The short answer
A roof window is best fitted by a professional. The job opens the roof, may cut a rafter, needs a flashing kit dressed to your tile or slate, and usually has to be notified to Building Control. A confident builder can do parts of it, but water tightness, the structure and the paperwork are where a first attempt goes wrong.
What the job really involves
It is easy to picture a window dropped into a hole. The work is closer to opening a small part of the roof and closing it again, in a day, with the weather watching.
The sequence is the same for any fit skylight window job. The roof covering is lifted around the opening, the underlay is cut back, and the rafters are checked. If the window is wider than the gap between two rafters, one is cut and the load is carried by new timbers fixed across, called trimmers. The frame is set on battens, levelled and fixed. The flashing kit goes on from the bottom upwards, each piece overlapping the last so water runs out and away. The felt or membrane is lapped to the frame. Then the tiles or slates are cut and relaid against the flashing, and the inside is lined and made good.
Every stage depends on the previous one. A frame fixed a few millimetres out of square makes the flashing gape. A flashing piece laid the wrong way round will pass a dry-day check and fail in the first long storm. That is why the job is usually done in one visit: the roof is open only while someone is there to close it.
The unit itself is also heavier than it looks. A large glazed sash and its frame need two people at roof level, and a third on the ground to lower them up safely. Our full sequence is in how a roof window is installed in a pitched roof.
Where a DIY job tends to come unstuck
The mistakes are rarely dramatic on the day. They show up months later as a stain on a ceiling, and by then the cause is hidden under the lining.
| Stage | What it needs | Where it slips |
|---|---|---|
| Structure | Rafters trimmed and supported | Cut without a trimmer |
| Flashing | Kit matched to the roof covering | Wrong kit, wrong order |
| Membrane | Lapped and sealed to the frame | Gaps at the corners |
| Lining | Insulated and airtight | Cold bridge, damp patch |
The first row matters most. A rafter is part of the structure that holds the roof up. Cutting one without carrying its load is a structural change, and it can sag a roof plane over time. Our note on when a roof window needs the rafters trimmed explains what the timber work looks like, and fitting between the rafters covers the case where no cut is needed.
The flashing is the second. The kit differs for plain tiles, interlocking tiles and natural slate, and each covering is cut differently at the edge. Read roof window flashings for tiles and slates to see how much the kit changes with the roof.
Inside, the lining decides whether the window is warm. Insulation has to meet the frame without gaps, and the lining has to be airtight against warm air from the room. Miss that and the glass and lining run cold, which brings condensation with it. The finish is shown in linings and making good the reveal.
Regulations, notification and planning
A DIY installer takes on the paperwork as well as the work. That part is easy to forget, and it is the part that comes back when the house is sold.
Cutting a new opening in a roof is building work, and it is covered by the Building Regulations. Several parts apply. Part L sets the thermal performance of the new window. Part K covers safety glazing. Part B applies if the window is in a loft room, where the stairs and escape routes matter. Part F covers ventilation. If the work is notifiable, it has to be notified to Building Control before or as it starts, and a completion certificate follows. A job with no notification leaves the homeowner with no record to show a solicitor later.
Planning is a different matter. Roof windows on a typical house fall under permitted development, which means no planning application, provided they do not protrude far beyond the roof plane or sit above the highest part of the roof. The rules are tighter in a conservation area, on a listed building and on some elevations, so the position is checked for the address before anything is ordered.
We handle the Building Control notification where the work is notifiable, and install to current Building Regulations. The work is covered by our 10-year workmanship guarantee. A window you fit yourself has no such cover, so check the terms of the manufacturer warranty too.
What you can sensibly do yourself
That does not leave the homeowner with nothing. Some of the work around the window sits comfortably with a capable DIYer, and doing it saves effort on the day.
- clearing the loft or room below so the opening is accessible;
- choosing the size, the glazing and the blind ahead of time;
- painting and decorating the lining once it is in.
Choosing well is where your time pays back. Our guide to standard roof window sizes and rafter spacing helps you pick a size that fits the existing structure. Quiet matters too, and the glazing choice goes in your quiet spec, so the glass suits the room before it is ordered.
Questions before you decide to fit one yourself
Is it cheaper to do it myself?
The labour costs are lower, but any error that lets water in costs more to put right than the fitting saved. For a view of the typical market figures, see rooflight and skylight costs.
Can I replace an old roof window myself?
The same points apply, with a like-for-like swap being a smaller job than a new opening. The structure is already there, but the flashing, the membrane and the regulation for glazing performance still apply. We can replace it in the same visit as any other work, so get a quote for the job.
Does it matter which brand I buy?
A little. Flashing kits are made for each range, so the window and the kit should match. See comparing roof window brands for the differences.
How long does a professional fit take?
A single window in a standard pitched roof is typically a day's work, with the inside made good afterwards. More than one window or a structural change takes longer. The wider service is on our roof window installation page.