Many people living in a house with a flat-roofed kitchen or extension have looked up at a dim ceiling and wondered whether a rooflight could simply be added. In most cases it can. The roof was built to keep weather out, though, and putting a hole in it changes how it works, so the job needs to be done in the right order and judged on the roof you have, not an ideal one.
What follows is the sequence we follow when a rooflight goes into a roof that is already finished and in use. It explains what we look at first, what makes the job simple or complicated, and what happens on site. It is written for the person who lives under the roof and wants to know what is going on above their head.
Looking at the roof before anything else
The first question is whether the existing roof should be cut at all. We look at its covering, its age and its condition. A roof that is firm, flat and well drained can take a new opening cleanly. One with soft spots, ponding water or splits at the edges is telling you something, and cutting into it would only add a new weak point.
We also look at the build-up beneath. Whether it is a warm roof with insulation above the deck, a cold roof with insulation between the joists and a ventilated void, or something older, the method changes. Our guide on fitting a flat rooflight to a warm roof describes the most common case.
Which roof coverings take a rooflight
All the common flat roof coverings can accept a rooflight, but each is dressed up the kerb differently. Rubber sheet is formed in a single piece and folded up the sides. Fibreglass is laid in layers over a resin base and runs up the kerb as a continuous skin. Felt is applied in layers and turned up with careful corners.
The sections on each type are in separate guides. See EPDM rubber roofs, GRP fibreglass roofs and felt roofs. The point to remember here is that the covering affects the method more than the rooflight does.
Checking what is under the ceiling
Before any cutting, we work out what is above and below the position of the new rooflight. Inside, there may be a ceiling light, a cable run, a pipe or a duct. Above, there is the deck, the insulation and the joists.
- Joists tell us where the structure must be trimmed.
- Cables and pipes may need to be moved first.
- An extractor duct or a flue passing through the roof may restrict the position.
- A steel beam in the ceiling may rule out a given spot.
Sometimes the best position for light is not the easiest position for the structure. At the survey we discuss the compromise, and a small shift in position often makes the job simpler without costing you much light.
Forming the new opening
An opening in an existing roof needs new framing. Joists that cross the opening are cut, and the cut ends are carried by trimmers fixed across the gap. This framing is what takes the load that the cut joists carried. It is done before the glass arrives, and it is the first of the hidden jobs that we photograph, so you can see what is inside the roof when you hand it over.
Putting a new opening into a roof is something Building Regulations care about, on two counts: the strength of the reframed structure and the heat the finished opening will lose. Where the job is notifiable we make the notification ourselves. An inspector visits, and the certificate reaches you at handover.
The ceiling below is cut after the roof is protected, not before. This keeps dust in the room to a minimum, and a screen goes up between the work and the rest of the house wherever the layout allows.
Building the upstand
The kerb is built next, and it is the heart of the job. A timber frame stands round the opening, insulated and covered so that it is at least 150mm above the finished roof. The height lifts the glass above standing water and gives the covering room to turn up the sides.
Insulation in and round the kerb is the main defence against condensation at the edge of the opening. If it is thin or interrupted at the corners, you may see damp patches on the reveal in cold weather. This is the detail that most separates a careful job from a quick one, and it is why we photograph it.
| Height | Usually at least 150mm above the finished roof |
|---|---|
| Frame | Timber, fixed to the new trimmers |
| Insulation | Rigid board inside and outside the kerb |
| Covering | The roof membrane dressed up and over the top |
Making the roof weathertight again
With the upstand built, the roof covering is dressed up its sides and across the top, then the rooflight is bedded and fixed. Joints at corners are where water finds its way in, so they are formed with extra care and checked before we leave.
Whatever stage the job has reached, the roof is closed in before we leave each evening. If rain is forecast and the glass is not yet on, a temporary cover goes over the opening. You should never be sitting under a hole in your roof overnight, and the plan for each day tells you what is happening and when.
Finishing the room
Inside, the reveal is lined, boarded and plastered. A splayed finish, widening towards the room, spreads the daylight and looks more natural than a plain box. We agree the detail with you at the survey so that the finish matches your ceiling.
Lighting cables for any lights round the opening are run before the plaster. Painting is a choice, and it is priced as a separate line. Floors, stairs and furniture stay covered throughout, the waste leaves with us, and the same team sees the job through from start to finish.
When the roof needs attention first
Sometimes the right reply to "can we add a rooflight?" is "not yet". A covering that is at the end of its life will not hold a new kerb, and it is poor value to build a fresh upstand into a roof that must be renewed within a couple of years. In that case the rooflight is best done with the new covering, when the whole surface is laid at once and the upstand is dressed into it as it goes.
We say so at the survey, with the reasons, and price the choices separately: a rooflight alone where the roof is sound, or a rooflight with a renewed covering where it is not. You decide, and nobody asks you to commit on the day.
- Ponding that stays for days after rain suggests the falls are poor.
- Soft boards near the edge suggest water has been getting in for some time.
- Several layers of old covering stacked one over another suggest the surface has been struggling for some time.
- Cracks in a ceiling below the proposed opening are worth tracing before anything is cut.
Living in the house while it happens
A rooflight into a finished roof is done around you, so it helps to know the shape of the days. Day one is protection and the opening: covers go down, the roof is cut and framed, and the temporary cover goes over the hole if the glass is not yet in. Later days cover the upstand, the covering and the glass, and the last is the inside finish.
Most households keep using the rest of the house. The room directly beneath is the one to empty or cover, and it is useful to move anything fragile out of it beforehand. Tell us if someone works from home, sleeps in the day or has pets that panic at noise, and we will arrange the loudest work around them.
Choosing the position on a flat roof
A flat roof gives more freedom than a pitched one, because nothing forces the rooflight to sit between rafters. The choice is yours, and it is worth spending time on. Stand in the room and picture where the sun enters at different times of day. A position near the back wall of an extension lights the part furthest from the garden doors. A position nearer the middle gives an even spread.
Think also about what is outside. A rooflight set close to a parapet gets less sun from that side. One under a tree collects leaves and may be shaded for much of the year. Position it away from the outlet where the roof drains, so water running to the gutter is never directed along the upstand.
Where an extension adjoins a neighbour, we also look at the party wall. Work close to a shared wall can be a matter for a party wall notice, and it is better to know before the date is fixed than after.
The handover, and keeping the rooflight well
When the glass is in and the inside is finished, we walk you through the room and up to the opening. You are shown how any opening mechanism works, given the makers' warranties along with the product literature, and handed the photographs of the hidden work taken before it was covered. The 10-year workmanship guarantee is confirmed in writing along with the Building Control paperwork where it applies.
After that, little is asked of you. Keep gutters and outlets clear, brush off leaves, and rinse the glass now and then with clean water. Check the ceiling reveal each winter for any sign of damp. Should something look wrong, ring us with the details and photographs, as early signs are far easier to deal with than late ones.
Rules to check first
On a house, a rooflight is usually permitted development when it stands no more than 150mm proud of the roof plane and stays below the roof's highest point. On a flat roof the glazed unit and its kerb are measured together against that 150mm, which is why the height of the upstand matters at the planning stage. Flats and maisonettes have no permitted development right for this and need planning permission. A listed building needs listed building consent.
Conservation areas are the exception to check: an Article 4 direction can take the right away on certain streets, and only the council can say whether yours is one. The skylight planning permission guide goes through each case. The planning authority is City of York Council inside York and its villages, or North Yorkshire Council for most of the towns beyond.
What the job costs and how big it can be
Adding to an existing roof usually costs more per rooflight than building one into a new roof, because cutting, trimming and making good all happen in a finished house. The roof's condition and the number of joists cut are what move the price most. Our flat rooflight cost guide goes through the lines of a quote, and flat rooflight sizes shows how the opening is chosen.
If you want more than one rooflight, it is generally cheaper to do them together, as the access, the protection and the weathering are shared. You can see the whole service on our flat roof rooflights page, and book a free survey with the form below.