A roof window is installed in a fixed order: survey and measure, put up access, strip back the covering, remove the old unit or cut the new opening, set and square the frame, fit the insulation collar and the underfelt collar, fit the flashing kit matched to the covering, re-cover the roof, then line and finish inside. On a like-for-like replacement the roof is open for hours, not days, and never overnight. The order never changes, because each stage depends on the one before it being right. The difference between a fit that lasts and one that leaks in three years is not the window. It is whether the middle stages were done in full or quietly skipped.
This page walks the whole sequence and names the details that disappear off a cheap quote. If you want the short version, the how are roof windows installed answer covers it in a few hundred words. This is the full account.
The full stage table
Read this first. It is the whole job on one screen, with the stage, what actually happens, what a cheap fit leaves out, and what that omission costs you later.
| Stage | What happens | What gets skipped on a cheap fit | Consequence |
|---|---|---|---|
| 1. Survey and measure | Read the data plate or measure the structural opening, check covering, profile depth, pitch, rafter type and exposure | Ordering off a phone description or a tape reading of the visible frame | Wrong size unit or wrong flashing kit turns up on the day, job stalls or gets bodged |
| 2. Access | Scaffold or tower with edge protection and a working platform at eaves level | Working off a ladder to save the access cost | Rushed work, poor flashing dressing, and a real safety problem |
| 3. Strip back | Slates or tiles lifted from the working area and stacked for reuse, underlay exposed and inspected | Stripping the minimum, not inspecting the trimmers or underlay | Rotten trimmers left in place, existing membrane damage missed |
| 4. Old unit out or new opening cut | Old window removed, or a rafter trimmed with doubled trimmers to the engineer’s detail | Cutting a rafter with no design on a roof that needed one | Structural movement, cracked ceilings, an opening that was never signed off |
| 5. Frame set and squared | Frame packed, levelled and squared in the opening on its brackets | Dropping the frame in without checking square | Sash will not pull evenly onto the gasket, draught and leak within a few years |
| 6. Insulation collar | Pre-formed insulation jacket wrapped round the frame, made continuous with the roof insulation | Left off entirely, or stuffed with loose offcuts | Cold bridge round the opening, black mould line at the head each January |
| 7. Underfelt collar | Pre-formed collar linking underlay to the frame so water drains over the head and back out | Felt cut rough and stapled instead | Driven rain gets under the tiles, past the lap, into the timber |
| 8. Flashing kit | Kit matched to the covering, skirt dressed into the profile, laps mechanically fixed | Whatever kit is in the van, or a flat kit on a profiled roof | Wind-driven leak at the bottom corners, the classic built-in fault |
| 9. Re-cover | Slates or tiles cut accurately and re-hung with head lap maintained, no exposed nail holes | Slates grinder-cut, nail holes sealed with mastic | Chipped covering that reads as a patch, sealant that fails in a few winters |
| 10. Line, test, hand over | Reveal lined and finished, hose test, sash checked closing evenly, guarantee issued | No hose test, no lining detail, gone before it rains | First real easterly finds the fault, and the fitter is unreachable |
The rest of this page takes the stages that matter most and explains why they are the ones worth watching.
Survey: the stage that prevents the other nine going wrong
Nothing is ordered until the roof is surveyed. We read the data plate or measure the structural opening directly, and if you are not sure what a size code means, the size codes page explains how the letter and number map to millimetres. At the same visit we log the covering type and its profile depth, because that decides the flashing kit family, not the window. We check the pitch, because a shallow slate roof at 25 to 30 degrees needs the laps and collars exactly right. We check whether the roof is a cut roof or trussed, because that decides whether a new opening needs an engineer. And we assess exposure, because a slope within a mile of open sea gets a different fixing specification from an inland estate.
All of that lands in one written price. The reason a proper survey saves money is that the discovering happens here, on paper, rather than halfway through the job when the roof is already open.
Strip back: the stage that finds the surprises
Slates or tiles come off the working area and are stacked for reuse. On a replacement the old unit comes out now, and for the first time the trimmers, the frame timber and the underlay are exposed. This is where a rotten trimmer surfaces, and the honest way to handle it is to photograph it and show you before anything is done about it. A trimmer that has been sitting under a leaking flashing for ten years may have gone soft, and there is no point setting a new window onto timber that will not hold a fixing. Cheap fits skip the inspection, drop the new window straight in, and pass the problem to you.
Rot found on strip-back is priced before we go on, never added quietly. If a trimmer has to come out and be replaced, you see it, you get a figure for it, and you decide. That is the opposite of the mid-job surprise bill, and it is the reason we open the roof with the covering stacked for reuse rather than skipped straight to the tip.
Squaring: millimetres that decide the seal
The frame is set into the opening on its fixing brackets, then packed, levelled and squared. Squareness sounds like a finish detail and it is not. A roof window sash pulls down onto a continuous gasket when you close the handle, and it can only seal evenly if the frame is square. A frame out by a few millimetres across the diagonal leaves the sash bearing hard on one edge and light on the other. Air draws through the light side, and on an exposed slope rain follows the air. It will not show on handover day. It shows in the third winter, reported as a draught or a leak, and it traces straight back to a frame that was dropped in and screwed off without a diagonal check.
The two collars nobody itemises
These are the details most often missing from a cheap quote, because both are separate items you can leave off without the finished window looking any different on the day.
The insulation collar
This is a pre-formed insulation jacket that wraps the outside of the frame and links the roof insulation to the window, closing the cold bridge that otherwise runs right round the opening through the full thickness of the roof. Leave it off and the reveal runs cold. In January that cold reveal collects room moisture as condensation, and the result is a black mould line, usually at the head, usually reported to us as a leak. It is not a leak. No flashing work cures it and no anti-mould paint holds it. The collar costs a small fraction of the window and it belongs on every fit, replacement or new.
The underfelt collar
The roofing underlay has to be connected to the frame so that any water getting past the covering is carried over the head of the frame and back out onto the tiles below. The underfelt collar is the pre-formed component that makes that connection cleanly. The bodge is to cut the underlay roughly, lap it over the frame and staple it. That holds until the first hard easterly, when driven rain gets under the covering, hits an unsealed lap and runs into the timber instead of over it. On a shallow slate roof this is a question of when, not if. We fit both collars on every job, and the underfelt detail sits underneath the flashing, so once the roof is re-covered you cannot see whether it was done. You have to trust the fitter, or watch the stage.
The flashing kit: match it to the covering or it leaks
A roof window does not keep water out on its own. The flashing kit does, and it has to match the covering. A flat, low-profile kit on a low roof, slate or plain tile up to about 8mm. A deeper, pleated kit on profiled tiles, concrete interlocking tiles and pantiles up to around 120mm profile. A combination or recessed kit where the window sits deeper for a flatter look, which conservation officers sometimes ask for. And a purpose-made twin kit with a designed gutter where two or more windows sit together, because you cannot make a twin by butting two single kits. Put the wrong family on and you build a leak in on day one. This is the single most common specification error we find on other people’s work, and the flashing kit explained page goes through all four families in detail.
Re-covering: where slate work shows
The covering goes back with the head lap maintained and no nail hole left exposed. On slate this is the fussy part. Slates are cut with a slate cutter, not an angle grinder, because a ground edge chips and reads as wrong for the life of the roof. Slates that have to be face-fixed, because the nail hole falls in the wrong place, get proper copper or lead tingles rather than a bead of sealant. Old Welsh slate is brittle and breaks easily, so breakage is allowed for and matching reclaimed slate is sourced rather than dropping a modern fibre-cement slate into a Victorian roof. The full slate interface is covered on our roof window installation on a slate roof page.
Line, test, hand over
Inside, the reveal is lined and finished where that is in scope. The standard detail is a vertical reveal at the head and a horizontal reveal at the sill, which throws daylight further into the room and lets warm air wash up the glass to reduce condensation. Then we hose test, check the sash closes evenly onto the gasket all the way round, walk you through the flashing detail rather than just the finished look, and hand over the guarantee and the glazing specification in writing. A fit with no hose test is a fit where nobody confirmed the thing works before the fitter left.
How long the roof is open
The question behind all of this is usually how long the roof is exposed. On a like-for-like replacement with sound trimmers, hours. The roof is watertight the same day, at the end of the flashing and re-cover stage, and it is never left open overnight. Two or three units on the same slope, still like-for-like, is one to two days. A new opening in a cut roof is about two days per window because the trimmers go in first. A new opening in a trussed roof depends entirely on the designed solution. If the weather turns and the roof cannot be closed properly, we sheet, secure and come back rather than press on and leave a fault under the covering.
Common questions about the process
Can I install a roof window myself?
We would not advise it, and we never advise anyone to go onto a roof. The stages that fail on a self-install are the hidden ones: the underfelt collar, the flashing laps and the squaring, none of which show on the day but all of which decide whether it leaks. The common DIY faults are set out on our DIY installation problems page.
How do I know both collars were actually fitted?
Watch the strip-back and frame stages, because once the roof is re-covered the underfelt collar is buried under the flashing and cannot be seen. This is exactly why they are the details cheap fits leave off. We fit both on every job and will show you them going in. If a competing quote is much cheaper than ours, the collars are the first thing to ask about.
Why does squaring the frame matter so much?
Because the sash seals by pulling down evenly onto a gasket, and it can only do that if the frame is square. A frame out by a few millimetres leaves the sash bearing hard on one edge and open on the other, which draws air and, on an exposed slope, rain. It does not show on handover day. It shows in the third winter as a draught or a leak.
Will you replace rotten timber you find during the job?
Yes, and we price it before we proceed. Rot in a trimmer usually surfaces at strip-back, when the old unit is out and the timber is exposed. We photograph it, show you, give you a figure, and you decide. We do not set a new window onto soft timber, and we do not add the cost quietly at the end.
Get a fixed price for the job done in full
Every stage on this page is in the price we quote, because leaving stages out is how the cheap fits on other people’s roofs became the leaks we get called to look at. We survey properly, specify the flashing kit for your covering, fit both collars, square the frame, and hose test before we leave. That work carries a 10-year workmanship guarantee, and the installers doing it have 25 years fitting roof windows behind them.
Request a fixed quote for your roof window, or read the whole approach on the main roof window installation page. If you are replacing an old unit rather than cutting a new opening, the roof window replacement page covers what changes between generations.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.