A leaking flat roof is one of the most common calls we get. The question every homeowner asks is the same: do I patch it, or replace the lot? Here's how we think about it on every job — and how you can frame the decision before getting a quote.
Start with the age and the material
Flat roof lifespan varies a lot by material:
- Traditional felt — 10–15 years on a well-installed three-layer build-up. Felt that's reached this age has usually had its life.
- EPDM rubber — 25–30+ years if installed properly with the right adhesive and detailing.
- GRP fibreglass — 25–30 years; tends to fail at edges and joints first.
- Modified bitumen (torch-on) — 15–20 years; performs well but the heat-on install needs experienced hands.
If the roof is past 80% of its expected life and you're seeing problems, repair is usually false economy. If it's still mid-life, repair can buy you years.
The repair-it signs
Targeted repair makes sense when:
- The damage is localised — a single split, blister, lifted seam or punctured patch
- The rest of the roof looks sound (no widespread bubbling, cracking or pooling)
- The underlying deck is dry — no soft spots when you walk on it
- The roof is less than ~75% of its expected lifespan
- There's no chronic ponding water (water sitting more than 48 hours after rain)
In these cases a quality patch repair, re-bonded seam or a new section can give you another 5–10 years easily.
The replace-it signs
Full replacement is usually the right call when you see:
- Multiple leaks at the same time — the membrane is failing across its area
- Widespread blistering, cracking or alligator-skin texture on felt
- Sagging or soft underfoot — the deck is likely water-damaged
- Persistent ponding because the falls (slopes) were never right
- The roof is at or past its expected lifespan
- You've already paid for two or three repairs in the last few years
The third bullet is the big one: once water gets into the timber deck, no top-side repair will fix the underlying rot. Replacing the membrane and the deck is the only real fix.
The grey area: hybrid jobs
A lot of jobs sit in the middle. Common hybrids:
- Re-cover — bonding a new membrane over the existing one. Quicker and cheaper than a full strip but only works if the original deck is sound and you're not stacking too many layers (most building regs accept a maximum of two layers).
- Partial replacement — stripping and replacing one section while patching the rest. Useful when only one corner has failed.
- Drainage upgrade — fixing ponding with tapered insulation boards under a new membrane. Solves the root cause, not just the symptom.
The five-year cost test
When the choice isn't obvious, we run a simple test with customers:
- Get a written quote for the repair and the replacement.
- Add the likely cost of two more repairs over the next five years to the repair number.
- Compare the total to the replacement quote.
If the five-year repair total is more than 60–70% of the replacement cost, replacement nearly always wins — because at the end of those five years you still own the old roof.
What we recommend
Get an honest assessment from a roofer who'll look at it properly — including the underside if it's accessible. A camera-on-a-pole is fine for the surface but doesn't show you what the deck looks like. A 20-minute proper inspection is usually enough to give you a clear recommendation.
At LM Roofing we won't sell you a replacement if a repair will do, and we won't patch a roof that needs replacing. Either way, you'll get a written quote with no obligation. Call 01473 350 488 or message us on WhatsApp and we'll come and take a look.
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