How Is a Flat Rooflight Fitted to a Warm Roof?

What makes a roof "warm"

In a warm roof, the insulation lies on top of the structural deck and under the waterproof membrane. The timbers below stay inside the warmth of the house, and there is no ventilated void to keep clear. Most new flat roofs on extensions are built this way, and many older roofs are converted to warm roofs when they are recovered.

The layers, from the bottom up, are the ceiling, the joists, the deck, a vapour control layer, rigid insulation boards and the waterproof covering.

Layers of a warm roof around a rooflight

Deck	Boards or ply carrying the roof, trimmed round the opening
Vapour control layer	Taken up the inside of the upstand
Insulation	Rigid boards, carried up the kerb to the glazing frame
Upstand	Insulated kerb, usually at least 150mm above the finished roof
Covering	Membrane dressed up and over the upstand

Forming the opening

The opening is set out from inside the room first, so that it lines up with the ceiling and the light well. The deck is cut back, and trimmers are fixed to carry the joists that have been interrupted. The size of the opening depends on the rooflight you have chosen, and flat rooflight sizes explains how the opening and the visible glass relate.

Building the upstand on the deck

With the opening framed, the upstand is built around it. It may be a timber kerb lined with insulation, or a ready-made insulated GRP or PVC kerb sized to the rooflight. Either way it stands on the deck and rises at least 150mm above the finished roof surface, which includes the thickness of the insulation and covering.

The key detail on a warm roof is continuity. The vapour control layer is lapped up the face of the upstand, and the roof insulation boards are cut tight against the outside of the kerb. Any gap here is a cold spot, and it can lead to condensation in the corners of the light well.

The membrane goes up and over

Once the insulation is in place, the waterproof covering is laid. Whether it is EPDM, GRP, felt or a single-ply sheet, it is taken up the outside of the upstand and finished at the top, where the rooflight frame will cover its edge. Corners are dressed carefully, because that is where the covering is under the most stress. The rooflight is then set on the kerb and fixed, and the frame is sealed to the upstand according to the maker's instructions.

If the roof is not yet a warm roof

On an existing cold roof, the insulation sits between the joists and the void above is ventilated. Fitting a rooflight there works differently, because the opening cuts across that void. If your roof is being recovered, you can convert it to a warm roof at the same time, and the rooflight can then be planned into the new build-up. We cover this question for felt roofs on our page about felt roofs.

Why the thermal line matters

A rooflight frame is usually colder than the roof around it, so the junction is where condensation is most likely to show. The cure is a continuous line of insulation from the roof boards, up the kerb and right to the underside of the frame. Where that line is broken, warm air from the room meets a cold surface and leaves damp patches on the plaster of the light well.

For this reason we check the fit of every insulation board against the kerb before the covering goes down. Small gaps are filled with the right material for the job, not with whatever is to hand, and the vapour layer is taped at every lap.

Choosing the kerb

The kerb is one of the choices you make at the quote. A site-built timber kerb can be shaped to an odd roof fall or an awkward opening. A factory-made insulated kerb arrives to size and is quick to fix. Either one can work on a warm roof. The better choice depends on the roof and on the rooflight, and we explain the reasoning at the survey so the quote is not a surprise.

The hidden work is photographed

Once the covering goes on, the upstand, the insulation and the vapour layer are out of sight for good. We photograph them before they are covered, and you receive those pictures at handover, alongside the product details, the makers' warranties, our 10-year workmanship guarantee and, if the job had to be notified, the Building Control certificate.

Regulations

Cutting into the roof calls for Building Regulations approval, which checks the framing and the heat loss, and a flat rooflight must achieve 2.2 W/m²K for existing homes under Part L 2021. A new extension roof is assessed as a whole, so the rooflight is shown on the same drawings as the rest of the roof. The planning guide covers permitted development, and the flat roof rooflight installation page describes the full service.