EPDM, GRP or felt: which flat roof material is actually best

Three materials cover almost every domestic flat roof in Leeds: EPDM rubber, GRP fibreglass, and torch-on felt. Everyone who fits one of them will tell you theirs is the best flat roof material. They are all partly right, because the three are good at different things.

Here is the version I would give a neighbour.

The short answer

• EPDM rubber for most garages, extensions and anything a reasonable size with a simple shape.
• GRP fibreglass where you need a hard wearing surface, a lot of complicated detail, or you want a seamless finish.
• Torch-on felt where budget is the deciding factor and the roof is not visible.

Now the detail, because the shape of your roof changes the answer.

EPDM rubber

A single sheet of synthetic rubber, bonded down. On most domestic roofs it goes on in one piece, so there are no seams at all across the field of the roof.

What is good about it. No joints means no joints to fail, which is where flat roofs almost always leak. It stays flexible in cold weather, so it copes with a building moving and with Yorkshire winters. It is fitted cold, with adhesive, so there is no flame anywhere near your house. Twenty to thirty years is a realistic life and the good manufacturers guarantee in that region.

What is not. It is soft. Drop a slab or drag something sharp across it and you can cut it. That is repairable, easily, but it means it is the wrong choice for a roof people walk on regularly. The standard black finish is exactly as attractive as it sounds, which is irrelevant on a garage and matters on a dormer you look at from a bedroom window.

Where I use it. Garages, extension roofs, dormers, anything simple and reasonably large. This is the default answer for most Leeds flat roofs and the one I fit most.

GRP fibreglass

Glass fibre matting laid over a ply deck and saturated with resin, then finished with a topcoat. It cures into one rigid, seamless shell.

What is good about it. Genuinely seamless, including up the upstands and around every awkward detail, so it suits roofs with a lot going on: multiple upstands, a rooflight, pipes coming through, an odd shape. It is hard. You can walk on it, put a balcony on it, stand a ladder on it. It comes in colours, so a dormer can be finished to look deliberate rather than patched. Twenty five to thirty years is normal.

What is not. It is rigid, and rigid materials and moving buildings do not always agree. On a timber deck that flexes, or a long run with no expansion detail, GRP can crack, and it usually cracks at an upstand or a corner where the movement concentrates. It also has to go on in decent weather, and it needs a properly built ply deck underneath, which adds cost. Fitting it badly is easy and the failures are expensive.

Where I use it. Balconies, walk-on roofs, small complicated roofs with lots of detail, and anywhere the finish is on show.

Torch-on felt

Modern torch-on is not the felt your grandad had on his shed. It is reinforced bitumen membrane, applied in two or three layers with the joints heat-welded.

What is good about it. It is the cheapest of the three by a clear margin. Fitted properly in three layers it will do fifteen to twenty five years, which is not far off the others. It is repairable by anybody, anywhere, cheaply. It handles awkward shapes well because it goes on in manageable pieces.

What is not. It has joints, and joints are where flat roofs fail. It goes brittle with age and UV, so the failure mode is cracking, usually at a lap or an upstand. Fitting it involves a naked flame on your roof, which is a real fire risk and is why some insurers ask about it. Single-layer torch-on, which is what a cheap quote often means, is a false economy.

Where I use it. Where budget is genuinely the constraint, and on roofs nobody sees. It is not a bad material. It is a cheaper material, and there is nothing wrong with choosing it deliberately.

The comparison, side by side

What actually decides whether a flat roof lasts

Here is the part that matters more than the material, and it is the reason I am wary of anyone who leads with the covering.

The deck. Whatever goes on top is only as good as what is underneath. If the existing deck is soft, sagging or rotten and somebody covers it anyway, you have paid for a nice new membrane on a failing structure. A proper job strips back and checks. A cheap quote does not, and that is usually where the price difference comes from.

The falls. A flat roof is not flat. It needs a slight slope so water runs off. Standing water, what the trade calls ponding, is what kills flat roofs early regardless of covering. If your existing roof holds puddles for days after rain, the falls are wrong and re-covering it without correcting them just resets the clock on the same problem.

The edges and upstands. Almost every flat roof leak I attend is at an edge, an upstand, or where the roof meets a wall. Not in the middle. Whoever fits it, ask how the edges and the abutment are being detailed.

The drainage. An outlet that cannot cope, or a gutter that is blocked, backs water up onto the roof and holds it there.

Get those four right and any of the three materials will do its job. Get them wrong and the most expensive covering on the market will fail early.

Leeds specifics worth knowing

Most of the flat roofing round here is on three things: garages on the interwar and postwar estates, back extensions, and dormers on loft conversions.

Garages on the estates at Seacroft, Middleton, Belle Isle and Whinmoor were mostly felted originally, and a lot of them are on their second or third covering. The decks underneath are often the original boarding and are frequently the actual problem. Worth insisting somebody looks before quoting.

Dormers on the terraces through Headingley, Hyde Park and Burley are usually small, visible from the street, and complicated at the edges where they meet the slate. GRP or EPDM both work; the detail where the flat roof meets the pitched slate is what decides whether it lasts.

Extension roofs on the semis out at Adel, Cookridge and Moortown tend to be bigger, simpler and out of sight, which is EPDM territory almost every time.

So which is the best flat roof material

For most people reading this, with a garage or an extension: EPDM. It is seamless, it is flexible, it is fitted cold, and it lasts.

If your roof is complicated, walked on, or on show: GRP, fitted by somebody who builds the deck properly first.

If the budget will not stretch and the roof is out of sight: three layer torch-on felt, and there is no shame in it.

And if anyone quotes you for a covering without going up to look at the deck and the falls first, get somebody else to quote as well.