Blisters and bubbles in waterproofing are one of the clearest signs that something isn’t right on a roof or terrace. They don’t always mean an active leak today, but they often point to trapped moisture, poor adhesion, or an unstable substrate that—under heat and movement—can eventually let water in.

What blisters are (and why they matter)

A blister is a localized “bulge” in a membrane or coating (bitumen felt, liquid-applied membranes, etc.). It typically forms due to:

Water vapor pressure pushing from below
Air or solvents trapped during application
Loss of adhesion caused by dust, dirt, or insufficient primer

A blister isn’t just cosmetic: it can rupture under foot traffic, hail, or heat and become a direct entry point for water.

The most common causes of bubbles on roofs and terraces

Moisture in the substrate before waterproofing

This is the #1 cause. If a membrane is installed over a damp screed (or without a vapor barrier when needed), sunlight heats the roof and vapor tries to escape—lifting the membrane.

Missing or wrong primer

For fully bonded systems (bitumen, polyurethane, etc.), an incorrect or poorly applied primer reduces adhesion and makes air pockets more likely.

Installation under poor conditions

Very high temperatures (surface “skins” too fast)
Applying on a hot surface under direct sun
High ambient humidity or dew risk

Contaminated or poorly prepared substrate

Dust, cement laitance, release agents, poorly bonded old paint, or efflorescence can prevent proper bonding.

Bad detailing at critical points

Weak detailing at parapets, drains, movement joints, or penetrations can allow small water ingress that later “inflates” nearby areas.

How to tell if it’s a serious issue

Before you “puncture and patch,” check:

Location: near drains, parapets, joints, or penetrations?
Scale: a few small blisters vs. widespread bubbling
Sound: does it sound hollow (debonded) when tapped?
Moisture: if opened, is there liquid water or damp odor?
Progression: if it grows in summer heat, vapor/moisture below is likely

In multi-owner buildings, problems often worsen during heat waves: the roof moves more, vapor pressure increases, and blisters multiply when the original build-up didn’t include proper vapor control or ventilation.

Fixes by scenario (and what NOT to do)

1) Isolated blisters and the rest of the system is sound

Typical approach:

Controlled cross-cut (without damaging the substrate)
Proper drying (not just a quick air-out)
Compatible patch and fully sealed perimeter
Reinforcement if there’s cracking or a weak detail

Avoid:

Sealing over the top without removing moisture (it will come back)
Using incompatible mastics with the membrane

2) Many blisters or widespread debonding

This usually needs a larger intervention:

Partial or full removal of the affected system
Check falls, substrate condition, and critical details
Add a vapor barrier (where required) and rebuild with the right system
Re-waterproof with a compatible solution (bonded, semi-bonded, or inverted build-up)

3) Blisters caused by ongoing substrate moisture

If moisture is coming from the structure itself (wet slab, poorly cured screed, previous leaks), you must:

Identify the real source (lateral ingress, condensation, moisture at junctions)
Fix it before closing everything again with a new membrane

How to prevent bubbles in a new waterproofing job

Verify substrate moisture and respect curing times
Match primer and system to the substrate and use (traffic/no traffic)
Reinforce critical details (drains, parapets, joints)
Avoid application on sun-heated surfaces
On complex roofs, consider ventilated or inverted build-ups where appropriate

When to call a professional

Blisters near drains or junctions
Hollow-sounding areas across larger zones
Interior staining, leaks, or persistent damp smell
Shared roofs where diagnosis and repair need proper reporting

If you’re seeing blisters or bubbles in your waterproofing on a terrace or roof, Impermyna can help you identify the real cause and propose a long-lasting repair—not just a quick patch.

Blisters and bubbles in waterproofing: causes and fixes (done right)