Flat roof waterproofing on extensions, garages and dormers

A small split or a failed seam can be repaired. A roof that is sound underneath but has an aged surface can sometimes be overlaid with a compatible membrane or liquid coating. When the deck is soft, the roof holds water for days after rain, or the insulation below is wet, the only lasting answer is to strip back to the joists, replace the deck and build the roof up again.

A roofer can tell a lot by walking the roof and checking for spongy areas, and more by making a small inspection opening. Ask what was found underneath before you choose, because a new membrane laid over a wet deck will trap the moisture.

Older flat roofs are often cold decks: insulation between the joists and a ventilated void above, under the deck. Without good cross-ventilation these are prone to condensation and rot. Most new and replacement flat roofs are built as warm decks, with rigid insulation laid on top of the deck and the waterproofing on top of that, so the timber stays warm and dry.

Replacing a flat roof that contains insulation usually means the insulation has to be upgraded, because the roof is a thermal element. Adding insulation on top raises the roof level, so the roofer must check the height of upstands, door thresholds, window sills and flashings at the wall. On an extension roof next to a bedroom window this can be a real constraint and should be solved in the quote, not on the day.

Modern reinforced bitumen systems are built up in layers and are forgiving on complicated roofs with many upstands. EPDM rubber comes in a single sheet and suits simple rectangles such as garages and single-storey extensions. GRP fibreglass cures to a hard surface that handles light foot traffic and neat edge trims. Liquid-applied systems are useful around awkward details and for overlays.

The membrane is only as good as the person laying it and the details at the edges. Ask how outlets, upstands, abutments and roof lights are finished, and whether the installer is trained or approved by the system manufacturer so that the product guarantee applies.

Turning a flat roof into a terrace or balcony is a different project. The Planning Portal notes that balconies and raised platforms often need planning permission and are not covered by permitted development, and the Building Regulations cover the guarding, the structure and fire safety. A standard flat roof is not designed to be walked on regularly, and the waterproofing under a terrace needs protection.

Roof lights set into a flat roof are a frequent source of leaks at the kerb. Old domed roof lights can also be fragile. If they are being replaced, the upstands and flashings should be part of the same job and the new roof lights should meet current thermal standards.

• overlay or full strip, and the state of the deck assumed in the price
• deck material and thickness, and how rotten joists will be priced
• warm or cold deck, insulation type and thickness, and any tapered falls
• membrane system, number of layers or thickness, and manufacturer approval
• upstands, drips, trims, outlets, abutment flashings and roof lights
• Building Regulations route, guarantee terms and waste removal

Once your phone number and email are confirmed, roofers near you see a description of the flat roof, not who you are. A single roofer can buy the request, paying Offrano; after that, it is off the market. We email you that roofer's name, address and phone number before it gets your details. You pay nothing to us.

If no roofer takes the request within 21 days, we withdraw it. You decide freely whether to accept any quote, and you are free to ask others. We only check that the buyer is a registered business, so ask to see flat roofs it has laid and check any manufacturer approval yourself.