How to fix roof rubber bubbles in hot weather – and how to stop them from happening again

Experiencing bubbles or blisters under your EPDM roof rubber? Don’t worry – it’s more common than you think, and usually nothing to panic about. Here’s how to handle it (and avoid it next time).

You’ve fitted the roof rubber – and then the heatwave hits.

You’ve done everything right. Your EPDM membrane is installed, it’s looking clean, smooth, and weatherproof. Then suddenly, as the temperature rises, you notice bubbles appearing under the surface.

It’s frustrating. It doesn’t look great. And if you’re working on a customer’s roof, it can feel like it reflects badly on you.

But here’s the good news: most roof rubber bubbles are temporary, preventable, and won’t cause long-term damage.

At Plytech, we’ve been helping roofers get it right for over 30 years. We supply everything you need to get the job done right the first time, from our trusted RubberCover EPDM membrane to expert advice and training.

In this guide we will walk you through what’s happening when bubbles appear, why it happens, and how to prevent it going forward.

Bubbles or blisters under your EPDM roof rubber?

If you’ve spotted bubbles or blisters forming after installation, especially in warmer weather, you’re not alone.

There are a few common reasons why bubbles appear under the membrane:

Moisture evaporating from water-based adhesive
Trapped solvent vapours from contact adhesive
Expanding air from gaps or dampness in the roof structure

Let’s take a closer look at each cause – and how to fix it.

What’s really going on – and what to do about it

1. Bubbles from water-based adhesive

Water-based adhesives are popular for their ease of use and safer handling. But in hot conditions, they can bubble.

Why it happens
As the temperature rises, the water in the adhesive begins to evaporate. If the OSB board beneath the membrane was damp, it may also release moisture. Once the EPDM is down, there’s nowhere for the vapour to go, so it forms blisters beneath the membrane.

How to fix it
The good news is that most of these bubbles will dissipate over time. The membrane is partially breathable, and as the moisture escapes, the adhesive will still retain its tack. You can then use a roller to press the membrane back down.

How to prevent it next time
Let the adhesive sit longer before laying the membrane, giving it more “open time” to evaporate moisture naturally.

Ensure the deck is completely dry before applying any adhesive.

2. Bubbles from contact bonding adhesive

Contact adhesive is essential for upstands and perimeter edges. It’s solvent-based – which means it needs to flash off before you bond the membrane.

Why it happens
If the adhesive hasn’t flashed off (i.e. it’s still wet or stringy), the vapours get trapped as the roof rubber goes down. This creates bubbles, and in hot weather, those bubbles can grow larger as the vapour expands.

How to fix it
Some bubbles will shrink back on their own. You can speed this up by weighting the area. In more severe cases, the affected section may need to be peeled back and re-bonded once the adhesive is touch-dry.

How to prevent it next time
Always allow the contact adhesive to flash off completely, it should feel tacky but dry, with no transfer to your finger.

Avoid over-applying adhesive or laying the membrane too soon.

3. Expanding trapped air within the roof structure

This issue often shows up in warm roof builds, where insulation is added below the membrane.

Why it happens
If the roof structure or insulation is damp, or if the boards are fitted with gaps, air gets trapped. As the sun heats the roof, that air expands – pushing against the membrane and causing it to bubble or lift. You might notice it’s worse during the day and settles again in the evening.

How to fix it
In many cases, this is cosmetic and will come and go. If the problem persists, you may need to reassess how the insulation or boards were fitted.

How to prevent it next time
Ensure the roof deck and insulation are fully dry

Fit insulation boards snugly with no gaps – use expanding foam if needed

Consider adding a Visqueen membrane below the timber deck as a vapour barrier

How Plytech helps you install roof rubber with confidence

Whether you’re fitting a simple garden room roof or a large commercial project, we help you get it right from the start.

Here’s how we support you:

Top-quality products
Including our RubberCover EPDM membrane and full range of adhesives, trims, and tools
Expert installation training
With our Plytech Approved Installer Programme
On-hand technical support
For questions before, during, or after installation

We don’t just sell materials – we make sure you know how to use them properly. That’s why professional roofers across the UK trust Plytech when it comes to roof rubber solutions.