Flat roofing: EPDM rubber, GRP and torch-on felt

Signs your flat roof needs attention

• Water standing on the surface for days after rain.
• Blisters, cracks or splits in felt or crazing in an old fibreglass finish.
• A soft or spongy deck underfoot, a sign the boards beneath are wet.
• Damp or mould on the ceiling below or drips in winter when it hasn't rained.
• Lifted edges, trims or flashings where the roof meets the wall.

How we build a flat roof

1. Strip back to the deck and check the joists and boards for rot.
2. Set the falls so water drains away. BS 6229 recommends designing to a fall of 1:40 so the finished roof still drains at 1:80 or better.
3. Build as a warm roof where possible: vapour control layer, rigid insulation, then the waterproof layer, so there's no cold void for condensation.
4. Lay the membrane, EPDM, GRP or torch-on felt, with trims, upstands and outlets detailed properly.
5. Flash into the wall with a lead cover flashing, then clean up and photograph the finished roof.

EPDM, GRP and felt compared

• EPDM rubber is a synthetic rubber membrane, usually laid in one piece on domestic roofs so there are no seams to fail. It stays flexible in cold weather and has a long service life when installed well.
• GRP fibreglass is laid wet on a timber deck and cures into a hard, seamless finish. It's good for roofs that get walked on and forms neat edges and details.
• Torch-on felt is a multi-layer system of reinforced bitumen membranes bonded with heat. It's robust and economical and modern felts far outlast the old single-layer roofs many garages still have.

We also lay lead flat roofs on bays, porches and period homes where lead is the right material; see heritage leadwork. Where a flat roof replacement renews most of the covering, building regulations may require an insulation upgrade and we'll tell you if yours does.

What affects the cost of a flat roof

Size, the system you choose, whether the deck and joists need replacing, insulation (a warm roof costs more upfront but cures condensation), edge trims and outlets and access. Small garage and porch roofs are often a day or two's work. You get a written, itemised quote after a free inspection.

A flat roof is only as good as its edges

Most flat roof failures we see are at upstands, outlets and wall flashings, not in the middle of the membrane. Because we're leadwork specialists, the junction with your house gets the same attention as the roof itself.