Chapter one
Flat-roofed extensions on Mannings Heath bungalows
A hamlet of bungalows has plenty of flat roofs bolted on, and most of their trouble starts at the edges.
After 1945 Mannings Heath grew rapidly as a dormitory hamlet, and the county history counts 350 to 400 houses there by 1979, many of them bungalows. Bungalows invite extension. A kitchen goes out the back, a garage is squared off, a porch is added, and each of those tends to be covered by a flat or nearly flat roof. Those additions are often the part of the house that leaks, not because flat roofing is a poor idea, but because the details around the edge are fiddly: where the membrane turns up the old wall, where it runs over the gutter edge, and where a rooflight or outlet is cut through it.
At a glance
- After 1945 the hamlet grew fast, with many bungalows
- Flat roofs need a fall of 1:80 once finished
- Felt, EPDM and GRP each have a different lifespan
1.1 Falls, ponding and age
Level is a myth on a flat roof. IKO, citing BS 6229:2018, gives a recommended minimum finished fall of 1:80 and advises designing for 1:40 to allow for tolerances and deflection. Puddles that linger for days mean the fall has gone, and standing water searches out any weakness in the membrane. Published lifespans vary by material: felt 10 to 20 years, EPDM 20 to 30 years or more, GRP 25 to 40 years. A sheet of EPDM is a single rubber membrane laid with few joints, GRP is a glass-reinforced plastic that is laid wet over a deck, and felt is built up in layers. Which suits an extension depends on its shape, the deck beneath and what already sits against the wall. A small porch roof and a full-width kitchen extension rarely want the same answer, and we say which we would choose for yours and why.
1.2 Patch, overlay or strip
A localised failure at an upstand can often be repaired without touching the rest. A roof with widespread blistering, splits across the field and a damp deck needs stripping to the boards. We lift the covering at a few points to see the deck, because a sodden board cannot take a new membrane. Our flat roofing page explains each system, while the gutters, soffits and fascias along the same edge are checked on the same visit. Older soffits can hide asbestos insulating board, and HSE advises presuming asbestos in any building from before 2000. If we suspect it, work stops until it is identified.
Flat roofing guide prices, per square metre
- Felt£70 to £100
- EPDM£90 to £130
- GRP£95 to £150
- PVC£95 to £140
- Lead£150 to £200
A guide cannot price your extension; the survey fixes the area and the details, and you receive the total in writing. Source: MyBuilder flat roof cost guide, updated 21/08/2026.
