing more. It is a one-day operation outside with the reveal following. Two of them in adjacent bays deliver more glass than one wider unit would have done, light a room from two points rather than one, and stay entirely inside the simple category of work. Setting out a pair so they read as deliberate rather than accidental is covered on two rooflights side by side on one slope, and the units themselves are on roof window installation.
Gloucester localities
Rear access is the constraint here far more often than the roof is.
Attic trusses, where the Innsworth loft was designed as a room
Some house types on recent estates were built with attic trusses rather than standard fink trusses, because the loft was always intended to become or already is a room. An attic truss has a clear rectangular zone in the middle by design, formed by a deeper floor chord and vertical members set out at the walls of the future room, with the webs pushed to the sides.
That changes everything. Inside the clear zone there is nothing structural to work around, the floor is already sized to carry a room, and an opening in the slope above is trimmed in the ordinary way. Head height in the middle is usually adequate and the knee walls are already in the right place. The check is worth doing before assuming anything, because attic trusses and fink trusses look similar from a loft hatch until you notice where the webs are. A house with attic trusses is a much easier and cheaper proposition for a conversion than an identical-looking house next door with standard trusses, and that is worth knowing before an offer is made on either.
Going wider than one truss bay on an Innsworth roof
Where the room genuinely needs a wider opening than the module gives, it is achievable, and it is a different job.
The affected truss is cut only after a beam has been designed to carry it, spanning to the trusses either side, with those trusses then taking load they were not individually sized for. That is a structural engineer’s calculation, it produces a specified member, frequently a deeper timber or a small steel with a bearing detail at each end, and it goes in with the Building Control application before any covering is removed. The trusses receiving the load may need doubling. None of that is exotic and it is regularly the right thing to do for a loft room. It is simply not the same as a narrow unit between two trusses, and it is described that way at survey rather than discovered halfway through. The structural side is on trimming rafters for a pitched roof rooflight.
Breather membrane underlay in an Innsworth loft, and how it is cut
Older roofs in the county have bitumen felt underlay, brittle and torn at the eaves. Innsworth roofs have a vapour-permeable breather membrane, which is a different material and wants different treatment.
A breather membrane is designed to let water vapour out of the roof space while keeping liquid water off the structure, and on many modern roofs it also does part of the ventilation job that eaves vents used to do on their own. Cut it carelessly around a new frame and both functions are compromised. The correct detail is the same principle as on any roof and the execution is more demanding: a tongue of membrane taken up over the head of the frame and fixed to the batten above, side pieces dressed into the frame’s own drainage channel so any water crossing the tile lands on the membrane and runs down outside the frame, and the cut edges lapped and taped to the membrane manufacturer’s instruction rather than left raw. That detail is invisible for the life of the roof and it is the one that decides whether a mark ever appears on a ceiling.
Dry verges, dry ridges and mechanical fixing on an Innsworth roof
Modern roofs are dry-fixed. The verge is a plastic or aluminium system clipped and screwed rather than bedded in mortar, the ridge sits on a dry ridge roll with mechanical fixings, and since the 2014 revision of BS 5534 the fixing regime for the field tiles is set by a calculation against wind uplift rather than by habit.
Two consequences for a new opening. Every tile lifted to form the opening goes back with its clip and its nail, because on an exposed site like this the calculation demands it and a tile relying on interlock alone is a tile that lifts in a February gale. And where the opening runs close to a verge, the dry verge units are removed and reinstated as a system, in sequence, rather than prised off and pushed back. Neither adds much time. Both get missed by people working from habit formed on older roofs.
The covering
Interwar hipped roofs: shallow pitches, and a hip that decides where an opening can sensibly go.
Interwar hipped tile and Severn Vale red brick, which is what the survey is looking at before it looks at the glass.
Large-format tiles at low pitch, and the flashing kit that goes with them
The covering on an Innsworth house is usually a large-format concrete interlocking tile, laid at the low end of its permitted pitch range, commonly somewhere between 17.5 and 25 degrees.
Pitch is a hard limit for a manufactured roof window. The major brands are rated from roughly 15 degrees upward, and between 15 and 20 degrees the standard flashing kit is not sufficient: a low-pitch kit with an extended apron and an additional underlay collar is required, because at a shallow angle wind pushes water further up the slope before it drains. The pitch is measured at survey with a digital level rather than guessed from the garden, since 17 and 22 degrees look identical from ground level and decide which kit is ordered. The tile profile also has to be matched properly. A kit for a deep-profile large-format tile is not the kit for a flat interlocking tile, and a mismatch leaves a gap that sealant is expected to bridge, which it will not do for twenty years.
Insulation at ceiling level, and the loft that was never meant to be a room
An Innsworth loft is a cold roof: 270 to 300mm of mineral wool laid between and over the ceiling joists, a ventilated void above it, and the trusses running through. The ceiling joists are the bottom chords of the trusses and they were sized for a plasterboard ceiling and the insulation, not for people or storage.
That matters in two ways for a rooflight. First, anyone working up there works off boards spanning the chords rather than treading between them, and the survey records where it is safe to stand. Second, the insulation line has to move locally around a new opening: up the rafter line, tight to the frame, continuous with the ceiling insulation where the two meet, with no void at the junction. A gap of nothing behind the plasterboard at the head of a roof window is a cold bridge, and cold bridges are where black spotting appears in the top corners of a reveal in February. The vapour control layer is taped to the frame rather than trimmed short of it. The geometry of the finished box is on rooflight reveals and why splaying them matters.
Rear access is the constraint here far more often than the roof is.
Ventilation paths in a modern Innsworth roof
Ventilation on these roofs is designed as a system, and it is easy to interrupt without noticing. Depending on the build there will be over-fascia or eaves vents drawing air in at the bottom, a ventilated dry ridge letting it out at the top, and a breather membrane doing part of the work along the way.
Forming an opening in the slope interrupts that path locally. Where insulation is taken up the rafter line around the new frame, a clear 25mm air gap is maintained above it so air still travels from eaves to ridge past the opening rather than dead-ending against it. Where extract ducting from a bathroom or a kitchen runs through the void, it is checked as well, because a duct discharging into the loft instead of through a tile vent puts a great deal of moisture into a space that is now going to have a cold glass surface in it. Both are recorded at survey. The wider condensation picture is on condensation, ventilation and rooflights.
On Innsworth roofs Tight rear access on terraced streets.
Windowless landings and internal bathrooms in Innsworth houses
Modern plans push circulation into the middle of the house. A first-floor landing with no external wall, an internal bathroom or en suite ventilated by a fan, a hallway lit only by a glazed panel in the front door: all of them are normal on this stock and all of them are dark.
A sun tunnel answers that without needing a room in the roof. A small dome on the slope feeds a rigid reflective tube, typically 250 to 350mm in diameter, running down through the loft to a diffuser in the ceiling below. The performance depends on the run: short and straight is much better than long with two bends, and every bend costs light. The route is set out at survey against the trusses, the bracing, the ducting and the water pipework it has to pass between, and on an Innsworth house with a shallow roof and a first-floor ceiling, that run is usually short. Detail is on sun tunnel installation.
Arrays, aerials and ducting already crossing an Innsworth slope
Recent houses arrive with things fixed to the roof. Photovoltaic arrays on the sunny slope, mounted on rails hooked over the trusses with cabling run under the tiles to an inverter in the loft. Tile vents serving bathroom and kitchen extracts. A soil vent pipe. Sometimes a flue.
Each of them constrains position, and each is dealt with rather than worked around blindly. An array is isolated by somebody qualified before anyone works on that slope, and where a panel has to come off and go back that is arranged with the installer who holds the warranty on it. A tile vent is not moved without deciding where the extract now discharges. A flue outlet brings Approved Document J into play: where the outlet sits within 2.3 metres horizontally of an openable rooflight, the flue has to terminate at least 600mm above the top of that opening. Frequently the sensible answer is to put the glass on the other slope, and a north-facing rooflight giving even, glare-free light all day is a better outcome for a kitchen or a home office than people expect.
On a hipped roof the usable field is smaller than it looks from the ground, which is why the survey measures rafters rather than tiles.
Tewkesbury Borough Council, and permitted development at an Innsworth address
Innsworth is not inside the city for planning purposes. The local planning authority is Tewkesbury Borough Council, and an application, pre-application enquiry or lawful development certificate for an Innsworth house goes there.
In practice most rooflights on this stock go in under permitted development. The limit that governs is the projection: an alteration to the roof of a house must not project more than 150mm beyond the plane of the existing slope, and must not be higher than the highest part of the existing roof. An in-plane roof window is comfortably inside it. A side-facing roof window in a room formed in the roof has the extra condition of obscure glazing and no opening part below 1.7 metres above the floor, which comes up on the closer-set plots here. Flats and maisonettes have no permitted development rights for roof alterations at all, and there are apartment blocks on this ground. Alongside that sit the warranty and the covenant noted at the top of this page, which are separate questions with separate answers. The planning rules are on do rooflights need planning permission.
Building Control on a house that is already well insulated
Forming an opening in an Innsworth roof is notifiable work. Structure covers the trimming and any beam carrying a truss. Conservation of fuel and power sets a limiting standard of 1.4 W/m2K for the unit, and any current manufactured roof window is comfortably inside that. Fire safety governs any escape opening at 0.33 square metres clear with 450mm minimum in both directions and the cill between 800 and 1100mm. Ventilation requires purge provision of at least one twentieth of the floor area. Safety glazing means a toughened outer pane and a laminated inner wherever there is habitable space below.
The point worth making on a modern house is that the existing fabric is good, so the new opening is the weakest thermal element in that part of the roof and the continuity of insulation around it carries more weight than it does on an older roof where everything is mediocre. The inspector will want the insulation seen tight and continuous around the frame before anything is boarded. Severn submits the notification, books the inspections and the completion certificate comes back in the homeowner’s name.
Getting a range and then a price for an Innsworth roof
Severn fits new roof windows and rooflights into Innsworth roofs and replaces units that have reached the end of their life, with a ten-year guarantee on the workmanship and a rule that the roof is never left open overnight. Innsworth is a few minutes north of Gloucester and sits well inside the thirty miles by road covered from the city, along with the other localities on areas we cover.
The useful first email carries a photograph of the roof from the garden, a photograph of the room from inside, the approximate age of the house, and if you can manage it a photograph taken from the loft hatch showing the trusses. That last one answers the biggest question on this stock before anybody visits. An indicative range comes back within a day or two, the survey follows, and the fixed price after it is itemised across unit, labour, scaffold and Building Control. The full process is on costs, and the written guides sit at guides.
Also covered: Gloucester localities