How to repair flat roof blisters

If you’ve spotted strange bubbles or raised bumps on your flat roof, you’re dealing with blistering.

Flat roof blisters are extremely common in the UK, especially on older felt, poorly installed GRP, or sun-exposed EPDM systems.

And if you’re searching

“how to repair flat roof blisters?”

Here’s the truth:

✔ Some blisters can be repaired easily

✘ Some blisters must not be touched

✔ The repair technique depends entirely on the roofing material

✘ Quick fixes or DIY sealants won’t solve the cause

Let’s walk through what you can safely repair and what needs professional attention.

1. What Causes Flat Roof Blisters?

Blisters form when air or moisture gets trapped under the roof surface and heats up, causing the top layer to expand.

Common causes:

•Poor installation

•Trapped moisture from rain during build

•Sun exposure overheating the roof

•Loose adhesion

•Weak points around joints and edges

•Old or worn-out felt

The key is to fix the damage and stop the underlying moisture from returning.

2. Identify the Type of Blister You Have

Flat roof blisters fall into three categories:

🔹 1. Dry blisters

Raised, air-filled, no water inside.

Can usually be repaired easily.

🔹 2. Wet blisters

Filled with moisture.

Often caused by leaks or trapped condensation.

More complex to repair.

🔹 3. Structural blisters

Large, deep blisters that affect the decking beneath.

These require professional repair or full replacement.

3. How to Repair Blisters on Different Flat Roof Materials

Each roof type needs a specific repair method.

A. Repairing Blisters on a Felt Flat Roof

Felt is the most common roof to blister.

Repair steps:

1.Cut an X-shape into the blister with a sharp blade

2.Gently lift the flaps

3.Allow trapped air and moisture to escape

4.Dry the area fully

5.Apply bitumen adhesive under each flap

6.Press the flaps back down

7.Add a new felt patch on top

8.Seal edges with bitumen

This is a temporary repair for older felt roofs — felt blisters often return.

B. Repairing Blisters on an EPDM Rubber Roof

EPDM rarely blisters, but when it does, the cause is usually poor bonding.

Repair steps:

1.Clean the blistered area

2.Apply EPDM primer

3.Install a rubber patch over the blister

4.Use a roller to remove trapped air

5.Seal the edges

DO NOT slice EPDM blisters — this damages the membrane.

C. Repairing Blisters on a GRP Fibreglass Roof

Most GRP blisters come from trapped air during installation.

Repair steps:

1.Sand down the blistered area

2.Remove any weak laminate

3.Clean with acetone

4.Apply new GRP resin

5.Add fibreglass matting if needed

6.Apply another layer of resin

7.Finish with fresh topcoat

A GRP blister repaired properly becomes as strong as new.

4. When NOT to Repair Flat Roof Blisters Yourself

Call a professional if:

✘ The blister is larger than your hand

✘ Water keeps leaking inside

✘ The roof deck feels soft

✘ Multiple blisters appear at once

✘ The roof is over 20 years old

✘ The blister reforms after repair

These signs usually mean deep moisture problems underneath the surface.

6. Is It Better to Repair or Replace the Roof?

Repair blisters if:

✔ The roof is under 10–15 years old

✔ Damage is small and localised

✔ No water has reached the decking

Replace the roof if:

✘ Blisters keep returning

✘ The roof is 20–25+ years old

✘ Water has soaked the boards below

✘ More than 20–30% of the roof is affected

Modern EPDM and GRP systems last much longer and almost never blister when installed correctly.

Need a Flat Roof Blister Repaired?

Correct Way Roofing provides long-lasting repairs for all flat roofs across Telford and the West Midlands.

We offer:

✔ EPDM, GRP & felt blister repairs

✔ Max repair cost £500

✔ Same-day emergency work

✔ Guaranteed repairs

✔ Free inspections

✔ Honest advice from a family-run company

📞 Call Correct Way Roofing for a free on-site inspection and honest price estimate.

Call Us Today