Replacing an old skylight or rooflight

There are two jobs hiding under one phrase. One is changing the sealed glass unit inside a frame that still opens, closes and sits square. That is a smaller piece of work, and it is not notifiable to Building Control. The other is removing the frame itself, which is the swap this page is about.

The glass-only route suits a rooflight whose frame is straight, whose seals still grip and whose flashing is sound. It does not suit a frame that has warped, a pivot that has worn, a timber sash that has softened or a unit that was never insulated to a decent standard in the first place. Fitting new glass into a tired frame buys you a clearer pane and the same cold, draughty surround.

If your rooflight has misted between the panes, our page on misted and failed rooflights explains which route usually makes sense and how we decide at survey. Where the frame is the weak part, the whole unit comes out.

Start with a label. Most roof windows carry a small identification plate on the frame, often at the top of the frame or on the sash, where it is easiest to see with the window open. It usually gives the maker, the model and a size code. VELUX windows, for example, use a letter for the width and two digits for the height, such as MK04 or CK02, and the pivot and opening type are shown by a short model code. If the plate has worn away or painted over, a photograph and a tape measure do the same job.

What we measure at survey:

• the outside frame width and height, taken in more than one place, since old openings are rarely square;
• the pitch of the roof and which way it faces;
• the roof covering, its age and its condition around the window;
• the depth from the glass to the ceiling, which sets the lining;
• how the existing window opens and whether you want that to change.

A measured photograph from you is a good start for a first conversation. The survey is what we price from, because the roof around the window is as much a part of the quote as the window itself.

Keeping the opening the same size is the simplest swap. The rafters either side have already been trimmed once, the ceiling has an opening that suits, and the new frame is chosen to fit. Even then, modern roof windows are not always the same outer size as the one you remove, and the flashing is sized to the frame, so there may be a small gap to close on the roof.

Going bigger or smaller is a different order of job. Making the opening wider means cutting rafters and introducing new timbers to carry the load, which is a structural alteration. Making it smaller means filling part of the old opening with framing, insulation and roof covering. Both are perfectly workable, and both change the lining and the ceiling edge as well as the roof.

A change of type is a third question. Many old rooflights are top-hung, and some owners now prefer a centre-pivot that opens further from the sill. Others want to move from a fixed unit to an opening one, or add an electric motor. The frame, the flashing and the lining depth all shift when the type changes, so we work out the whole opening at survey, not one component at a time.

Old rooflights usually fail at the edges long before the glass lets go. The flashing is the set of metal pieces that dresses over the roof covering and seals the frame to the roof. It is made to suit the covering: profiled roofing such as interlocking concrete tiles takes one kit, plain tiles or thicker slate take another, and thin slate takes a third.

VELUX flashing kits begin ED. The EDW kit is for profiled roofing, EDZ suits flat coverings, meaning plain tiles or thicker slate, and EDL is for thin slate. The correct kit has to match the covering you have, and a swap is the moment to find out whether the existing covering is the one the previous installer assumed.

Roof coverings around an old window are the other half of this chapter. Slates and tiles that were cut years ago to fit the old frame may be cracked, slipped or lifted. A new frame sits at a slightly different height and may overlap them differently. We lift and relay the surrounding coverings as part of the work and, if a few slates or tiles are too brittle to refit, we tell you at survey, not on the day.

Taking out one rooflight or roof window and fitting another is notifiable building work. Windows count as controlled fittings, so the new one must satisfy Part L (conservation of fuel and power), and the job is either notified to Building Control or self-certified by an installer registered with a competent person scheme. We make the notification, and it is included in the written quote. A sealed glass unit changed on its own, with the frame left in, is not notifiable.

If the opening changes size, the structural side comes in as well, because rafters that are cut or trimmed amount to a structural alteration. If the work touches a loft room that serves as a bedroom, we also look at whether the window is meant to act as an escape window, since Approved Document B sets size and position rules for those.

Planning is usually simple for a like-for-like swap on a house, because rooflights normally fall within permitted development, in Class C, Part 1, Schedule 2 of the GPDO. The conditions are a projection of 150mm or less from the roof plane and a position no higher than the roof's highest part. A few situations change that:

• a listed building requires listed building consent for external changes, a swap included;
• a conservation area allows Class C, though an Article 4 direction may take it away;
• flats and maisonettes sit outside householder permitted development;
• national parks have restricted rights in some cases.

Where it genuinely varies, check with the local planning authority first. You can read the national rules at the Planning Portal. Our guide to rooflights, planning permission and Building Regulations in Devon goes through the questions we get asked most often.

An old skylight swap always has an inside half. The lining around the existing window may be tired, discoloured by condensation, cracked at the corners or finished in a style that has dated. The new frame may also sit at a different depth, which changes where the lining meets the glass.

The lining is the short tunnel between the frame and the ceiling face. In many older houses it was formed in a hurry, with a square return and a skim of filler that has since opened along the joints. A swap is the cheapest moment to rebuild it properly. We can form a new lining, splay it wider at the ceiling where the roof build is deep enough to allow it, and finish it to match the room.

The ceiling is the other side. A lath and plaster ceiling in an Exeter Georgian terrace behaves differently from plasterboard in a 1930s semi in Heavitree, and a cob cottage in the Culm Valley wants lime plaster where a new-build in Cranbrook takes a board and skim. We note which you have in the finish schedule before work starts. The page on lath and plaster ceilings explains why those need extra care.

The finish schedule is the written note attached to each quote. For a swap it describes the room, the ceiling and roof construction, the way the new lining is formed, its finish and whether we hand it over prepared for your own paint or painted. It makes the inside half of the job visible before anyone starts. If you want to see how the lining is built up, read how a rooflight is finished inside.

A like-for-like swap is usually a shorter job than a new opening, but it still involves the roof, the loft and the room, so the order matters. The day tends to run like this:

1. The room is protected: furniture moved or covered, floor sheeted, a dust screen across the doorway.
2. The old sash comes out, then the frame, with the surrounding flashing and the roof coverings lifted around it.
3. The new frame is fixed, levelled and weathered with the correct kit, and the coverings are relaid.
4. Insulation and the vapour control layer are lapped onto the frame from inside.
5. The lining is formed and plastered or boarded, then left ready for decoration or finished as the schedule says.
6. The room is cleaned and the opening checked from below.

Where the lining and plaster need to dry before decoration, finishing continues after the roof is closed. Our answer on how much mess fitting a rooflight makes covers how we keep dust to the work area.

Weather decides the roof half of the day. A rooflight is not taken out in rain, and if the forecast turns we reschedule the roof stage instead of leaving an opening to the sky. Once the new frame is on and flashed, the roof is closed, and the interior can continue regardless.

Not every old skylight is a pitched roof window. Three other types turn up in Devon houses:

Domed plastic rooflights

Common on flat roofs over kitchens, bathrooms and extensions from the 1970s onward. They are usually bedded on a timber or GRP upstand. A swap often replaces the upstand with an insulated kerb and a modern flat rooflight with better glazing, which changes the ceiling opening too.

Old timber or steel-framed lights

Victorian and Edwardian roofs sometimes carry small fixed lights in cast iron or timber. These can be conservation-style units, and in a conservation area or on a listed house the swap needs a closer look at what is allowed.

Sun tunnels

A dome on the roof, a reflective tube and a diffuser in the ceiling. Replacing one is often quicker than a roof window, though the tube length and the ceiling collar still matter.

In each case the same questions apply: what is the roof covering, what is the ceiling, what is the size of the opening and what will the inside look like when the job is finished. Our page on roof window installation is a good place to read about the pitched types in more detail.

You do not need a failed window to justify a swap. These are the signs we hear about most often:

• condensation or a cloudy film between the panes of a double-glazed unit;
• a frame that has swelled, twisted or begun to soften, so the sash no longer closes evenly;
• a sash that will not stay open or a pivot that has worn;
• cold air at the frame edges, or a lining that feels cold to the touch;
• a window that was fitted before current insulation standards and now looks small and dim against the rest of the room;
• flashing that has corroded or lifted;
• a lining that has cracked, stained or dated.

None of these means the whole unit has to go on the day you notice it. Several together, though, usually point to the whole unit rather than a patchwork of small jobs.

Take two photographs, one of the window from inside and one of the roof around it from outside, if you can do that safely from a window or the ground. Note the size code on the frame if you can find it. Then send them through the quote form, by WhatsApp or by email to info@brightviewskylightsexeter.co.uk.

We will arrange a survey, measure the opening, the roof and the ceiling, and send a written quote with the finish schedule before any work starts. If you are weighing up other options at the same time, it also helps to read what separates a skylight from a rooflight, so the words on the quote match the product you want.