Can a rooflight be fitted to a flat roof?
Yes. A flat roof is the most accommodating place in a house to put daylight, and it is where the majority of new glass goes across Essex. The question is never really whether it can be done. It is what the deck is made of, which way the joists run, how the roof drains, and how high the glass has to sit above the finished surface to stay dry through a wet February.
This page works through what has to be true before the answer is a firm yes, and what a survey looks at to get there. The wider process is covered on flat roof rooflight installation.
Yes, and the deck underneath decides how easy it is
Every flat roof is a deck spanning between walls, a layer of insulation, and a waterproof covering over the top. A rooflight interrupts all three. The covering has to be cut and re-terminated, the insulation has to be carried around the opening without leaving a cold line, and the deck has to have something spanning across the hole where the boards used to be.
On a timber joisted deck, which is what sits over almost every domestic extension in the county, all three are routine. On a concrete deck, common on nineteen sixties flat roofed garages and on some Basildon and Harlow new town housing, the opening is a different conversation and usually wants an engineer.
What counts as flat, in degrees
Flat is a trade word rather than a geometric one. In practice it means anything below about ten degrees, and the products designed for it are sold on that basis.
Above ten degrees you are into roof window territory, where the unit is trimmed between rafters and weathered with a flashing kit chosen for the covering. Below ten degrees the unit sits on a raised kerb and the covering is dressed up against it. The two families are not interchangeable, and a flat rooflight fitted into a genuinely pitched slope will sit proud and look wrong from the garden.
The coverings found on Essex flat roofs
Four coverings turn up on domestic flat roofs, and each terminates against a kerb differently.
- Torch-on felt, still the default on older extensions and garages. It is dressed up the kerb in two layers with the cap sheet lapped over. Detail on felt roofs is covered separately.
- GRP, laid wet and cured in place, which forms a continuous laminate up the side of the kerb with no joint at the angle.
- Single ply membrane, hot air welded, which suits large modern extensions and takes a very clean upstand termination.
- Mastic asphalt, found on interwar and municipal work, poured in two coats and skirted up the kerb with an angle fillet at the base.
The covering does not decide whether a rooflight is possible. It decides how the perimeter is built and who is doing that part of the work.
Forming the opening in a joisted deck
Joists in a domestic flat roof typically run at four hundred or six hundred centres, spanning the short way across the extension. An opening removes a length of one or more joists, and the load they were carrying has to be redirected.
The standard solution is trimming. Trimmers go in across the ends of the cut joists, the joists either side of the opening are doubled up to carry the extra load, and the whole thing is hung on joist hangers rather than skew nailed. An opening up to around one and a half metres wide is usually within reach of timber trimming. Beyond that, or where the opening runs across the span rather than along it, the load starts asking for steel.
Where on the roof plane the unit can land
Position matters more than people expect, and it is set by water rather than by the view from below.
A flat roof falls towards an outlet or a gutter, and a rooflight kerb sitting directly in that path becomes a dam. Water backs up against the upstand on the high side, sits there, and finds any weakness in the detail. The unit therefore wants to sit clear of the main drainage run, or the falls around it want tapering so that water is directed past the kerb on both sides rather than into it.
Parapets complicate this further. A parapet roof drains internally, so there is nowhere for water to leave except the outlet, and the margin for error around a kerb is smaller.
Warm deck and cold deck build-ups
The insulation position changes the whole detail. In a warm deck, insulation sits above the structural deck and below the covering, with a vapour control layer under it. The kerb is built up through that insulation and insulated on its outer face, so the thermal line stays continuous.
In a cold deck, insulation sits between the joists with a ventilated void above it. Cutting an opening in a cold deck interrupts the cross ventilation of that void, and a rooflight kerb can create a dead pocket where moist air stalls and condenses on the underside of the deck. Where a survey finds a cold deck, the ventilation path around the new opening is designed rather than assumed.
| Build-up | Insulation position | What the kerb has to do |
|---|---|---|
| Warm deck | Above the deck, under the covering | Insulated on the outside face, continuous with the roof insulation |
| Cold deck | Between the joists, void above | Preserve cross ventilation of the void on both sides |
| Inverted | Above the covering, under ballast | Height measured from the ballast surface, not the membrane |
Fitting into an existing roof rather than a new one
On a new extension the rooflight is designed in, the opening is formed as the roof is built, and the covering goes on once around a kerb that is already there. That is the cheapest and cleanest version of the job.
Into an existing roof the sequence reverses. The covering is stripped back around the intended opening, the deck is opened up, the trimming goes in, the kerb is built, and the covering is made good and dressed up the new upstand. It is entirely normal work, and it is one to two days on a typical extension. The roof is closed and watertight at the end of every working day.
What the survey settles before the answer is firm
Six things, and none of them can be settled from a photograph alone. What the covering is and how much life it has left. Which way the joists run and at what centres. Where the falls go and where the water leaves. Whether the deck is warm or cold. What the ceiling below is doing, since a splayed reveal needs depth. And how materials get up and waste comes down.
Sunspire fits new rooflights into flat roofs and replaces units that have reached the end of their working life. A range comes by email once the basics are known, and a fixed written figure follows the survey.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements