A raised, bubble-looking patch on a Winchester commercial TPO or EPDM roof usually isn’t damage from a single storm — it’s trapped moisture or air working its way up through the membrane, and it tends to show up right as summer heat cycles into cooler fall nights. If you’re seeing blisters now, this is the season that makes them most visible.
What Causes Blistering on a Flat Commercial Roof?
Blistering happens when moisture or air gets trapped between the roof membrane and the substrate beneath it, then expands as the sun heats the roof, pushing the membrane upward into a visible bubble.
The moisture usually got in during installation — incomplete adhesive coverage, a substrate that wasn’t fully dry, or trapped air pockets under the membrane. Once it’s sealed in, every hot afternoon and cool night cycle makes the trapped air or vapor expand and contract, and over time that repeated movement can grow a small blister into a larger one, or eventually split the membrane at the high point.
Is a Blister Always a Leak?
Not immediately, and that’s what makes it easy to ignore. A blister that’s still intact is usually just trapped air or vapor, not an active leak. The risk is mechanical: the membrane at the top of a blister is stretched thinner than the surrounding roof, and foot traffic, hail, or just enough thermal cycling can eventually split it. Once it splits, a blister that was cosmetic becomes an active entry point.
This is different from the shrinkage and tenting we see at EPDM corners and terminations — that’s the membrane pulling away from its edges, while blistering is trapped material pushing the membrane up from underneath.
How We Assess and Repair It
- Check whether the blister is isolated or part of a wider adhesion failure across the field of the roof
- For a small, isolated blister: cut it open, let it dry, and patch with compatible membrane and adhesive
- For widespread blistering: this usually points to an installation-wide issue, and patching individual blisters becomes a losing game against the ones that haven’t shown yet
- Infrared moisture scanning to find trapped moisture that hasn’t blistered visibly yet, before it spreads under insulation
Typical cost to repair isolated blistering on a commercial flat roof runs $400-$1,200 for a handful of spot repairs, but widespread blistering that points to a systemic adhesion problem can run considerably more once it’s scoped with an infrared scan. That’s a general range — the real cost depends on how many blisters there are and whether the membrane underneath is still sound once opened up.
See our TPO & EPDM roofing page for how we handle membrane systems overall.
Frequently Asked Questions
Should I pop a blister myself to check it?
No. Cutting into a membrane without the right patch materials and adhesive on hand can turn an isolated bubble into an active leak point. Have it assessed and repaired in the same visit.
Does blistering mean the whole roof needs to be replaced?
Not usually, if it’s caught early and isolated. Widespread blistering across the whole field is a different story and often signals an adhesion or moisture problem that’s worth a full assessment rather than spot patches.
Why do blisters seem worse in fall?
Blisters are driven by thermal cycling — the bigger the swing between hot days and cool nights, the more the trapped air or moisture expands and contracts, so fall’s wide temperature swings tend to make existing blisters more visible.
Source: EPDM Roofing Association — Technical Bulletin #101.
Seeing blisters on your commercial roof? Call or text us at (859) 310-1209, or
Last updated: September 20, 2026
Operated by Central Roof Repair.