How Much Does a Flat Roof Cost in South London in 2026?
If you’ve spotted a damp patch on the ceiling or noticed water pooling on a flat roof over your kitchen extension, the first question is almost always the same: is this a quick repair, or am I looking at a full replacement? The honest answer depends on the roof’s age, the covering it’s made from, and how long the problem’s been left unaddressed before anyone noticed.
We repair and replace flat roofs across South London every month, usually on extensions, dormers, garages and bay windows, so these figures come from jobs we’ve actually priced and carried out, not a national average pulled from a spreadsheet.
Repair Cost: When a Patch-Up Is Enough
A straightforward repair is usually the right call when the roof structure underneath is sound and the problem is a single, identifiable area of damage.
- Small patch repair (split, blister or tear): £200–£500
- Full re-seal of lap joints or an upstand: £300–£1,000
- Chimney or skylight flashing repair: £300–£600
Repairs are priced on access and roof condition as much as size — a flat roof you can reach easily from a ladder costs less to fix than one that needs scaffolding over a side return or a tricky rear extension.
Replacement Cost by Material
Once a flat roof is past patching — usually once it’s over 15–20 years old or has more than one active leak point — full replacement becomes the better long-term investment. Costs are priced per square metre and vary significantly by material:
- Felt: £50–£90 per m² — the cheapest option, but the shortest lifespan at around 15–20 years
- GRP (fibreglass): £80–£130 per m² — durable, seamless, and increasingly the standard choice on South London extensions
- EPDM (rubber): £70–£110 per m² — excellent lifespan of 40+ years, minimal maintenance
- Green/living roof: £120–£200 per m² — higher upfront cost, but adds insulation value and is increasingly popular on new extensions
For a typical 20–25m² extension roof, that puts a full replacement in the region of £1,500 to £3,500, depending on material.
Signs You Need a Replacement, Not a Repair
It’s tempting to keep patching a problem roof to put off the bigger spend, but there’s a point where that stops making financial sense. Watch for:
- Multiple leaks appearing in different areas, not just one spot
- Visible sagging or ponding water that doesn’t drain after 48 hours
- A covering that’s visibly cracked, blistered or lifting at the edges across most of the roof, not just one corner
- A roof already past 20 years old, regardless of how it currently looks
If more than one of these applies, a repair is usually a short-term fix that costs you twice — once now, and again in a year or two when the real problem resurfaces.
Why Going Cheap on a Flat Roof Costs More Later
This is worth being blunt about: a flat roof is one of the easiest jobs to get a cut-price quote on, and one of the most expensive to get wrong. A poorly fitted felt roof at the bottom of the price range can fail within a few years, and by the time it does, water has often already reached the timber deck underneath — turning a roofing job into a carpentry and plastering job too.
We’ve been called out to more than one South London property where a “bargain” roof from a year or two earlier had already let water into the structure below. The roof itself is rarely the expensive part of getting this wrong — the damp repair, replastering and sometimes replacing damaged joists afterwards is what actually costs homeowners money.
How Long Does the Work Take?
- Repairs: usually completed within a day
- Standard extension or garage roof replacement (up to 25m²): 1–2 days
- Larger or more complex roofs, including those with multiple upstands, skylights or chimneys: 3–5 days
Weather is the main variable — flat roofing work needs a dry window to go down properly, so we always build a small buffer into the programme rather than rushing a roof on in the rain.
Getting an Accurate Price
Every figure above is a realistic range, not a fixed quote. The only way to know exactly what your roof needs is a proper inspection — checking the deck underneath, not just the surface covering, since a roof can look fine from below and still be failing where it matters.
If your flat roof sits over a recent or planned extension, it’s also worth reading our guide on house extension costs in South London, since roofing is usually priced as part of that wider project.
Frequently Asked Questions
How long does a flat roof last?
Felt typically lasts 15–20 years, GRP and EPDM 30–50 years depending on installation quality. A roof installed properly in the first place is the biggest factor in how long it actually lasts, more than the material itself.
Can you repair a flat roof in winter?
Small repairs can be done year-round in dry conditions, but full replacements are best scheduled for drier months where possible, since most materials need a dry deck to bond properly.
Is EPDM or GRP better for a flat roof?
Both are strong long-term options. EPDM is a single rubber membrane with very few joints to fail; GRP (fibreglass) creates a fully seamless, rigid surface. We’ll recommend one over the other based on your roof’s shape and how it drains.
Do I need planning permission to replace a flat roof?
Usually not, if you’re replacing like-for-like. It’s only likely to become a planning issue if you’re changing the roof’s height, pitch, or adding something like a roof terrace.
Got a leak or a roof that’s due for replacement? Our in-house team covers flat roof repair and replacement across Central London, South London and Surrey — get in touch for a free inspection and no-obligation quote.