Weathering a Rooflight Into EPDM, GRP and Single-Ply Roofs
Weathering a rooflight into a flat roof means dressing the roof covering up the upstand and over its top edge so the rooflight frame sits on top of the covering, and terminating the covering correctly for whichever membrane the roof is made of. There are four coverings we meet in Suffolk and north Essex, EPDM rubber, GRP fibreglass, single-ply PVC or TPO, and built-up felt, and each is dressed, terminated and warranted differently. A single-ply membrane is hot-air welded, EPDM is bonded with preformed corners, GRP is laminated over a fillet, felt is torched in stepped layers. An installer who dresses all four the same way is wrong three times out of four, and the biggest hidden trap is compatibility: patching one system with another, or mixing PVC and TPO, voids the manufacturer’s warranty and often does not bond at all.
This page sets out the correct termination for each membrane and where the compatibility lines are. The upstand it all dresses onto is specified on rooflight kerb construction, and the wider job is on flat roof skylight installation.
The principle that applies to all four
Before the differences, the rule they all share. The covering comes up the full height of the upstand and is dressed over the top edge, so the frame sits on top of it. Water arriving at the frame has to climb the vertical face, cross the wrapped head and pass under a compressed bedding line before it reaches anything it can damage. The covering must never be stopped short on the face and the gap filled with sealant. Sealant is a gasket, not a waterproofing layer, and a detail that relies on it has a service life of about five years. Every failure below is a departure from that principle in the language of a particular material.
EPDM rubber
EPDM is a single-piece synthetic rubber sheet, usually 1.2mm or 1.5mm, bonded to the deck with contact adhesive, often laid as one continuous sheet with no joints. At an upstand it is bonded up the vertical face with a wall-grade adhesive rather than the deck adhesive, wrapped over the top edge, and either terminated under the frame or held with a mechanical termination bar.
The critical detail is the corners. Internal corners are formed with preformed moulded corner pieces, bonded and rolled, not by folding the sheet and hoping. A folded EPDM corner creates a stress line that opens over a few thermal cycles, and a site-folded corner is the single most common leak point we find on other people’s EPDM work. The second failure is a starved vertical face, where the adhesive was applied too thin on the upstand and the sheet peels away from it. EPDM is our default recommendation for a new extension roof carrying a rooflight, because a joint-free covering has nowhere to fail across the field, and the only risks left are at the corners, which preformed pieces handle.
GRP fibreglass
GRP is a glass-reinforced polyester laminate applied wet on site over an OSB3 deck and cured in place, finished with a pigmented topcoat, giving a hard seamless surface. Its weakness is that it is rigid, and rigid materials on a timber deck that moves seasonally crack if the deck is not detailed with expansion allowance.
At an upstand, GRP is laminated continuously up the vertical face and over the top edge, with the reinforcement mat cut and lapped so the corners get extra layers rather than fewer. The one rule that governs GRP corners is that it must be laminated over a fillet at the internal angle, never into a sharp 90 degree corner. A sharp internal angle in GRP is a crack waiting for its first hard frost, and it fails as a fine line down one face after a cold snap. On a retrofit into existing GRP we cut back to sound laminate, key the surface properly and bond in new laminate rather than smearing topcoat over the joint, because topcoat is a finish, not a structural bond.
Single-ply, PVC and TPO
Single-ply membranes are mechanically fixed or adhered sheets, hot-air welded at the seams. They are the standard on commercial and larger domestic flat roofs, and they are excellent because a hot-air weld is a homogeneous joint, the sheet fused to itself, rather than a glued lap. Systems from the main manufacturers come with proprietary preformed corners and pipe collars.
At an upstand, single-ply is taken up the face, welded at the corners with preformed pieces, and terminated under a manufacturer’s termination bar with a compatible sealant behind it, then the frame is set over that. A cold weld, one that looked welded but was not fully fused, is the failure mode, and it opens quickly under any standing water. The compatibility rule is absolute here and it is the one that costs people money: PVC and TPO are different plastics and they do not weld to each other. If your roof is PVC, the upstand detail is PVC, and a TPO patch on it will not bond and will not be covered. We check what is actually on the roof before ordering anything.
Built-up felt
Torch-on or pour-and-roll felt in two or three layers is the most common covering on flat roofs built before about 2005, so it is what we find on most replacement jobs. Modern high-performance polyester-reinforced felt is a genuinely good material. The 1980s oxidised bitumen on a lot of Ipswich extension roofs is not, and by now it is well past its service life.
At an upstand, felt is dressed up the face in layers, each stepped so the laps do not stack, with a reinforcing angle fillet at the internal corner and the cap sheet turned over the top edge. Corners are cut, folded and torched properly, or they are patched with mastic, and the difference is visible within ten seconds. Where existing felt is oxidised, brittle and crazing, we say so at survey, because bonding a new rooflight detail into a covering with two years left in it is not work we will sell. When that roof goes, the rooflight gets blamed.
The comparison, side by side
| Covering | How it joins to itself | Upstand termination | Corner detail | Where it fails |
|---|---|---|---|---|
| EPDM rubber | Bonded laps, or none if one sheet | Wall-grade adhesive up the face, over the head | Preformed moulded corners, bonded and rolled | Site-folded corners, starved adhesive on the vertical |
| GRP fibreglass | Continuous laminate, no joints | Laminated up the face and over the head | Laminated over a fillet, extra mat | Cracks at sharp internal angles and deck movement |
| Single-ply PVC or TPO | Hot-air welded, fused | Welded up the face, proprietary termination bar | Preformed welded corners | Cold welds, incompatible patch material |
| Modern felt | Torched or bonded laps | Stepped layers up the face, cap over the head | Angle fillet, cut and torched corners | Stacked laps, mastic-filled corners |
How each of these degrades once water is allowed to stand against it is on ponding water around a flat rooflight.
Compatibility, and how warranties are voided
The most expensive mistakes on flat roof rooflight work are compatibility mistakes, because they often do not show until years later and they void the covering warranty in the meantime.
- PVC and TPO do not weld together. They are different plastics. A TPO patch on a PVC roof, or the reverse, will not fuse, and the manufacturer will not honour a system warranty on mixed materials.
- EPDM adhesives are not interchangeable. The deck adhesive and the wall-grade adhesive do different jobs, and using the deck grade on the vertical is why sheets peel off upstands.
- GRP topcoat is not a bond. Smearing topcoat over a joint in existing GRP looks like a repair and holds for one season. New work has to be cut back and laminated into sound material.
- Bitumen products attack some single-ply. Certain PVC membranes are degraded by contact with bitumen, so a felt-based patch or a bitumen sealant on a single-ply roof can do slow harm.
- Mixing systems ends the warranty. A covering manufacturer’s guarantee is written for their system installed to their detail. Introduce another maker’s product at the rooflight and the guarantee on the whole roof is at risk.
We detail the rooflight in the same system the roof is made of. Before we order anything, the survey confirms what is actually on the roof, because the upstand detail for a PVC roof is a PVC detail and nothing else. Matching the system is what keeps both the rooflight and the roof warranty intact.
Why none of this is a repair
When a rooflight leaks at the covering, the answer is to strip the covering back around the opening and re-dress it correctly in the matching system, not to run a bead of mastic over the failed corner. Mastic on a failed termination is a five-year detail at best, and on a roof that is itself near end of life it delays the recover the roof needs. We do not offer resealing or patching on any rooflight. Where the covering has failed at the upstand we re-dress it properly, and where the covering itself is expired we are honest that the roof needs recovering first, timed as set out on recovering a flat roof and rooflight together.
Common questions about rooflight flashing and membranes
Can you fit a rooflight into any membrane type?
Yes, into EPDM, GRP, single-ply and sound felt, using the correct termination for that system. The rule is that the detail matches the covering: a PVC roof gets a PVC detail, an EPDM roof gets preformed EPDM corners. We confirm what is actually on the roof at survey before ordering, because mixing systems voids the warranty and often does not bond.
Will fitting a rooflight void my flat roof warranty?
Not if the rooflight is detailed in the same system as the roof, to the manufacturer’s detail. It is mixing materials that voids a warranty, for example a TPO patch on a PVC roof. We match the system and use the maker’s preformed corners and termination bars, which keeps both the rooflight guarantee and the roof warranty intact.
Why do EPDM and single-ply corners leak?
Because they were folded on site instead of formed with preformed pieces. A folded corner creates a stress line or a cold weld that opens over a few thermal cycles. Both EPDM and single-ply systems supply moulded or weldable preformed corners for exactly this reason, and a corner made with them, bonded and rolled or hot-air welded, is where the detail should be strongest, not weakest.
Can you patch the flashing rather than replace it?
We do not offer patching. A mastic patch over a failed termination is a five-year detail that hides the real problem, and on an ageing roof it delays the recover the roof needs anyway. Where the covering has failed at the upstand we strip it back and re-dress it correctly in the matching system, which is the only repair that lasts.
Get the covering detail done properly
Tell us what the roof is covered in if you know, whether it is a new opening or a leaking existing unit, and where the property sits. We will survey it, confirm the membrane, dress the rooflight in the matching system with preformed corners and the correct termination, and give you one written fixed price. Every installation carries a 10-year workmanship guarantee, and the installers have 25 years fitting rooflights behind them.
Request a fixed quote for flat roof rooflight work in Ipswich, Suffolk or north Essex. If water is already getting in at the kerb, start with leaks at the rooflight kerb.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.