Roofing

5 Signs Your Flat Roof Needs Replacing — Not Just Patching

Patching buys time. But there's a point where every additional repair is money thrown at a roof that needs replacing — and we'd rather be straight about that than keep returning every six months.

Here are the five signs we use on every flat roof inspection to call replacement vs repair.

1. The roof is over 20 years old and it's felt

Three-layer felt past 20 years has typically lost its bitumen elasticity, oxidised, and is now brittle. Patches bond poorly because the substrate flexes around them. If your roof is felt and over 20, plan for replacement, not repair.

2. Visible ponding more than 24 hours after rain

Water sitting on the roof 24+ hours after a downpour means falls have failed — usually from rotted decking or compressed insulation. No surface repair fixes a structural fall problem; the deck has to come up.

3. Multiple bubbles, splits or blisters

One blister we'll patch. Five blisters across a 20m² roof tells us the membrane has lost adhesion to the deck across the whole surface. New patches bond to the loose layer and peel off in months.

4. Internal damp on the ceiling below

Once water is showing internally, the felt + insulation + decking are already wet. Even if we stop further water entry today, the existing trapped moisture continues to damage timbers. Strip and replace is the only honest fix.

5. You've had three or more repairs in two years

Each successive leak appears in a different spot because the membrane is failing across the whole roof. At that point continued patching costs more over 5 years than a full EPDM or GRP replacement with a 20-year guarantee.

What replacement actually involves

For a typical 25–30m² extension or garage roof: strip the old covering, check & replace any rotten decking, fit new firrings to correct falls if needed, install new insulation, then EPDM rubber (1-day install) or GRP fibreglass (2–3 days). Total cost £2,500–£4,500 with a 20-year manufacturer guarantee.

Read our full EPDM vs GRP comparison to see which system suits your roof.