Raised bubbles in the membrane
Blisters on a flat roof
A blister is a pocket of trapped air or moisture between roof layers. Many are harmless for a while; once the skin splits, they are leaks.
- Common on bitumen roofs
- Watch, then repair
- Split blisters leak
What blisters look like
Blisters are raised bumps from the size of a coin to a dinner plate. Press one gently with a gloved hand from a safe position and it may feel spongy or hollow, sometimes with a crackle. They are most visible on mod bit and built-up roofs, but some single-ply membranes show similar bubbling when adhesives were applied poorly.
The roof looks fine from a distance, which is why they are often missed until they crack. A raised line or a patch of surface granule loss on top of a blister is the first sign that the skin is wearing through.
Why blisters form
Moisture trapped during installation
Rain, morning dew or damp insulation under a membrane is sealed in, then turns to vapour in July sun.
Poor adhesion between plies
Dust, cold weather or too little adhesive leaves unbonded patches that fill with air.
Vapour from inside the building
Humid indoor air rises into the roof assembly. Without a vapour control layer it condenses under the membrane.
Heat
Dark roofs in full sun expand the trapped gas, so blisters often swell in summer and flatten in winter, repeating the stress each year.
How urgent are they?
A small, intact blister on a roof otherwise in good condition can be photographed and watched. A blister that is growing, has a cracked or abraded top, or sits near a drain, seam or wall is worth repairing this season. Water getting in through the split can saturate insulation beneath and make a small repair into a bigger one.
Ontario freeze–thaw hurts here: water inside a split blister freezes, expands and widens the opening.
What you can safely do yourself
Mark and photograph
Note the locations and sizes from a window or from the ground with binoculars, so you can compare after a season.
Do not pop them
Cutting or pricking a blister lets water in and does not remove the cause. It also makes the repair harder.
Do not smear tar on them
Roof cement over a cracked blister hides the problem and traps moisture. It often leaks again in a few weeks.
How a blister is repaired
Check how wet it is
If the area under the blister is dry, a minor repair is possible. If insulation is damp, it needs to come out.
Cut it open and dry it
The blister is cut in a clean pattern, the layers are peeled back and the area is dried.
Re-bond the layers
Fresh adhesive or heat re-bonds the plies, then a new patch is laid over the area with edges that overlap sound material.
Look for the reason
If several blisters line up along a wall or a vent, moisture may be coming from inside. We flag that so the cause is addressed.
When blisters point to replacement
Dozens of blisters across the field, or blisters combined with soft spots underfoot, usually mean the roof has trapped moisture throughout. Patching one by one is a losing game. If the system is also nearing the end of its range (15 to 25 years for mod bit, 20 to 30 years for built-up), plan a replacement and ask about a vapour layer. See modified bitumen and built-up roofing.
Typical costs
| Item | Starting price |
|---|---|
| Small leak repair (patch, seam or puncture) | from $350 per repair |
| Roof inspection with written photo report (house) | from $250 per roof |
| Modified bitumen (torch-on or SBS) replacement | from $8 per sq ft |
Starting prices in CAD before 13% HST, based on typical Ontario contractor rates (mostly Toronto and the GTA). The final price depends on roof size, access, the layers to remove, the membrane, drains and flashings, and is confirmed in your written quote.
Blister questions
Are blisters a sign of a failing roof?
One or two are not. A roof covered in them, or ones that keep returning after repair, are.
Why do blisters appear after a new roof?
Usually trapped moisture or poor adhesion at install. A short check in the first year shows if they are starting.
Can a coating cover blisters?
No. Coating over a blister traps it and does not stop it growing. Repair first, then consider coating.
Leak, damage or an old flat roof?
Send a few details and photos. We reply by email with next steps and a written quote.