A roof window is installed by stripping the covering back around the opening, setting and squaring the frame between the rafters, wrapping it in an insulation collar, linking the roofing underlay to the frame with an underfelt collar, then fitting a flashing kit matched to your slate or tile. On a like-for-like replacement the roof is watertight the same day.
That is the short version. The order matters more than any single step, because each stage protects the one before it, and the two stages most likely to be skipped are the two that keep water out of the timber. Here is how a competent pitched-roof fit runs from start to finish.
The sequence, stage by stage
- Survey first. The covering type and its profile depth decide the flashing kit, the pitch decides whether the unit is even rated for that slope, and the rafter type decides whether anything structural is needed. None of this is guessed from the ground.
- Access goes up. Scaffold or a tower with proper edge protection, in place before the roof is touched. Nobody freehands a roof window off a ladder, and no homeowner should be up there at all.
- Strip back the working area. Slates or tiles come off around the opening and are stacked for reuse. On a replacement the old unit comes out here, and the trimmers and underlay are exposed and inspected. Rot in the trimmers surfaces at this point, gets photographed, and gets priced before anyone carries on.
- Set the frame. The frame drops onto its fixing brackets, then gets packed, levelled and squared in the opening. Squareness is not cosmetic. A frame a few millimetres out of true will never pull the sash evenly onto its gasket, and that uneven seal is a draught now and a leak in three winters.
- Fit the insulation collar. This is the detail below.
- Fit the underfelt collar. So is this.
- Flashing kit and re-cover. The kit matched to your covering goes on, the skirt is dressed down into the slate or tile profile, laps are mechanically fixed, and the covering is cut and re-hung with its head lap maintained. The roof is watertight when this stage ends.
- Line and finish inside. Reveal lining, plasterboard and skim where that is in scope, usually a vertical reveal at the head and a horizontal one at the sill so light throws further into the room.
- Test and hand over. Hose test, check the sash closes evenly all round, walk you through the flashing detail, hand over the guarantee and glazing spec in writing.
The insulation collar: the cold-bridge detail
The insulation collar is a pre-formed jacket that wraps the outside of the frame and joins the roof insulation to the window, closing the gap that would otherwise run cold right around the opening. It is supplied as a separate item, and it is the first thing left off a sharp quote.
Leave it out and you get a continuous cold bridge around the perimeter of the window, straight through the depth of the roof. In practice that shows up as a black mould line on the reveal, usually at the head, usually in January. Most people report it to us as a leak. It is not a leak. The reveal is running cold enough that room moisture condenses on it, and no flashing work and no anti-mould paint will hold it back. The collar costs a small fraction of the window and belongs on every fit, replacement or new.
The underfelt collar: the detail that beats driven rain
The roofing underlay, felt or breather membrane, has to be connected to the window frame so that any water getting past the covering is carried over the head of the frame and back out onto the slates below. The underfelt collar is the pre-formed piece that makes that connection cleanly.
The bodge is to cut the felt roughly, lap it over the frame and staple it. That holds until the first hard easterly, when rain driven under the tiles hits an unsealed lap and runs into the timber instead of over it. On a shallow-pitch Ipswich slate terrace this is a question of when, not if. Get the two collars right and most of the calls we get to look at other people’s leaks would never have happened.
What this means for a fit near you
Around Ipswich the covering changes the job. Victorian and Edwardian terraces off Norwich Road and Spring Road are natural slate, laid shallow, so the flashing lap and the underfelt collar have to be exactly right. Post-1970 estates at Kesgrave and Martlesham are trussed roofs, where a new opening cannot simply be cut and needs a designed solution. On the coast at Felixstowe and Harwich the fixings and flashing move up to marine grade. The sequence above stays the same. The specification bolted onto it does not.
This describes what trained installers do, not a weekend job. A roof window fit puts people at height on a stripped roof with weather on the way. If you want to know why it is a poor DIY candidate, our DIY roof window installation problems page is blunt about it.
Related reading
This is the quick version of the fit. For the full detail, including the survey and ordering stages, see our roof window installation process page. If you want to understand why the flashing kit has to match your covering, read the flashing kit explained. And if your question is really about the roof being open, our roof window replacement time page answers that directly. All of it sits under our main VELUX window installation guide.
Get a fixed price
Send a photograph of the roof from the garden showing the covering and one of the window from inside, and we can usually tell you what the job needs before anyone comes out. Then we survey, confirm the opening and the flashing kit, and give you one written price backed by a 10-year workmanship guarantee. The installers doing the work have 25 years fitting roof windows behind them.
Request a fixed quote for roof window installation in Ipswich, Suffolk or north Essex.
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.