Blisters and Bubbles on a Flat Roof: What They Mean and When They Need Repair
| # | Table of Contents |
|---|---|
| 1 | What a Roof Blister Is |
| 2 | What Causes Blisters |
| 3 | Which Flat Roofs Blister |
| 4 | Leave It or Repair It? |
| 5 | When Blisters Point to a Bigger Problem |
| 6 | What Property Owners Should Do |
| 7 | FAQ |
What a Roof Blister Is
Flat and low-slope roofs are built in layers: insulation or cover board, sometimes a base sheet, and the waterproof membrane on top, all bonded together. A blister forms where part of that bond is missing. The trapped pocket then pushes up into a dome, from coin-sized to several feet across. Some blisters run in long lines where sheets overlap.
A blister is not a hole. Many stay closed and watertight for years. The risk is what happens when one is stepped on, cracks from movement, or keeps growing until it tears.
What Causes Blisters
NRCA's technical guidance puts it simply: "A blister needs a void to begin or, at a minimum, a disbonded area." The common ways that happens:
- Moisture in the assembly. Wet insulation, a damp cover board or moisture on the deck when the roof was installed can prevent a proper bond. NRCA notes that substrate moisture "can create voids or the disbondment necessary for blister formation."
- Gaps in the adhesive or bitumen. Uneven coverage or bitumen applied too cool leaves unbonded spots.
- Heat cycling. NRCA notes blisters need temperature change to create the pressure that makes them grow, and that water vapor makes those pressure swings much larger than air alone. A Central Texas roof that heats up every afternoon and cools at night gives a blister a daily push.
Once a blister starts, that pressure stresses the edge of the pocket, the bond fails a little further out, and the blister gets bigger.
Which Flat Roofs Blister
Blisters show up on most bonded roof systems. NRCA's article describes them between the base sheet and cap sheet of modified bitumen roofs, on built-up roofs, and on single-ply membranes that are adhered directly to the substrate.
Single-ply roofs such as TPO are installed two main ways. When the membrane is fully adhered, a bubble is a true blister. When it is mechanically attached, the sheet is fastened along its edges rather than glued down across its surface, so wind can lift it between the fastener rows and make it look puffy. That is usually billowing, a different issue from a blister, and it is worth having someone who knows the system tell you which one you are looking at.
If you are not sure what your roof is, our flat and TPO roofing page explains the systems we work with.
Leave It or Repair It?
Not every blister needs to be cut open. NRCA's guidance is that a closed blister can often be left alone, and that cutting into a sound blister can create a leak where there was none. The same guidance, drawn from NRCA's repair manual, recommends repair when a blister:
| Condition | What it usually means |
|---|---|
| Is open, split or punctured | Water can now get under the membrane. Repair it. |
| Has lost much of its surfacing (granules or coating) | The membrane is exposed to sun and weathering. Repair it. |
| Shows exposed reinforcement or cracking around it | The membrane is fatiguing. Repair it. |
| Is in a walkway or near rooftop equipment | Likely to be stepped on. NRCA says repairing it may be prudent even if closed. |
| Has pulled apart a seam by about a third of the seam width or more | Consider repair; seams are where leaks start. |
| Is small, closed and not growing | Usually monitor it and recheck after hot weather. |
When repair is needed, the blister is cut out and the area is patched following the membrane manufacturer's instructions. A patch over a blister whose cause is wet insulation fixes the symptom, not the source.
When Blisters Point to a Bigger Problem
One or two blisters are a maintenance item. Many blisters, blisters that keep coming back after repair, or blisters together with soft spots underfoot and ceiling leaks inside the building suggest moisture trapped in the insulation across a larger area. At that point the question becomes how much of the roof is wet, because that decides whether repairs, a partial replacement or a full replacement makes sense.
Signs that point that way:
- New blisters appearing every summer.
- The roof feels spongy or crunchy when walked on.
- Water stains on ceiling tiles below, especially after storms.
- Ponding water that sits in the same low spots for days.
What Property Owners Should Do
- Photograph the blisters with something for scale and note their location.
- Keep foot traffic, including HVAC technicians, off them where possible.
- Check them again after a few hot weeks to see whether they are growing.
- Have any open, cracked or seam-adjacent blister repaired promptly.
- If there are many, ask for an inspection that looks at the insulation, not just the surface.
For a commercial building, apartment property or home with a flat roof section, our commercial roofing and flat roofing teams can inspect blistering, tell you which ones can wait and which cannot, and document the roof's condition with photos.
Source: National Roofing Contractors Association, Professional Roofing, "Back to the start" by Matt Dupuis, Ph.D., P.E. (September 2024), including guidance drawn from the NRCA Repair Manual for Low-slope Roof Systems. Always follow the membrane manufacturer's repair instructions for your specific system.
Frequently Asked Questions
Are blisters on a flat roof a problem?
Not always. A closed, intact blister can often be monitored. Blisters that are open, cracked, losing surfacing, in walkways or pulling apart a seam should be repaired.
What causes bubbles on a TPO roof?
On a fully adhered TPO roof, bubbles are usually blisters caused by an unbonded area, often from moisture or incomplete adhesive coverage. On a mechanically attached TPO roof, a puffy look is often billowing from wind uplift between fastener rows, which is a different issue.
Should I pop or cut open a roof blister?
Do not cut a closed blister yourself. NRCA guidance is that closed blisters can often be left alone, and cutting one opens a path for water. Open or damaged blisters should be repaired following the membrane manufacturer’s instructions.
Why do roof blisters get bigger in summer?
Heat makes the air and water vapor trapped inside expand. NRCA notes that water vapor makes the pressure swings much larger than air alone, so blisters can grow through repeated hot days.
Do blisters mean my flat roof needs replacing?
A few blisters usually do not. Widespread or recurring blisters, especially with soft spots or interior leaks, can mean wet insulation over a larger area, which is when replacement becomes a real option.
