▽ +09.00Problem · Roofs
Blisters in the roof waterproofing
Raised bubbles in an exposed membrane or coating that grow in the heat of the sun.
What you see
- Raised domes in the membrane or coating
- Blisters that grow by day and shrink at night
- Cracked or punctured blisters
- Soft spots underfoot
Possible causes
Moisture trapped under the layer
The sun turns it into vapour that lifts the layer.
A poor bond
Dust, damp or unfused areas leave voids.
Air or solvent trapped between coats
Coats too thick, or recoated too soon.
Water entering through a defect
A failed lap or detail feeds water into the void.
How it is diagnosed
The diagnosis confirms the cause before any repair is chosen.
Map the blisters
Size, spread and location: near details or in the open roof.
Open a test blister
Is it dry or wet inside, and which interface failed?
Check substrate moisture
Readings around the blistered zones.
Check laps and details
Look for any entry point feeding water.
Possible solutions
The fix follows the diagnosis; these are the common options and their systems.
Cutting and patching isolated blisters
After drying, with a material compatible with the layer.
Re-waterproofing on a dry substrate
When blistering is widespread.
Protection and insulation
An inverted build-up keeps the membrane cooler and protected.
Systems
What not to do
- Puncturing blisters and leaving them open
- Coating over blisters
- Re-waterproofing over a wet screed
General educational content; final values and requirements follow the manufacturer’s TDS and the approved project specification.
Common questions
Does every blister mean a leak?
No, but blisters weaken the layer and can burst, so their cause must be found.
Can blisters be repaired without redoing the whole roof?
Yes, if they are few: each blister is cut open, dried and patched with the same system. If they are widespread there is trapped moisture under the layer, and the answer is redoing it with proper preparation and venting where needed.
Technical inspection
Need a diagnosis for your case?
Send photos, a description and the location; our engineers identify the cause and the fix after an inspection.