A flat roof on an extension or garage can be reliable if it is built and maintained properly. Most problems come from old felt that has cracked, poor drainage, rotten decking or weak edge details. Modern EPDM rubber and GRP fibreglass roofs are the most common replacements. A new extension usually needs building regulations approval and may fall under permitted development, while replacing an existing flat roof covering may need approval if it is a large job on a heated room.
How a flat roof is built
A flat roof isn't completely flat. It should have a slight slope, called the fall, so water runs to a gutter or outlet. From the inside out, it usually has:
- joists: the timbers that carry the roof
- deck: boards fixed over the joists, often plywood or OSB
- insulation: above the deck (a warm roof) or between the joists (a cold roof)
- waterproof covering: EPDM, GRP or felt
- edge trims and upstands: where the roof meets walls, gutters and edges
Choosing a covering
EPDM rubber
A single sheet of synthetic rubber, bonded to the deck. It has few or no joints, copes well with heat and cold, and is easy to repair.
GRP fibreglass
Glass fibre matting and resin laid in layers to form a seamless, hard finish. It can take light foot traffic and gives a neat look.
Felt
Modern torch-on or cold-applied felt systems, built up in layers, are much better than older felt roofs. Felt is a familiar choice and can suit many garages.
For a side-by-side comparison, read our guide to EPDM vs GRP vs felt.
Common flat roof problems
- Cracked and blistered felt: old felt dries out and splits in the sun.
- Ponding: water sitting on the roof because the fall is poor or the deck has sagged.
- Rotten decking: water that has been getting in for a while rots the boards, which is the main reason a new covering fails early if the deck isn't checked.
- Leaking upstands and flashing: where the roof meets the house wall.
- Blocked outlets and gutters: leaves and moss stop water draining.
If several of these show up at once, replacing the whole roof is often better value than patching.
Looking after a flat roof
A little upkeep helps any flat roof stay watertight:
- clear leaves, moss and debris from the surface, outlets and gutters, especially in autumn
- look for splits, blisters or lifting edges after storms
- check the joint where the roof meets the house wall
- keep trees cut back so branches can't scrape or drop onto the roof
- avoid walking on the roof unless it is built for access
- deal with small problems early, before water reaches the deck
Planning permission
Many single-storey rear extensions on houses can be built under permitted development, without a planning application, if they meet the limits. In England these include:
- a depth of up to 3 metres from the original rear wall for semi-detached and terraced houses, or 4 metres for detached houses (larger extensions are possible through the prior approval process)
- a maximum height of 4 metres
- eaves no higher than 3 metres if the extension is within 2 metres of a boundary
A flat roof cannot be used as a balcony or roof terrace under permitted development, because balconies, verandas and raised platforms are excluded. In conservation areas, side extensions are not permitted development. Flats and maisonettes don't have these householder rights, and on some newer estates the rights have been removed by planning conditions. Detached garages and outbuildings have their own height limits. Always check with your local council before you build. See our guide to planning permission for roof work.
Building regulations
Almost all new extensions need building regulations approval. Small detached garages, and some carports and porches, can be exempt, depending on size, materials and distance from the boundary.
For replacing an existing flat roof, the rules depend on the size of the job and the room below. Re-covering a large part of the roof over a heated room is likely to need approval and better insulation. A roofer registered with a government-authorised competent person scheme can handle the building regulations side for you, or you can apply to building control. Our guide to building regulations when you re-roof has the detail.
Party walls
If a new extension involves building on or up against the boundary, cutting into a shared wall, or digging foundations close to a neighbour's building, the Party Wall etc. Act 1996 may apply. You may need to give your neighbour written notice before work starts. This is separate from planning and building regulations.
In Essex, Hertfordshire and Cambridgeshire
Flat-roofed extensions, garages and dormers are part of everyday life in many local towns.
- New towns: Because so many Basildon homes have flat-roofed garages, porches, dormers and extensions, flat roof replacement is one of our most common jobs there. A lot of Harlow's housing has low-pitched or flat roofs, and in Stevenage we add proper falls and check the deck underneath.
- Terraces and rear extensions: Many Watford terraces and semis have rear extensions, dormers and garages with flat roofs.
- Bungalows and dormers: Many Rayleigh bungalows have had dormers added, often with felt flat roofs, and the joins between dormer and main roof are a common source of leaks.
- New estates: In Cambourne and Northstowe, permitted development rights have been removed on some estates, so extensions may need planning permission.
How First4Roofcare can help
We replace and install flat roofs on extensions, garages and dormers, checking the deck and drainage first. Find out about our EPDM flat roofs and GRP fibreglass flat roofs, or book a free survey to talk through your project.