Moisture trapped at install
Mopping asphalt over damp felt or insulation seals water in. Summer sun turns it to vapor, the vapor expands, and the plies separate. Classic on built-up roofs laid on a dewy morning.
Built-up roofingRoof Blisters, at a glance
Flat roof blisters form when air or water vapor gets trapped between plies of built-up or modified bitumen, or between a membrane and its insulation, then expands on hot days. On St. Louis roofs that top 150 degrees in July and drop below freezing in January, they grow fast. A small dry blister can be left alone or cut and patched. A field of blisters over wet insulation usually means a section has to come off. John Beal Roofing tells you which with a free inspection. Call (314) 429-6000.
We get the call every summer. Somebody goes up to check a unit and the roof feels like a waterbed in spots. Bubbles the size of a dinner plate, some as big as a car hood. On a gravel built-up roof you’ll sometimes see the gravel heaved into a ridge.
It looks alarming. Sometimes it is. The blister itself isn’t the leak — it’s a symptom that something got trapped during the install or worked its way in later. What matters is what’s inside it and whether it’s starting to split.
What Causes Them
Why St. Louis Flat Roofs Blister
Voids in the plies
Skip-mopped areas and torch-applied mod bit with cold spots leave dry pockets. The air expands every afternoon and shrinks every night, and the blister grows a little each cycle.
Modified bitumen roofingWet insulation below
A leak at a curb or seam soaks perlite or fiberboard, which keeps feeding vapor up into the membrane. These blisters come back no matter how many times you cut them.
Wet insulationNo vapor control
Humid interiors — pools, kitchens, laundries — push moisture up into the roof all winter. Without a vapor retarder it condenses under the membrane.
Vapor barriersCoating blisters
Acrylic or silicone sprayed over a damp or dirty surface bubbles within a season. That’s an adhesion failure, and the loose coating has to be scraped back before anything else.
Silicone vs. acrylic coatingsSingle-ply bubbles
On fully adhered TPO or EPDM, bubbles mean the adhesive let go or wasn’t flashed off right. A mechanically attached sheet billowing in wind is a different problem.
Infrared moisture scans
Diagnosis
Cut It, Watch It, Or Replace It
We start by pressing on it. A dry blister feels like a balloon. A wet one squishes, and sometimes water weeps out of an old nail hole when you step near it. Then we find out what’s under it.
An infrared scan after sunset shows where wet insulation is holding heat, and core cuts confirm it. If the blisters sit over dry board and the plies are intact, plenty of them can be left alone and checked every spring and fall.
A blister that’s cracked, sitting in a walkway or holding ponded water gets repaired. On built-up and mod bit that’s an X-cut, the flaps dried and set back down, then a patch that laps well past the cut. On single-ply it’s a cut-out with a heat-welded or taped patch.
When blisters cover a big share of a section and the cores come back wet, patching is throwing money at it. That section needs a tear-off to the deck, new insulation and new membrane. Recovering over wet board just traps the moisture, and the new roof blisters too. Our flat roof repair crews make that call on the roof with you, not from the truck.
Common Questions
Are roof blisters an emergency?
Usually not. An intact blister isn’t leaking. It gets urgent when it splits, when it’s in a walkway or near a drain, or when there’s a leak inside right below it.
Can I just pop the blister?
Please don’t. Puncturing it lets water straight into the plies. A blister has to be cut, dried, re-adhered and patched properly, or left alone.
Why do new blisters keep showing up every summer?
Something’s still feeding them, usually wet insulation from an old leak or interior moisture with no vapor retarder. Fixing the source matters more than patching the bubbles.
Will insurance cover roof blistering?
Blisters are generally treated as an installation or age issue, not storm damage. Hail can bruise a blister open, though, and that damage may be part of a claim.
Do you inspect blistered roofs for free?
Yes. We check the blisters, pull cores where they matter, and tell you whether to leave them, repair them or plan for a replacement.
Free Blistered Roof Inspection
Tell us about the roof and a dispatcher will call to schedule.