Flat roofs

Why are bubbles or blisters appearing on my flat roof?

Short answer: Blisters may indicate trapped air or moisture, loss of adhesion or movement between layers. Their significance depends on the system, size, location and whether they are splitting.

What this means in practice

Flat-roof performance depends on the whole build-up: structural deck, vapour control, insulation, falls, waterproofing, outlets, kerbs and edge details. Choosing a branded membrane while ignoring trapped moisture or poor drainage simply locks the original fault beneath a new surface.

What to check next

  • Do not puncture blisters casually
  • Check for trapped moisture and deck defects
  • Assess whether local repair or renewal is justified

When should you ask a roofer to inspect it?

Arrange a survey for recurring leaks, soft or deflected decking, persistent ponding, split details, blistering or an unknown build-up. Openings may be needed before anyone can responsibly confirm whether an overlay is suitable or the roof must be stripped.

Safety first: do not climb onto a roof without suitable access, training and protection. Keep clear of loose coverings, unstable ceilings and water near electrical fittings.

Technical content reviewJ & L Welch Roofing & Paving Services Ltd · Over 20 years of combined roofing experienceReviewed 28 August 2026 · How our answers are produced

Would you like a property-specific answer?

Request an inspection and written quotation from J&L Welch.

No deposit is required. Payment is due on completion within seven days, with the agreed scope and price confirmed in writing.

Request a quotation