Short answer: Yes, a rooflight on a felt or EPDM roof is routine, and GRP is no different in principle. The unit itself sits on a kerb, and what changes from one covering to the next is how the waterproofing is dressed up that kerb and sealed to it. We look at the covering at survey, because its type and its condition decide how the rooflight skylight is tied in.
Why the covering matters more than the glass
Ask where a flat rooflight is most likely to let water in and the answer is almost never the glass. It is the junction where the roof covering meets the kerb. Glass and frame are made in a factory to a set tolerance. The covering is laid on site, over a deck that moves a little with the seasons, and it has to turn up a vertical face and finish under the frame. That is the detail that needs the care.
So when someone asks whether a skylight on a rubber roof is possible, or whether a rooflight on a felt or EPDM roof will cause trouble, the honest answer is that the roof type is not a barrier. Each covering has an established way of meeting a kerb. The job is to use the right one, and to start from a covering that is sound enough to be joined to.
Torch-on felt
Felt is the covering on a great many Liverpool flat roofs, from the back additions on Victorian terraces to garage roofs and 1960s and 1970s extensions. Modern torch-on felt is laid in layers. A base layer goes down first and a cap sheet over it, each welded by heat to the layer beneath. Where it meets a kerb, the felt is turned up the face of the kerb as an upstand, with extra reinforcement at the corners, and the top edge is finished under the rooflight's frame or an apron flashing.
Because felt is heat-welded, the work near a timber kerb needs thought. A naked flame next to timber and insulation is a risk, so careful detailing and a controlled method matter, and some roofers use cold-applied or self-adhesive felts at the kerb for that reason. If the existing felt is old, cracked or blistered, there is little point joining new work to it. Better to renew the covering around the opening, or the whole roof if it is due.
EPDM, the rubber membrane
EPDM is a synthetic rubber sheet, supplied in large pieces and bonded to the deck with adhesive. The roofer can often cover a whole small roof with one sheet, so there are few seams. At a kerb, the sheet is wrapped up the face of the upstand and bonded to it, with preformed or cut corner pieces and sealing tape for the joins. The top edge is fixed under the frame, which then sheds water over it.
EPDM does not take heat, so there is no torch to worry about. It does need clean, dry surfaces to bond to, and the adhesive and tapes are specific to the membrane. A rooflight fitted to an existing EPDM roof is a straightforward job when the membrane is sound. We use the membrane maker's own accessories so the junction is compatible.
GRP, fibreglass
GRP is glass-reinforced plastic, laid wet with resin over the deck and left to cure into one continuous shell. It is common on flat roofs of recent years, partly because it has no joins in the field of the roof. At a kerb, the GRP is laminated up the upstand, so the covering and the kerb are one skin. The rooflight frame then sits on or over that skin.
Because GRP cures into a rigid shell, a hole cut for a rooflight later is a bigger decision than on felt or EPDM. The cut edge must be sealed and the new upstand laminated into the old shell, and the resin needs the right conditions to cure, so we plan for dry days. Where a GRP roof is being laid new, the best order is to have the kerb in place first so the laminate runs over it in one go.
| Covering | Fixed to kerb by | Watch for |
|---|---|---|
| Torch-on felt | Heat-welded layers | Flame near timber |
| EPDM | Bonded sheet and tape | Dirty or damp surfaces |
| GRP | Laminated resin and matting | Weather while curing |
Who does the covering work, and when
This is where plans often go wrong. A rooflight and a roof covering are two trades whose work meets at one line, and the order matters. On a new extension, the usual sequence is the structure, then the kerb, then the covering dressed up to it, then the rooflight set in place. If the flat roofer has already finished before the opening is cut, the covering has to be cut and made good, which is a weaker result than doing it in order.
When we fit a rooflight to an existing roof, we agree at survey who does what. We build or fit the kerb and the rooflight, and the covering must be reinstated around it. If your roof has life left in it, we can work with the covering as it is. If it is near the end, we will say so and suggest renewing it at the same time, since the opening is the best moment to do so. Check any guarantee on the roof covering before work starts, as the manufacturer may have conditions on who can alter it. Should the work be notifiable, we also take on the Building Control notification.
How the kerb is built, including its height and insulation, is set out in our kerbs and upstands guide, while the flat roof rooflight installation page covers the wider options. A written, fixed-price quote follows the survey, and the guarantee on our workmanship runs for 10 years and is in writing. To go ahead, request a free quote or message us on WhatsApp.
Covering questions we hear at survey
Do I need a new roof before I add a rooflight?
Not necessarily. A sound covering can be worked into, and we say so at survey. If it is cracked, blistered or lifting, it is sensible to renew it while the roof is open anyway.
Can the same rooflight go on any of the three coverings?
The rooflight and kerb can be the same. What differs is the waterproofing dressed to the kerb, which is made to suit the membrane on your roof.
Will cutting into my roof affect its guarantee?
It can, which is why it is worth checking the paperwork for the covering first. Tell us what you have and we will take it into account when planning who carries out the covering work.