A flat roof on an Eastbourne extension should use GRP fibreglass or EPDM rubber with a minimum 1:80 drainage fall, costing £1,400–£3,500 depending on size. Both systems last 25–50 years when correctly installed. Building Regulations require specific insulation values — a warm-deck build-up is usually the correct approach.
Rear extension flat roofs built as part of the 1970s–90s residential expansion across Polegate, Langney, Hampden Park, and Hailsham used torch-on felt as the primary covering specification. That felt had a design life of approximately 15–20 years in moderate conditions. In coastal East Sussex — persistent moisture, UV intensity, and thermal cycling — the practical lifespan was often shorter. The majority of these extensions are now well past their first covering replacement cycle and many are on their second or third felt re-cover.
Why Felt Extensions Keep Failing in Eastbourne
The underlying problem with repeated felt failures on Eastbourne extensions is usually not the felt itself — it's the drainage. East Sussex housing regulations for extensions in the 1970s and 1980s were inconsistently applied, and many rear extensions were built with inadequate or zero drainage fall. A flat roof with inadequate fall ponds water. Ponded water accelerates membrane degradation, creates hydrostatic pressure at any weakness in the membrane, and in winter provides a reservoir for freeze-thaw damage. Applying a new felt covering to an extension with inadequate falls solves nothing — it will fail again in 8–12 years for the same reason.
A flat roof must be designed with a minimum finished fall of 1:80 to ensure positive drainage and avoid ponding.
Source: BS 6229 code of practice for flat roofs
Deck Condition — The Critical Variable on Older Extensions
When we strip an old felt flat roof on an Eastbourne extension, the condition of the timber deck beneath determines the full scope and cost of the job. Three possible conditions:
- Sound deck. The OSB or plywood substrate is dry and structurally intact. New membrane can be applied directly. This is the cheapest outcome.
- Partially waterlogged deck. One or more sheets of OSB or plywood have absorbed water from a slow leak and are softened or delaminated. These sheets must be cut out and replaced before new membrane. Adds £200–£600 typically depending on extent.
- Comprehensively failed deck. Multiple sheets waterlogged or structurally failed, or the underlying joists showing moisture damage. Full deck replacement required — adds £500–£1,500 and occasionally reveals joist issues that require structural repair.
We include deck condition assessment in every flat roof survey and price deck replacement transparently in the quote. A contractor who doesn't inspect the deck before quoting is pricing a job they haven't properly assessed.
Free Flat Roof Survey — Eastbourne Extensions
We'll assess deck condition and drainage falls before quoting. Call 01323 365960 or request a callback.
Call 01323 365960GRP vs EPDM for Eastbourne Extension Flat Roofs
For most Eastbourne rear extensions — typically 15–40m² — we recommend GRP fibreglass as the primary specification: seamless, no joints, UV resistant, and carrying a 25-year manufacturer warranty. EPDM is specified on larger extensions where substrate movement is a concern or where the extension has timber-framed walls that move seasonally.
In both cases, correcting the drainage falls is included in the work scope. We add a poured screed or counter-batten layer as required to achieve the minimum 1:80 drainage fall before any new membrane is applied.
Flat Roof Extension Replacement Costs in Eastbourne (2025)
| Extension Size | GRP Cost | EPDM Cost |
|---|---|---|
| Small (under 15m²) | £900–£1,500 | £750–£1,200 |
| Medium (15–25m²) | £1,400–£2,400 | £1,200–£2,000 |
| Large (25–40m²) | £2,200–£3,800 | £1,900–£3,200 |
| Deck replacement (if required) | +£200–£1,500 depending on extent | |
| Falls correction | +£150–£500 depending on method | |