What a felt blister is telling you
When you notice blisters in a felt roof, it helps to separate what you can see from what may be happening underneath. The issue here is raised bubbles or swollen areas in bituminous roofing felt. With blisters in a felt roof, a small visible defect can sometimes have a simple explanation, while a wider pattern may point to something that needs a closer inspection.
Before arranging work, note whether the blister is firm, soft or cracked, whether it grows during hot weather, and whether there are only one or two bubbles or many across the roof. For blisters in a felt roof, this sort of simple information often tells more than a close-up picture because it shows when and where the defect is active.
Why bubbles form under roofing felt
The main possibilities are air or moisture trapped beneath the felt, poor bonding between felt layers, and water entering at a nearby defect and travelling under the surface.
Other causes worth considering are heat expanding trapped vapour. With blisters in a felt roof, the important thing is to match the cause to raised bubbles or swollen areas in bituminous roofing felt, rather than treating it as a general roofing fault. For blisters in a felt roof, that distinction can prevent work being carried out on a nearby detail that happens to look worn but is not responsible for what you are seeing.
Do not pop it like a bubble
From a safe position, you can do not puncture the blister yourself. You can also photograph its size over time from a safe position.
It is also useful to look for splits at the blister edge after very hot or cold weather. For blisters in a felt roof, write down what you can see and, where relevant, what the weather was doing at the time. A short sequence of photographs of blisters in a felt roof from the same safe viewpoint can show whether the condition is changing. When checking blisters in a felt roof, avoid walking on the roof or leaning out of windows; the aim is to collect useful information without creating another problem.
A split blister can become a leak
This becomes more urgent if the blister has split open, water is entering below it, and large areas are debonding. With blisters in a felt roof, those signs suggest that waiting for a convenient time could allow more water entry, falling material or further damage.
If those warning signs are absent, blisters in a felt roof may not need an emergency call-out, but it is still worth arranging a proper look before the condition gets worse. In this case, change over time matters: raised bubbles or swollen areas in bituminous roofing felt that has been stable for a long period is a different situation from something that has appeared recently or is becoming more obvious after each spell of bad weather.
The surrounding felt matters too
A roofer looking at blisters in a felt roof should check whether moisture is trapped beneath the felt, then inspect nearby laps and edges for the original entry point.
They may also need to judge the condition of the surrounding felt before recommending a patch. Because this page is about blisters in a felt roof, the inspection should stay focused on that problem and the nearby details that could realistically produce it. For blisters in a felt roof, ask what they think has failed, what evidence supports that view, and whether the proposed work is local or part of a wider defect. A clear explanation of blisters in a felt roof is more useful than a quotation that simply lists materials.
Repair the cause beneath the blister
Depending on what is found, a suitable repair might cut out and properly patch a local failed area where the system is otherwise sound, renew a larger section if adhesion is poor across the roof, or fix the source of trapped moisture before covering it again.
For blisters in a felt roof, the best option is the one that deals with raised bubbles or swollen areas in bituminous roofing felt and leaves the surrounding roof working as it should. If wider work is proposed for blisters in a felt roof, ask why a local repair would not be reliable, what condition the adjoining materials are in, and whether access costs make it sensible to combine closely related repairs. The answer should make sense for blisters in a felt roof, not read like a standard reroofing recommendation.
This guide is general information about blisters in a felt roof. The cause of blisters in a felt roof at an individual property can only be confirmed by inspecting the relevant roof area.