Blistering vs Cracking: How to Spot and Interpret Flat Roof Damage 6th July 2026
Published by the team at Avant Garde Roofing Solutions
Over 20 years of installing flat roofs in the Midlands
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If you have noticed anything unusual with your flat roof, such as blistering across the surface or cracking in the membrane, it’s understandable that you would want to know what this means. But is it a cause for concern or not? And should you take action?
For over two decades, we’ve witnessed far too many instances of blistering and cracking being misinterpreted – some people ignore a problem that needs addressing, and others worry unnecessarily about something that’s quite innocuous. Let’s clear the air.
What Are Blisters Caused By?
Blisters are bubbles that appear on the surface of the roof.
This may seem alarming, but the problem is almost always one of moisture or air being trapped under the roof membrane and expanding once the roof’s surface warms up with the heat of the sun.
Older felt roofs are the worst culprits for blistering because either the layers of felt were not sufficiently bonded during installation or moisture had entered and became trapped underneath the roof membrane. In summer, when the temperature rises, the moisture turns into vapour and causes bubbles on the surface of the roof.
What’s important to note about blistering is whether or not these blisters are intact.
Blisters that are intact (i.e., not broken) are not ideal, but at least they aren’t letting water into the roof structure at the moment. An experienced roofer will be able to determine whether they’re stable or worsening. Broken blisters are a completely different story because once the membrane is broken, you’ll have an opening in your roof structure and water will seep inside and move through all the layers.
Recently we were asked to examine a commercial unit in Bedford, whose owners had been monitoring blisters for two years thinking that they were harmless. By the time we visited the property, three blisters were broken and the deck underneath was water damaged. What could have been a simple fix became a partial replacement.
Don’t procrastinate. Get your blisters inspected.
What Are Cracks Caused By?
Unlike blisters, cracks usually mean the deterioration of your flat roof and nothing else. This type of problem is caused by the continuous expansion and contraction of materials (warm days, cold nights, and typical British weather) for many years.
Felt roofs show this pattern as “river beds” on their surfaces. Fibreglass and GRP roofs tend to develop cracks along edges and seams.
Location is crucial when it comes to cracking:
Surface crazing is the first indication of cracking that means that the material has lost its elasticity. Although the membrane is still functional at this point, you need to address the problem in order to prevent further damage.
Deep or structural cracks mean that there is moisture inside your roof membrane already. This is no longer a cosmetic issue.
Cracks at seams and edges are very serious. Flashings, upstands, and seams between roof and walls are the weakest points on any flat roof anyway. Cracks in these areas provide quick access to the membrane for the water.
Blistering vs Cracking: Which Issue Is More Urgent?
Generally speaking, cracking is more urgently demanding than intact blisters. The truth is, however, that both issues require a professional assessment rather than ground-level assumptions.
Our experience shows that neither problem occurs in isolation. Roofs that have significant blisters usually also have problems with their bond or moisture content. Cracked roofs are almost certainly too old and can’t be effectively repaired in those areas. Neither a quick seal nor patchwork solutions solve the problem.
What Should You Do?
Here’s our advice on what to do if you notice either of the above:
Don’t try to repair it yourself using some sealant or tape – this only hides the problem and makes proper repairs harder.
Don’t leave it until winter. Both blistering and cracking will develop faster in these conditions, and water damage accumulates quickly once it has started.
Do arrange an inspection to understand what the problem really is. Not all roofers are objective and some of them would suggest repairing the maximum possible amount of work. We believe that the client should know the actual state of his/her roof.
In some cases, repair will be enough. In others, replacement will be more sensible solution. You need to know what your roof needs.
Have you found blisters or cracks on your roof? Contact us by calling 0800 456 1104 or via our website.