Felt vs GRP for a flat roof

Felt is cheaper today. GRP is cheaper over twenty years. The case for each, with the numbers.

Garage, extension or bay roof leaking? You will be offered two things in the West Midlands: torch-on felt or a GRP fibreglass system. Both are legitimate. They just suit different situations and budgets.

Torch-on felt

Modern three-layer torch-on felt is a long way from the single-layer mineral felt that gave flat roofs their bad name. Expect a 15–20 year life and roughly £65–£95 per square metre fitted on a typical garage. It handles slight movement in the deck well, it can be laid in cooler weather than resin, and small damage is easy and cheap to patch.

The downsides: it has seams, which is where failures start; it needs a heat source on the roof; and it is less happy with ponding water than resin is.

GRP fibreglass

GRP is a seamless glass-reinforced resin laid over a new OSB deck with bonded trims. Expect 25–30 years and roughly £95–£140 per square metre. No joints means nothing to unzip, the edge trims are neat, and it takes light foot traffic well, which matters if you have a roof you actually walk on or a balcony.

The catch is that GRP is fussy. It needs a dry deck, an ambient temperature above about 5°C, and it does not tolerate a moving or springy deck — movement shows up as cracks along the joists. On a big roof, expansion joints have to be designed in.

The twenty-year maths

On a single garage of about 18m²: felt at roughly £1,400 that you may replace once in that period, versus GRP at roughly £2,100 that you probably will not. That is why we usually recommend GRP where the deck is sound and the roof is visible or walked on, and felt where the deck moves, access is awkward, or the budget needs to work today.

Whichever you choose, the deck matters more than the covering. Laying anything over soft, wet boards is money thrown away.

Signs yours needs doing rather than patching

• Blisters or bubbles across the surface, or felt you can push a finger into.
• Splits at the upstands and around the edge trims.
• Standing water that never drains, or a deck that feels spongy underfoot.
• More than two or three repairs in the last few years.

We survey the deck before quoting either option, and tell you which one we would actually spend our own money on for your roof.

What about EPDM rubber?

EPDM is the third option and it is a good one on the right roof. It is a single sheet of rubber membrane, so on a small roof with a simple shape it can be laid in one piece with no seams at all, and it stays flexible in the cold. It costs somewhere between felt and GRP. Where it struggles is complicated shapes with lots of upstands, pipes and corners, because every detail is a bonded joint made on site — the exact opposite of GRP, which handles fiddly details very well.

Insulation and building regulations

If you are replacing more than half of a flat roof covering, building regulations generally expect you to bring the insulation up to current standards at the same time. In practice that means a warm-roof build-up: insulation boards over the deck with the waterproofing above them. It adds height at the edges and to the door thresholds, so it is worth designing rather than discovering halfway through. On a garage roof this usually does not apply; on a habitable extension it almost always does.

Drainage and detailing

• Aim for a fall of at least 1:80 so water actually leaves the roof rather than sitting in the middle.
• Outlets should be at the low point and big enough — undersized outlets are the most common cause of ponding.
• Upstands need to be at least 150mm above the finished surface at abutments.
• Trims must be mechanically fixed and bonded, not just sealed with mastic.

Guarantees

We guarantee our GRP roofs for twenty years and our three-layer torch-on felt for fifteen, both on workmanship and materials, provided the deck was replaced or verified as sound as part of the job. That last condition matters: a guarantee over somebody else’s wet deck is not a guarantee anybody can honour.