Bubbles in SBS Membrane: Causes, Prevention & Repair Guide

Quick Answer: Why Do Bubbles Appear in Torch-Applied SBS?

Most blistering in torch-applied SBS is a substrate problem, not a torch problem. When bubbles appear after installation, contractors usually blame the flame — but moisture in the substrate, a skipped primer coat, and damp reinforcement together cause roughly 80% of blister failures. The torch only reveals these problems during bonding. If blisters appear, diagnose the root cause before re-heating the area: re-torching a moisture-driven blister will not fix it.

Blister, Wrinkle, or Lifted Edge — Know the Difference

Not every surface defect is the same problem, and each one needs a different response:

Defect	Typical Cause	Correct Response
Blister (bubble)	Moisture vapor trapped between membrane and substrate	Cut, dry, patch — or remove and re-install if widespread
Wrinkle (fold)	Membrane laid on uneven surface, or shrunk after bonding	Lift, re-stretch and re-bond the affected section
Lifted edge (curling)	Insufficient heat at the lap, or cold application	Re-heat and re-roll the lap to full contact

Why Blisters Form: The Moisture Mechanism

The physics is simple: when the substrate or the reinforcement carries moisture, torching turns that moisture into vapor. The vapor needs to escape, but the molten bitumen seals the path — so the pressure lifts the membrane off the substrate into a dome. Once the material cools, the dome remains as a blister, and a probe will usually show that the membrane has separated from the substrate underneath it.

Three root causes account for nearly all blister failures:

1. Moisture in the substrate (most common). Concrete, screed and plywood substrates hold moisture from curing, rain or condensation. If the surface is not dry before application, torching vaporizes that moisture directly under the membrane. Test substrate moisture before starting — a simple plastic-sheet test or moisture meter tells you more than visual inspection.
2. A skipped or incomplete primer coat. The primer bonds the membrane to the substrate and seals dust. Where primer is missing, the membrane can lift, and any residual surface moisture concentrates at the unbonded area.
3. Damp polyester reinforcement. Rolls stored on damp ground or exposed to humidity absorb moisture into the carrier. The moisture flashes off during heating, creating localized bubbles inside the sheet itself.

The Correct Installation Sequence (Overview)

Proper procedures prevent the problem. The key point: steps 4 and 5 are one continuous motion — the flame heats ahead while the roller presses behind; they are never performed as separate passes.

1. Surface preparation — clean, dry and dust-free substrate, moisture-tested.
2. Primer coat — full-surface cold primer, dry before membrane lay-up.
3. Dry-lay — position and square each roll before bonding.
4. Torch heat — heat evenly until bitumen flows; do not scorch.
5. Roll & press — press from the centre out to expel trapped air.
6. Lap seal — 100mm side laps / 150mm end laps, melt line visible.

Full step-by-step procedures, tool requirements and safety rules are provided in the installation manual available on our SBS Waterproofing Membrane product page.

Repair or Replace? The Decision Guide

Situation	Action
One or two small isolated blisters	Cut a cross in the blister, dry the cavity, re-heat the surrounding bitumen and press a torch-welded patch over the cut
Several blisters concentrated in one area	Investigate the substrate first — if moisture is present, remove and re-install the affected membrane after drying and priming
Widespread or recurring blistering	Remove the affected section, dry and prime the substrate, re-install. Re-torching will not help: the vapor is still trapped

A repair is only as good as the diagnosis. If blisters recur in the same area after a patch, treat it as a substrate problem, not a workmanship problem.

Frequently Asked Questions

Why do bubbles appear after torch-applying SBS membrane?

Blisters in torch-applied SBS are almost always a moisture or preparation problem, not a torch problem. The most common causes, in order: moisture trapped in the substrate, a skipped or incomplete primer coat, and damp polyester reinforcement. The torch does not create these bubbles — it exposes them as the bitumen melts. Test the substrate moisture and confirm primer coverage before re-heating the area; re-torching a moisture blister will not make it adhere.

Can you fix bubbles in SBS membrane or must you replace it?

Small isolated blisters can be repaired by cutting a cross, drying the cavity, heating the surrounding bitumen, and pressing a torch-welded patch over the cut. Widespread or continuously recurring blistering indicates a substrate moisture problem, and the affected membrane should be removed and re-installed after the substrate is dried and primed. Repairs must follow the membrane TDS and the project specification.

What is the correct overlap for torch-applied SBS membrane?

The standard requirement is 100mm for side laps and 150mm for end laps. The lap area must be heated until the bitumen flows and a 5-10mm melt line extrudes from the edge, then pressed with a roller. Confirm the exact lap widths with the membrane TDS and project specification, as some specifications require wider laps for sloped or critical roofs.

How long should I heat SBS membrane with a torch?

There is no fixed time — the correct heat level is reached when the bitumen begins to flow and the membrane surface takes on a glossy, molten appearance. Underheating leaves an incomplete bond; overheating until the surface chars creates brittleness and future cracking. Work in sections and move the torch ahead of the roller at a steady pace rather than counting seconds.

Can SBS membrane be applied in cold weather?

Yes, with extra steps. The environment and substrate must be above the minimum application temperature in the TDS, the substrate should be pre-heated to remove surface frost, and work sections should be kept short so melted bitumen does not set before rolling. The -20°C / -25°C low-temperature rating of Type I / Type II describes in-service performance, not application temperature.

For the material decision before installation, see our SBS vs APP Modified Bitumen Membranes guide. For thickness selection and system design, see 3mm vs 4mm vs 5mm SBS Membrane.