Guide · Flat Roof Rooflight Installation
Single-ply, felt and GRP roofs around a rooflight
The covering on a flat roof decides how a rooflight is weathered. There is no flashing kit for a flat roof in the way there is for a tiled slope: the covering itself is the flashing, dressed up the face of the kerb, over the top and terminated under the frame. So the material on the roof sets the detail, the sequence, the trades involved and the season the work can happen in.
Three coverings account for nearly all domestic flat roofs in Essex: single ply membrane, reinforced bitumen membrane, and glass reinforced polyester. This page is about how each behaves at a rooflight upstand and what each asks for. It sits under flat roof rooflight installation.
The three coverings you meet on Essex flat roofs
Single ply is a factory made sheet, typically 1.2 to 1.5mm of PVC or TPO, laid in wide rolls and joined by hot air welding. Reinforced bitumen membrane is the modern descendant of built-up felt, laid in two or three layers with a torch or with self adhesive backing. GRP is a liquid system: chopped strand mat laid onto a plywood deck and wetted out with catalysed resin, then topcoated.
They are not interchangeable. Each has a different substrate requirement, a different way of turning a corner, a different working temperature and a different relationship with the timber underneath. The material already on a roof usually decides what goes back on it, which is why the covering is the first thing a survey identifies.
Single ply membranes at a kerb
The membrane is brought up the face of the kerb in one piece where the sheet width allows, and a separate corner piece is welded in at each of the four corners. At the top the sheet is turned over the kerb and mechanically fixed with a termination bar or a fixing plate, then the rooflight frame lands over it.
The important quality of single ply at an upstand is that the vertical face and the horizontal roof are the same material with a welded joint between them, so there is no change of product at the corner. The weak point is entirely the weld. A weld is a manufactured joint made on site with a hot air gun, and it is either continuous or it is not.
Hot air welding, and the lap that matters
A single ply weld fuses the two sheets into one. Done at the right temperature and speed, the joint is stronger than the sheet. Done cold, it looks identical and holds nothing. That is why welds are probe tested along their length with a blunt hook, corner by corner, before anything covers them.
Lap widths are typically 40mm on a machine weld and 60mm hand welded, and hand welding is what happens at every rooflight because a machine cannot follow a corner. The four corners of a kerb are therefore the four most hand made metres on the entire roof, and they are where a competent installation and an indifferent one diverge.
Reinforced bitumen membrane, layer by layer
A two or three layer system. A vapour control layer or perforated underlay first, then an intermediate sheet, then a mineral finished cap sheet. At an upstand the layers are stepped so they do not all terminate on the same line, and a separate upstand sheet carries the vertical face with an angle fillet at the base.
The angle fillet is the detail people forget. A triangular timber or bitumen fillet at the internal angle between the roof and the kerb stops the membrane having to make a sharp ninety degree bend, because a sharp bend in a bitumen sheet is a stress line that will eventually crack. It costs a length of timber and it is the difference between a twenty year detail and a ten year one.
Torch applied detailing at an upstand
Bitumen systems bond with heat, and heat near a rooflight frame is a real constraint. Frames are aluminium with thermal breaks and gaskets, and a torch flame will damage both. So the sequence is that the covering is completed to the top of the kerb before the unit is set, or the top edge is finished with a self adhesive or cold applied product where the frame is already in place.
There is also a fire safety dimension. Hot works on a timber kerb wrapped in insulation deserve care, a fire watch afterwards and, in practice, a preference for torch free detailing in the last few hundred millimetres. Self adhesive upstand sheets exist specifically for this and are the sensible choice at every rooflight.
GRP laminate, and how it is built up
GRP is built in place. An 18mm WBP plywood deck is laid, trims are fixed at the edges, chopped strand mat is laid over the whole area including up the kerb, and catalysed resin is rolled through it until the mat is fully wetted out and consolidated. When it has gone off, a pigmented topcoat is applied.
The result is a jointless covering with no welds and no laps at all, which is its great advantage at a rooflight. The upstand, the corners and the roof plane are one continuous laminate. Its disadvantage is that it is rigid, it is chemically cured, and both of those properties make it fussy about conditions and about movement.
Trims, and where GRP differs most
Single ply and bitumen systems terminate against a bar or under a frame. GRP terminates against a trim, a preformed GRP or aluminium section bonded into the laminate, and the trim becomes part of the covering rather than a cover over it.
At a rooflight kerb that means the top of the upstand is usually finished with a bonded top hat or a simple over-the-top laminate turned onto the kerb head, then dressed in before the unit lands. Getting a clean, flat, fully consolidated laminate over a kerb head is skilled work, because the mat wants to lift off an external corner while the resin is still mobile.
What each covering asks of the substrate
| Single ply | Reinforced bitumen | GRP | |
|---|---|---|---|
| Deck required | Ply, OSB or insulation faced board | Ply, OSB or insulation faced board | 18mm WBP plywood only |
| Moisture tolerance of deck | Dry preferred | Dry preferred | Must be dry, under 12 per cent |
| Working temperature | Down to about 5°C | Down to about 5°C | Above 5°C and rising |
| Joint method at kerb | Hot air weld | Torch or self adhesive lap | No joint, continuous laminate |
| Movement tolerance | High | Moderate | Low |
Compatibility, and what can go over what
Bitumen and single ply do not get on. Plasticisers migrate out of PVC membranes into bitumen and the membrane degrades, so a PVC single ply laid directly over an old felt roof needs a separation layer between them. TPO is less sensitive but the same separation is normal practice.
GRP goes onto plywood and effectively nothing else. It cannot be laid over an existing felt or membrane covering, and it cannot be laid onto OSB reliably. So a change to GRP means stripping back to a new deck, which changes the scope of any job where a rooflight is being introduced into an existing GRP roof or where somebody has specified GRP over what is already there.
Movement, and how each covering handles it
Timber roofs move. They expand and contract with temperature, they take up and give off moisture, and they deflect under load. A covering has to absorb that without transferring it into a stress line.
Single ply is flexible and mechanically fixed or adhered, so it accommodates movement easily and is the most forgiving of the three. Bitumen membranes are moderately flexible and rely on the fillet and the layering to spread stress. GRP is rigid and bonded, so it depends entirely on the deck being stable: joints in the plywood need to be well supported and the boards need to be fixed at close centres. Around a rooflight kerb this matters because the kerb is a stiff object in a flexible field, and the covering has to make the transition.
Temperature, season and what can be laid when
GRP is the constrained one. Resin cures by an exothermic reaction and the catalyst ratio is adjusted to the ambient temperature. Below about 5 degrees the cure is unreliable, in cold damp conditions the topcoat can bloom, and a shower during cure ruins the surface. Practically, that means an Essex November GRP roof is a weather gamble.
Single ply and bitumen are more tolerant, working in cold conditions provided surfaces are dry and free of frost. Since Sunspire closes and weathers a roof at the end of every working day regardless of forecast, the covering choice affects how a winter job is sequenced: a membrane can be run up and terminated in an afternoon, where a laminate needs a cure window.
Termination heights and the top edge fixing
Whatever the covering, it terminates over the top of the kerb and under the rooflight frame, and it is mechanically held at that point rather than relying on adhesion alone. Adhesives and welds hold well in shear and less well in peel, and wind at the head of an upstand is a peeling force.
The termination line should sit at least 150mm above the finished roof surface. Where a covering has been brought only part way up the face and finished with a mastic bead at the top, water running down the face has a route behind it, and that arrangement is one of the most common findings on flat roofs with rooflights added later.
Corners, and where a covering is most likely to fail
Every covering fails at the same four places: the external corners of the kerb. Single ply needs a preformed or hand fabricated corner welded in. Bitumen needs the layers cut and lapped so the corner is covered twice. GRP needs the mat properly consolidated round an arris that wants to push it away.
Because corners are where all three systems are at their most hand made, they are also where a covering deserves a look during any autumn clearing. Debris collects there, water sits there longest, and any lifting shows itself there first.
Every flat roof covering is strongest in the middle of the sheet and weakest at the four corners of the kerb.
Working into an existing covering
Adding a rooflight to a roof that is already covered is a different exercise from building one into a new roof. The covering has to be cut back beyond the kerb line, the deck exposed, the opening trimmed, the kerb built and then the covering brought back and joined to what is already there.
That join is the whole question. Single ply welds cleanly to itself if the existing sheet is sound, clean and the same material; a twenty year old PVC sheet with a weathered surface needs cleaning back and testing before anyone relies on it. Bitumen bonds to bitumen readily. GRP bonds to GRP with abrasion and preparation. Where the existing covering is close to the end of its service life, the sensible sequence is to renew the covering and form the opening in one operation rather than joining new work to old.
Service life against the life of the glass
A sealed glazing unit has a working life in the region of twenty to twenty five years. A single ply membrane is typically warranted for 20 to 25 and often runs longer. A good three layer bitumen system is similar. A well laid GRP roof on a properly supported deck is comparable again, and a thin GRP laminate on a flexing deck is not.
The useful conclusion is that the covering and the glass are roughly contemporaries. If the roof covering is already fifteen years old and a new rooflight is going in, it is worth pricing the covering renewal at the same time, because doing both together costs less than doing each separately and it puts the two components on the same clock.
Choosing a covering when the rooflight is the priority
If the daylight is the point of the project and the covering is simply what keeps it dry, single ply is the straightforward default. It is flexible, it welds to itself, it deals with kerbs and corners cleanly, it works in most weather and it copes with movement in a timber roof.
Bitumen makes sense where the existing roof is bitumen and sound, and where the budget favours a familiar system. GRP suits new decks on well built structures where a continuous jointless finish and a walkable surface are wanted, particularly on a roof that will be seen from a window above. Whichever is chosen, the covering is specified before the rooflight is ordered, because the detail at the head of the kerb is different in each case and the frame has to suit it.
Keep reading
More on flat roof rooflight installation
Flat rooflight falls, drainage and ponding
Read the guide →Walk-on rooflights and where they make sense
Read the guide →Flat rooflights on a cold roof build-up
Read the guide →How many rooflights a flat roof extension needs
Read the guide →Flat rooflight standard sizes and made to measure
Read the guide →
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