EPDM, GRP or felt: which flat roof should you choose?

EPDM is a rubber sheet, glued down cold, so there's no flame near timber fascias or window frames. On a single garage it usually goes on in one piece, with no joins, and it stays flexible through frost and summer heat.

Its weak spots are just as clear. A dropped tool or a sharp stone can puncture it, so it isn't a floor. Rain sits in any dip in the deck. Most EPDM leaks start at the edges, where a trim lifts.

GRP is resin and glass mat made into one rigid skin on the roof itself, so it follows any shape: dormers, roofs crowded with vent pipes, edges everywhere. Once cured it's hard enough to walk on, and the topcoat comes in colours.

The trade-offs come from being rigid and made on site. If the deck flexes, GRP cracks, usually along board joints. Resin needs a dry, mild day to cure. And a dry patch in the glass mat shows up a few winters later.

Felt has a bad name from old roofs laid with hot bitumen, which blistered and cracked within a decade or two. Modern torch-on is two or three polymer-modified layers under a mineral-finished cap sheet. A damaged top layer still has one underneath, and it patches easily.

The drawback is the gas torch. An open flame beside timber fascias, a wooden window or a neighbour's roof is a real fire risk, which is why many roofs near timber are now done in a cold-applied system instead. And felt's laps (the overlaps between strips) are where it eventually lets water in.

Start with the roof, not the material. If a felt roof has a small leak but is otherwise sound, it's a repair, not a reason to change system. The choice only opens up when the old covering has failed across the roof and the deck is going soft, because then it all comes off, whatever goes back on. From there, pick the system for how the roof is used and what surrounds it. These four questions usually settle the choice:

I don't publish prices: each flat roof is priced after I've looked at it, with a free written quote, and you pay the price we agreed. The system matters less than these:

• Size and access: a garage off the drive is a different job from a roof above a conservatory. If scaffolding is needed, I arrange it and it's in the quote.
• The deck: rotten boards under the old covering must be replaced first.
• Details: every upstand, outlet and vent pipe takes time, whichever system you choose.
• Insulation: if the Building Regulations apply (below).

If I find more work once the job has started, I talk it through with you first.

Replacing a flat roof covering counts as renovating the roof under Approved Document L, the government's guidance on energy efficiency (gov.uk, Approved Document L). If more than half the roof, or more than a quarter of the building's whole outer envelope, is renovated, the whole roof should be insulated to a U-value of 0.16 W/m²K where that's practical, and a new roof to 0.15. Below that, re-covering normally needs no building control application.

That usually means a warm roof: insulation boards on top of the deck, under the new covering, which keeps condensation out of the timbers. On a flat roof I replace, the insulation goes in as a warm roof, and I help you with the building control notification.

Any new flat roof I fit comes with my written 20-year guarantee; repairs get at least 5 years.