Flat roofing

The three we fit

EPDM rubber is a single sheet with very few joints, and it stays flexible through the temperature swings. On a plain rectangular extension or garage roof it is usually what we would suggest. Thirty to fifty years, and roughly £90 to £130 a square metre fitted.

GRP fibreglass is laid wet and cures into one continuous skin, so it forms itself around pipes, upstands and awkward shapes better than a sheet does. It is also the harder-wearing choice for a balcony or anything that gets walked on. Twenty five to thirty years, £110 to £160 a square metre. It needs dry weather to cure, which can mean waiting.

High performance felt still has its place, particularly on a budget or where a roof is due to be replaced along with the extension in a few years. Twenty to twenty five years, and cheaper up front.

Where flat roofs actually fail

Almost never in the middle, which is the part everybody looks at. It is the edges, the upstands where the roof turns up against a wall, and the outlets. That is where the movement is and where the workmanship shows.

If yours is holding water days after rain, the fall is wrong or the deck has sagged. Laying a new covering over the same flat deck just reproduces the problem in a newer material, so where the fall has gone we build it back with tapered insulation or new firrings and show you the levels. And we lift the old covering to look at the boards before anything goes over them - wet chipboard from the seventies has no business under a new roof.

Rough costs

A typical single-storey extension roof of around fifteen square metres runs £1,600 to £2,400 in EPDM or £1,900 to £2,900 in fibreglass, both including tearing off the old covering and taking it away. A single garage is £1,800 to £2,800. Replacing a failed deck adds £25 to £45 a square metre, and bringing the insulation up to current standards while the roof is open typically adds £400 to £900 - usually money well spent, since a flat roof only gets opened up once every few decades.

A few questions we get

Can it be repaired instead of replaced?

A split, a lifted edge trim or a failed outlet, yes - and if the rest is sound that is what we will recommend. What does not work is patching a covering that has reached the end, because each patch just moves the failure a metre to the left.

Why does mine hold water?

A flat roof is not actually flat - it should be laid to a fall of at least 1 in 80. Older extensions round here were often built with a nominal fall that has since been lost as the timbers settled.

Can I put a terrace or solar panels on it?

Sometimes, but that is a structural question before it is a roofing one. Tell us at the survey and we will specify accordingly and say whether an engineer needs to look at it.