Blisters are one of the most common flat-roof problems we're called to inspect. A few small blisters are usually cosmetic — but large or growing ones are a sign of a fundamental issue. Here's what to look for.
Are flat-roof blisters always a problem?
Small blisters (under 50mm) that aren't growing are usually fine to leave. Anything larger, growing, or near a seam needs investigation.
Can I burst a blister myself?
Don't — bursting it creates an open hole that immediately leaks. Either patch over the dome (temporary) or call a roofer to cut, dry and re-bond properly.
How long does flat-roof blister repair last?
Patch repairs on an otherwise sound roof: 5–10 years. If blistering is widespread, patching is a stopgap — full replacement gives 25–30 years.
If the original deck wasn't fully dry on install, moisture vaporises in sunshine and lifts the membrane.
Cut and patch individual blisters; full overlay or replacement if widespread.
Cold or damp install conditions stop adhesive curing properly; the membrane never fully bonds to the deck.
Single blisters — cut, dry, re-bond. Widespread — strip and re-lay.
Lapped seams in felt or EPDM let water sit between layers, then heat lifts the area.
Re-seal seams with appropriate adhesive and pressure-roll. Replace if seams are widely failing.
Old bitumen felt becomes brittle and forms multiple small blisters as it nears end of life.
End-of-life sign — full replacement with EPDM or GRP usually the right answer.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.