Single-ply, felt and GRP roofs around a rooflight

Identifying what is already on your roof

Stand at an upstairs window with a camera and zoom in on the edge.

Reinforced bitumen membrane, still widely called felt, is dark, slightly soft underfoot, and usually finished with green or grey mineral granules. You can normally see the joins as parallel lines every metre or so, sometimes with a bead of bitumen squeezed out along them.

Single ply is a thin plastics sheet, most often light grey, occasionally dark, laid in wide widths with welded seams that look like a fine line rather than a lap. It feels hard and it drums slightly if you tap it.

GRP is a continuous resin surface with a slight sheen and no visible joins at all across the field, with moulded plastics trims at the edges and at any upstand. If the edge trim looks like a piece of moulded profile rather than a folded metal drip, it is almost certainly GRP.

Why the covering dictates the perimeter detail

Every rooflight installation ends the same way: the waterproofing has to travel up the outside of the kerb, over the top, and terminate above the level water can reach.

What changes is how the material makes that turn and how it makes the corner. Single ply is welded, so a corner is a preformed piece or a hot-air weld. Bitumen membrane is bonded with heat and overlapped, so a corner is a set of folds. GRP is laid wet over a former, so a corner is moulded in place and cured hard.

Three materials, three sets of skills, three sets of failure modes. An installer who is fluent in one is not automatically fluent in another, and it is a fair question to ask which systems a firm actually works with.

Single ply at a kerb

Single ply is the most controllable of the three at an opening, which is why it is the default on new work.

The field sheet is brought up the outside face of the kerb in one piece where the geometry allows, and over the top. The corners are formed with preformed internal and external corner pieces from the same manufacturer, hot-air welded in. Every weld is then probed with a seam tester, which is a blunt hook run along the edge of the lap: it catches on anything unwelded.

The top of the sheet is mechanically fixed along a line, not left to adhesion, and the fixing line is covered by the unit’s flange or by a proprietary termination bar with sealant behind it.

The advantage at an opening is that a weld is a homogenous joint. Two pieces of the same polymer become one piece. There is no adhesive to age and no bond to fail.

Bitumen membrane at a kerb

Reinforced bitumen membrane is built in layers, and at a kerb that layering is what does the work.

The base layer runs up the kerb face. A reinforcing strip is dressed into the internal angle where the kerb meets the deck, because that angle is where movement concentrates. The cap sheet then runs up over both, lapping well past the angle, and terminates at the top of the kerb under the unit.

Corners are cut, folded and bonded, and this is where the craft lives. A neat felt corner is a specific sequence of cuts that leaves no point where three thicknesses meet in a way that traps a void. A rushed one leaves a cone of unbonded material that will eventually let water in.

The system tolerates a wide range of conditions and it is easy to lap new to old, which is why it is still the right answer on plenty of existing roofs.

GRP at a kerb

GRP is laid as chopped strand mat wetted out with catalysed resin, and it cures into a rigid shell that is bonded to the deck across its whole area.

At a kerb, the mat is dressed continuously from the deck up the kerb face and over the top, so the upstand and the field are literally one piece of material. There is no joint at the base of the upstand at all, which is the strongest thing about the system.

The edges are finished with moulded GRP trims bonded and laminated in, and a topcoat is applied over everything. Done well, the result is an unjointed shell with a fifteen to twenty five year expectation and no laps to worry about.

The catch is that it is rigid. It has to be laid on a stable, dry, correctly fixed deck, usually new OSB or ply of a specified grade, and it does not tolerate movement in the substrate.

The corners: where each system actually fails

Failures cluster at the four corners of a kerb, whichever system you have, because that is where the material has to change direction in two planes at once.

In single ply, the failure is an incomplete weld at a preformed corner, usually because it was welded too cool or too fast. It is invisible and it is found with a seam probe, which is why probing matters.

In bitumen membrane, the failure is an unbonded fold: heat applied to the surface but not into the lap, leaving a void that fills.

In GRP, the failure is a dry area in the laminate, where the mat was not properly wetted out at a tight internal angle and there are air pockets under the resin. It shows later as a whitened area and eventually as a crack.

All three are workmanship, not material. All three are avoidable by taking the extra twenty minutes at each corner.

Cutting a new opening into an existing covering

The sequence is the same in principle and different in practice.

On single ply, the covering is cut back around the intended opening to expose deck and insulation, the kerb is built, and new sheet is welded to the existing sheet. Welding to old single ply is entirely possible but the surface has to be cleaned back to sound material, and you need the same polymer family: a PVC offcut will not weld to TPO.

On bitumen membrane, the covering is cut back, the kerb built, and new membrane torched or bonded in with generous laps onto the sound existing material. This is the most forgiving of the three for a localised alteration.

On GRP, you cannot weld or bond a new sheet to a fully cured one with any confidence at a lap. The joint has to be ground back, keyed and laminated over with new mat, and the honest position is that a large alteration to a GRP roof is often better done as a recovering of the whole area.

Compatibility, and the one that catches people out

Bitumen and PVC single ply do not get on. Plasticiser migrates and the joint degrades. Where a new single ply detail meets an old felt roof, a separation layer is required, and the manufacturers publish exactly what.

Solvents in GRP resin will attack polystyrene insulation, so the insulation under a GRP roof has to be a compatible board or an overlay has to be used.

These are not obscure points. They are the first thing a competent fitter checks and the first thing a rushed one misses, and the consequences appear two or three years later, out of sight, under the covering.

A comparison of the three at an opening

Temperature, humidity and the Oxfordshire calendar

GRP is the fussiest. The resin needs a dry substrate and an ambient temperature in a workable band, and the catalyst ratio changes with the weather. Below about 5C the cure is unreliable, and above about 25C in direct sun the resin goes off in the bucket before it reaches the roof. A damp autumn morning in Oxford is not GRP weather.

Single ply welds well in the cold as long as the surface is dry, because the heat comes from the gun rather than from the day.

Bitumen membrane is workable across the widest range, which is one reason it persists on domestic work. None of the three will go down onto a wet deck, and none of them should.

How each affects the length of the job

Single ply and bitumen membrane both reach weathertight the same day the opening is cut, which is why we can hold to the rule that the roof is never left open overnight.

GRP needs cure time between laminate and topcoat, and that pushes a job across days. It is still done with the opening closed each evening, but the sequence has to be planned rather than improvised, and a wet forecast moves the start date rather than the finish date.

On a straightforward replacement into a single ply or felt roof, the covering work around the opening is a matter of hours. On a GRP roof it is the item that sets the programme.

Age of the covering, and whether cutting into it is sensible

A covering with five good years left in it is worth cutting into and detailing around. A covering at the end of its life is not, because you are about to spend real money making a perfect joint to a sheet that is going to be lifted anyway.

The signs are straightforward. Bitumen membrane that has lost its granules over large areas, gone shiny, and started to craze is finished. Single ply that has gone chalky and brittle at the laps is finished. GRP that has crazed across the field or lost its topcoat down to the mat is finished.

Where the covering is close to done, the sensible plan is one visit: new covering and new rooflight together, with the kerb built properly and the falls corrected while the roof is open. It costs more on the day and less over ten years.

Insulation, and what sits under each system

Single ply is laid over rigid insulation boards, either mechanically fixed through them or adhered, in a warm roof build-up with the vapour control layer beneath.

Bitumen membrane is bonded to an overlay board on top of the insulation, and that overlay is what a torch can be worked against safely.

GRP requires a deck rather than a board, so a warm GRP roof is insulation, then a new deck fixed through it, then the laminate. That extra deck is one of the reasons a GRP warm roof build-up is thicker than the alternatives, and it matters when headroom or a threshold height is tight.

How this interacts with kerb height and vapour control is covered on upstands, kerbs and warm roof build-ups.

What the guarantee covers on each

Our ten year workmanship guarantee covers what we did: the kerb, the termination, the laps and welds we made, the fixings, the flashings and the unit installation.

The covering manufacturers publish their own material expectations, and those are separate. Where we tie new work into an existing covering, we guarantee our joint and we are explicit that the old sheet either side of it is what it is.

That distinction is worth having in writing on any quotation, from anyone. A guarantee that is vague about where the responsibility stops is a guarantee that will be argued about.

What we look at before quoting a covering detail

We identify the system, take a photograph of a lap and an edge, and check the condition of the existing material within a metre of the proposed opening, because that is the zone the new work has to bond to.

We check the deck for movement by standing on it, and we look at the perimeter build-up depth to work out what insulation is present.

Then we say which of three things is on offer: a local detail into a sound covering, a local detail plus a strong recommendation to plan a recovering soon, or a covering and rooflight done together. The range goes out by email. The fixed price follows the survey, and it names the system by manufacturer rather than saying “flat roof covering” and leaving it open.