Can a Flat Rooflight Be Fitted to a GRP or EPDM Roof?

GRP stands for glass reinforced plastic, a fibreglass roof that is laid as a single unbroken sheet and common on newer flat extensions and garage tops. EPDM is a synthetic rubber sheet, supplied in large sections and bonded to the deck, with few joints. Both are single-layer coverings, which is what makes them popular on the flat-roofed extensions of newer estates in Bristol and South Gloucestershire.

They behave differently under tools. GRP is hard and brittle once cured, so cutting must be clean and the edges are finished with resin. EPDM is flexible and tends to be dressed up and around an upstand with adhesive. In both cases the covering and the rooflight must work as one waterproof arrangement, and that is the point to get right.

Felt and other built-up coverings are a different case again, and the detailing follows the same principle: the covering rises up the kerb and is finished with the correct edge trims. If you are unsure what your roof is, a quick look from a ladder at the survey will tell us, and the sheen, thickness and seam pattern give it away.

A flat rooflight does not sit on the membrane. It sits on a kerb, which is a timber or insulated upstand built around the opening and rising above the roof surface. The roof covering is carried up the outside of that kerb, and the rooflight frame then sits on top. The rooflight kerb guide explains the build-up in detail.

For a GRP roof, a new section of laminate is bonded to the existing and carried over the kerb, then topcoated for a uniform finish. For EPDM, a new piece is bonded round the upstand and sealed with the maker's own accessories. Using the covering's own system, rather than a mix of products, is what keeps the warranty position clear.

Insulation plays its part inside the kerb. A kerb that is hollow and uninsulated becomes a cold bridge between the room and the weather, so we build or buy it with insulation inside, which keeps the room edges warmer and eases misting on the glass.

Cutting into a continuous membrane is a one-way step. The opening is made, the kerb built and the covering finished, and during that window the roof is open to weather. We plan the work so the roof is weathertight at the end of every day, with a temporary cover where a coat of resin or a bonded sheet needs longer to cure. The same crew carries the job through, which keeps the handover between the carpentry and the membrane free of gaps.

The age and condition of the covering matters. A roof that is already tired may be better dealt with at the same visit, because opening it up for the rooflight exposes the deck and shows what lies beneath. If it is sound, the new unit simply joins it. If not, a new covering round the opening, or over the whole roof, can be priced as an option on the written quote.

The deck underneath is the other check. Boards that are soft or sagging must be sorted before a kerb is built on them, since a kerb needs a firm base. The roof also needs to drain properly around the new upstand, which is why the flat rooflight fall guide is worth reading alongside this one.

Size choices come into it as well. A larger opening means a larger kerb and more covering to turn up, as set out in our what size flat rooflight answer.

Access matters too. Fresh resin and adhesive need dry conditions, so we pick a spell of settled weather for the covering stage, and tell you in advance which day the roof will be open. Inside, floors and furniture are protected while the ceiling opening is formed.

At the survey, which is free, we examine the covering, test the deck and look at how rainwater leaves the roof. The written, itemised quote then names the kerb, the covering approach and the unit, with opening options shown. A fresh opening cut through the roof is building work, and whole-unit swaps are too, so we handle the Building Control notification where it applies. Our workmanship guarantee runs for 10 years on the installation. For the full range of work, see flat roof rooflight installation.