In brief
Yes. A flat rooflight can be added to a roof that has already been covered. The covering is cut back, the joists are trimmed to form an opening, a kerb is built and insulated, and the covering is brought up the kerb and sealed. It is more work than fitting during construction, because the roofer is needed twice, so it saves effort to decide on the rooflight before the roof is laid.
- The finished covering is cut back and the structure opened up below it
- The joists around the opening are trimmed and supported before anything is lifted out
- A new kerb is built, insulated and covered by someone who works with your roof material
- The covering is made good up the upstand so the join is sealed
- Planning the rooflight before the roof is laid removes a whole visit
How an opening is formed in a covered roof
The work begins on the inside of the roof as much as the outside. Once the position is marked from the room below, the covering and insulation are cut back, and the deck and any ceiling lining are opened to expose the joists. The joists that cross the opening cannot simply be removed: they are cut, and new trimmers take their weight around the gap, which are fixed into the joists on either side.
In most houses this is straightforward carpentry, but where the roof is a more engineered build, such as one with steel or with closely spaced joists, the structure may need checking by an engineer first. At the survey we look at what holds the roof up, and the quote tells you if anything beyond ordinary trimming is needed.
Kerb, insulation and covering
With the opening trimmed, the kerb is built around it from timber, to the height the maker specifies for the unit. It is insulated so the upstand does not become a cold bridge, and then covered. The covering has to rise up the outer face of the kerb and be finished at the top in the way the product requires, which is where most of the skill lies.
How the covering is joined to the existing roof depends on what it is. EPDM is bonded or lapped, GRP is laid over and laminated onto the new upstand, and felt is overlapped in layers. The roof covering guide sets out each one. If the existing covering is old or near the end of its life, that is a good time to decide whether to carry on with it or renew the roof around the rooflight.
Why it is simpler to plan the rooflight first
When the rooflight is included in the build, the opening is formed while the joists are bare, the kerb goes up before the covering, and the roofer lays one continuous membrane up the upstand. There is no cutting back, no join between old and new and no second visit. The roof is also left sealed when the first visit finishes instead of being opened a second time.
That is the reason we ask builders and architects for the position early and put the kerb and opening dimensions on paper ahead of the decking. If your extension is still on paper, this is a better moment to settle the rooflight than after the roof has been signed off. If it is already finished, adding a unit is still entirely possible, it just involves more steps.
The trades and the order of work
Adding a rooflight to a covered roof involves the same people as in new work, but the sequence is tighter because the room below is usually finished or in use. A carpenter forms the opening and the kerb. A roofer makes the covering good around it. Our crew fits the rooflight once the covering is dressed up the kerb. An electrician runs the cable first if the unit has an electric vent, and the plasterer lines the reveals last.
Protecting the room is part of the plan: dust sheets, a covered temporary weathering over the opening each evening and a clear route for materials. We agree this with you before the work starts so you know which days the room cannot be used.
The rooflight itself is ordered once the opening size is agreed. How long the unit takes to arrive is up to the maker, and the quote names it, so the carpentry and the roofer's visit can be arranged to finish as the unit arrives.
Weather is the other thing to plan around. The covering is opened only as far as can be closed again the same day, so a dry spell matters more here than on a new build, and a day can be moved if rain is forecast.
Rules and checks for existing roofs
A new rooflight in an existing flat roof is usually treated under the rules for altering an existing home, and Building Regulations apply, including the thermal limit for the glazing. Notification to Building Control, where the work needs it, is something we take care of. Permission is a separate question, and for most houses the rooflight rules under permitted development are explained in the planning guide. Where you live decides which rules apply, so we read the local position and relay it, and the decision is the council's.
Extra restrictions can apply to listed buildings, conservation areas and National Park land, so check before you order. If a flat roof has a warranty from its installer, ask them what they need before anyone cuts into it, so that the covering's own cover is not put at risk.
Next steps and where the price comes from
A survey shows the roof material, the layers beneath it and what the opening will involve. The quote that follows sets out the unit and glass, the maker's delivery time and the tasks left to other trades. Because the work includes carpentry and the covering as well as the glass, the price is built from several parts, and the flat rooflight cost page explains each of them.
If you are not sure about size, the choice for a kitchen is covered in sizing a rooflight for a kitchen extension, and the full range of units is on our flat roof rooflights page.
