Single-ply, felt and GRP roofs around a rooflight

Four coverings, and why the kerb detail is the difference between them

On the flat, all four systems do the same thing. Water lands, runs to a fall, and leaves. Any of them will do that for decades.

At an upstand the differences become concrete. One system welds, one melts, one cures chemically, one is folded. One turns a right angle happily, two need a fillet, one needs a fillet and a bandage. One can be dressed over a curved kerb head, the others cannot. One will not go down below five degrees of air temperature.

So the covering is not a finish applied at the end. It sets constraints on the kerb before the kerb is built.

Single-ply: a sheet membrane, welded rather than stuck

Single-ply is a factory-made sheet, usually PVC, TPO or EPDM, delivered in rolls and joined on site. PVC and TPO are joined by hot air welding at around 450 to 550 degrees, which fuses the two sheets into one material at the lap. EPDM is joined with tape or adhesive.

It is thin, typically 1.2 to 1.5mm, light, and it is either mechanically fixed through the insulation into the deck, fully adhered, or held down by ballast. Because the sheet is a single continuous material with welded laps, a well made single-ply roof has no chemical curing to go wrong and no drying time.

Where single-ply stops at a kerb

The membrane is dressed up the outside face of the kerb, over the top, and terminated so that no cut edge is left exposed to weather.

Two details matter. The four internal corners at the base of the kerb are formed with pre-made corner pieces welded in, because a folded sheet at an internal corner leaves a fold that cannot be welded through. The top termination is either taken over the kerb head and clamped under the rooflight frame, or held with a termination bar and a compatible mastic.

Every weld is probe tested with a blunt seam probe run along the edge. It is a crude test and a completely reliable one: a weld that has not taken will open under the probe.

Reinforced bitumen membrane: layers bonded in sequence

What most people call felt is now a polyester or glass fibre carrier saturated with modified bitumen, laid in two or three layers with staggered laps. On a warm deck it is commonly a vapour control layer, insulation, a bonded underlayer and a torch-applied mineral surfaced cap sheet.

Modern grades bear little relation to the roofing felt of the 1970s that gave the material its reputation. A properly specified two layer torch-on system on a rigid deck is a long-lived roof. Its virtue at a kerb is that it is applied in a molten state and can be worked into an awkward shape by hand.

Felt at an upstand: the upstand piece and the reinforcement at the angle

The layers do not simply run up the kerb. Each is taken up in a separate cut piece, lapped over the horizontal layer below it, so that water running down the upstand is always shed onto the outside of the next sheet down.

An additional reinforcing strip is bonded over the angle where the flat meets the upstand, because that is the line that sees the most movement and the most standing water. At the top, the cap sheet is taken over the kerb head or terminated under a metal trim.

Torching at a timber kerb is done with care and with the naked flame kept off the timber. On a rooflight job the upstand detailing is often completed with a self-adhesive or cold-applied grade for exactly that reason.

GRP: laid wet, cured in place, one continuous piece

Glass reinforced polyester goes down as chopped strand glass matting saturated with catalysed polyester resin, rolled out to remove air, and left to cure into a single rigid laminate with no joints anywhere. A topcoat is applied over it for ultraviolet protection and colour.

The complete absence of laps is the attraction, and it is a real one. It also means the covering is rigid, and a rigid covering on a moving substrate finds the point of highest stress and cracks there. On a rooflight roof that point is a kerb corner.

GRP at a kerb: trims, overlamination and the temperature it will not cure below

GRP kerb detailing uses preformed trims. An internal angle trim at the base of the upstand, a simple edge trim at the head, and both bonded in and then overlaminated with matting and resin so the trim becomes part of the laminate rather than a separate component.

The constraint is the weather. Polyester resin needs an ambient and substrate temperature above roughly five degrees to cure properly, the substrate has to be dry, and the catalyst ratio is adjusted for the conditions on the day. A cold, damp week in a Gloucester January is not a GRP week, and an honest programme says so rather than laying it and hoping.

Liquid applied membranes, and the kerb with no straight lines

Liquid systems, usually polyurethane or PMMA, are brushed or rolled on with a reinforcing fleece embedded in them, curing to a continuous elastomeric coat. They are more flexible than GRP and they will follow any shape at all.

That makes them the practical answer where a kerb is an odd shape, where a rooflight sits hard against a parapet or an abutment with 100mm to work in, or where several details converge in one corner. PMMA cures in around an hour and will go down at low temperatures, which occasionally decides the specification in winter. They cost more per square metre, so they are frequently used just at the details with another system across the field.

Lead, and the roofs behind a Cheltenham parapet

Lead is still the covering on a great many Regency and Victorian flats in Cheltenham, Gloucester and the market towns, and where a roof is listed or in a conservation area it often has to stay lead.

It works by folding rather than bonding, which means it must be allowed to move. Bays are limited in size, commonly around 1.5 metres in the direction of fall, with welted or rolled joints between them, and the sheet is fixed only at its head so the rest can expand. A rooflight kerb in a lead roof is dressed with a separate lead upstand piece taken up the kerb and secured under a clip or a cover flashing, and the bay layout is planned around the kerb rather than adjusted afterwards.

The fillet at the base of the upstand, and the angle each covering can turn

No covering likes a sharp internal right angle at the foot of a kerb. The material is stretched on the outside of the bend and compressed on the inside, and any debris that arrives settles precisely in that line.

An angle fillet, typically 50mm on the face and cut from timber or from insulation depending on the build-up, turns one ninety degree bend into two of forty five. Bitumen systems and single-ply both want one. GRP takes a preformed trim that does the same job. Lead is bossed or welded to shape and needs a tilt rather than a fillet. It is a small item and it is the one most often left off a kerb that somebody else built.

Which surface the rooflight frame actually sits on

The unit does not sit on the covering. It sits on the top of the kerb, and the covering is dressed up and over that top surface first, so that the frame lands on waterproofed timber rather than on bare plywood.

That sequence has a consequence for the kerb height. The covering, any fillet, any insulation returning onto the kerb and any trim all take up thickness, and the 150mm measured from the finished waterproofing to the top of the kerb has to survive all of it. Where a kerb is built to 150mm above the deck rather than above the finished covering, the height is short before anybody arrives.

Cutting into an existing covering, and the joint between old and new

Putting a rooflight into a roof that is already covered means opening the covering, forming the kerb, and then marrying the new work back into the old.

How well that goes depends almost entirely on the system. Single-ply welds to itself if it is the same polymer, clean and not too weathered, and a weld to a fifteen year old PVC sheet needs the surface preparing first. Bitumen bonds to bitumen readily. GRP will bond to GRP where the topcoat is abraded back to the laminate. Lead is dressed and welted into the existing bays.

Where the existing covering is at the end of its life, the sensible answer is a new covering across the whole roof at the same time, because the roof is open anyway and the alternative is a new detail surrounded by an old one.

Compatibility, and what happens when two systems meet

Materials that are individually sound can be a poor combination. Plasticised PVC and bitumen is the well known one: plasticiser migrates out of the PVC where the two are in contact, the membrane stiffens and eventually becomes brittle. A separation layer keeps them apart.

Polystyrene insulation and hot bitumen is another. Some polyurethane sealants and some single-ply polymers are a third. On a rooflight job this matters because the kerb detail is often where a different material is introduced: a trim, a tape, a mastic, a preformed corner. Each one has to be the manufacturer’s own or a stated equivalent, and that is one of the four invisible details the ten year workmanship guarantee stands behind.

Thermal movement, and the corner that goes first

A dark flat roof surface in Gloucestershire reaches well over fifty degrees on a July afternoon and drops below freezing on a January night. Everything on that roof expands and contracts through that range, and the deck below it moves too as timber takes up and gives off moisture.

Flexible systems absorb the movement. Rigid ones transfer it to whatever restrains them, and a kerb is a restraint. That is why a hairline crack in a GRP roof shows up at a kerb corner first, and why the corner detail on a rigid system is overlaminated rather than simply trimmed. It is also why the deck under a GRP roof is specified as tongued and grooved boarding laid to a discipline rather than any sheet material that happens to be to hand.

What each system needs from the weather on the day

Programming a rooflight installation around a Gloucestershire forecast is a real part of the work, because the roof is never left open overnight.

Single-ply will go down in the cold and can be welded in low temperatures with an adjusted machine speed, though not in rain. Torch-on bitumen needs a dry substrate and is workable through most of the year. GRP wants dry, above five degrees, and rising rather than falling temperatures. Liquid PMMA copes with the cold better than almost anything else. Lead is the least weather sensitive of the four.

In practice this means the covering on your roof influences which week the work happens in, and it is better to say that at quotation stage than to discover it in November.

Gloucestershire roofs, and the coverings you actually find

Victorian rear additions through Gloucester, Tredworth and Kingsholm mostly carry bitumen membrane laid over an earlier covering, sometimes three deep, on softwood boarding that has taken on a wave. That build-up gets stripped rather than added to when a rooflight goes in.

Cheltenham’s parapet flats are lead, often original, and often listed. The bay layout and the shared outlet dictate where a kerb can go, and the conservation position may dictate the unit as well, which is covered on the conservation rooflight installation page.

Estate housing at Quedgeley, Bishops Cleeve and Abbeydale has GRP or single-ply on garage roofs and rear extensions from the last twenty years, generally in good order. Forest of Dean roofs around Coleford and Cinderford get more rain and more leaf fall from the surrounding woodland, and single-ply with a well sized outlet copes with that better than most.

Where the covering hands over to the unit

There is one line on every flat roof where responsibility changes hands: the top of the kerb, where the covering ends and the rooflight frame begins.

Doing the kerb, the covering and the unit as one continuous operation keeps that line inside a single piece of work, which is how it is handled here. Where a roofing contractor has already covered the roof, the sequence is agreed in advance: what height the kerb is built to, how far the covering is dressed, what trim is used, and who is on site the day the glass arrives. Written down before anybody starts, it takes five minutes. Left unsaid, it is the gap the water finds.