Bubbles Or Blisters On A Flat Roof

What's happening?

A blister is a raised patch where the covering has come away from the layer below and air or moisture is trapped in the gap. On bitumen felt it usually starts at a point where the layers were not fully bonded, or where the deck or the layer underneath was damp when the felt was laid. Sun then warms the trapped air and moisture, which expands and lifts the felt further, and each hot spell can grow the blister. A blister is not a leak while the surface is whole, but it is a thin, stretched spot that foot traffic, hail or thermal movement can split. GRP and single-ply membranes can bubble too, from moisture under the layer or a failed bond. A new roof with blisters usually points to the way it was laid.

What usually causes it?

• Moisture or damp in the deck or insulation when the covering was laid
• Layers of felt that were not fully bonded together
• Air trapped as the felt was torched or stuck down
• Heat from the sun expanding trapped air and moisture under the surface
• Water entering through a small split and spreading between layers
• Foot traffic or hail damaging a blister that is already stretched

Is it dangerous?

Not by itself. A blister becomes a leak once it splits. Large or growing blisters, blisters in a cluster and any blister near an upstand or edge are worth having looked at before the next winter.

Can I fix it myself?

Look at the blisters from a window or the ground and photograph them every few weeks to see whether they are growing. Do not stand on them or press them, and do not pierce one with a knife to let air out: that makes a hole in the weatherproofing. Keep gutters and outlets clear so water does not add to the problem.

When should you call a roofer?

Call a roofer if the blisters are growing, have split, are clustered, or sit on a roof laid in the last few years. A recently laid roof with blisters should go back to the contractor first. A roofer can cut out and patch a blister, but widespread blistering points to a re-cover.

What will the roofer actually do?

• Inspect the roof and map the blisters, noting size, number and any splits
• Check the deck and insulation below for damp
• Cut out the blister, dry the area and patch with new layers bonded and lapped correctly
• Seal any adjoining laps, edges and upstands
• Advise a re-cover where the blistering is widespread or the felt has failed

Who fixes this

• Flat roof repair
• Roof leak repair

Questions people ask

Sources & References

1. Designing Buildings: flat roof
2. NFRC: choosing a roofing contractor
3. HSE: roof work