How to Fix a Blister in a Torch-On Membrane on a Flat Roof

Blisters in a torch-on bitumen membrane are a common flat roof headache in New Zealand, especially after hot summers or where moisture has been trapped during installation. Small blisters can often be repaired by a competent DIYer, but larger or widespread blistering usually signals a deeper problem that needs a licensed waterproofing professional.

What Causes Blisters in Torch-On Membrane?

Blisters form when air or moisture becomes trapped between the membrane layers — or between the membrane and the substrate — and then expands under heat. Common causes include:

• Moisture in the substrate (plywood or concrete deck) at the time of installation
• Inadequate adhesion during torching, leaving unbonded pockets
• Poor ventilation beneath the roof deck, causing vapour pressure to build up
• Age and UV degradation of older membranes

Before you repair anything, press the blister gently. If it feels firm and dry, it likely contains trapped air and may be stable. If it feels soft, spongy, or wet, water has entered and you may have underlying deck damage — get a professional assessment before proceeding.

Safety First

Working on a flat roof carries real fall risks. Under WorkSafe NZ guidelines, any work at height requires appropriate edge protection or fall restraint. At minimum:

• Never work alone on a roof
• Use non-slip footwear
• Avoid working in wet or windy conditions
• If your roof is higher than one storey, consider hiring a professional

Torch-on work also involves an open-flame LPG torch. If you are not experienced with a roofing torch, do not attempt to re-torch the repair yourself — use a cold-applied bitumen repair product instead (see below).

Tools and Materials You'll Need

Step-by-Step: Repairing a Small Blister

1. Clean and Dry the Area

Sweep away any grit, dirt, or moss. Allow the area to dry completely — ideally after several dry, sunny days. Moisture trapped under a patch will cause the repair to fail.

2. Cut the Blister Open

Using a sharp utility knife, make two cuts in an X pattern across the centre of the blister. Fold back the four flaps carefully without tearing them off. If you find wet or rotted substrate underneath, stop here and call a professional — the deck may need replacing before any membrane work.

3. Dry and Prime the Substrate

If the substrate is sound and dry, brush out any debris and apply a bitumen primer to the exposed area and the underside of the flaps. Allow to tack off as per the product instructions.

4. Press Down the Flaps

Once the primer is ready, press the flaps back down firmly into place. Use a seam roller to bond them as flat as possible.

5. Apply a Patch

Cut a patch of self-adhesive torch-on membrane (or cold-applied bitumen patch sheet) at least 150 mm larger than the blister on all sides — this generous overlap is critical for a weathertight seal. Peel and stick, or torch on if you are experienced, pressing firmly from the centre outward to avoid new air pockets.

6. Seal the Edges

Apply a bead of bitumen lap sealant around the perimeter of the patch and smooth it down. This prevents water tracking under the patch edge.

7. Inspect After the First Rain

Check the repair after the next rainfall. Any lifting or bubbling means the patch hasn't bonded properly and you should call a waterproofing professional.

Does This Work Require a Building Consent?

Like-for-like repair of an existing membrane is generally exempt from building consent under Schedule 1 of the Building Act, but you should confirm this with your local council before starting, particularly if the roof area being repaired is large or if the deck structure is being altered. MBIE Building Performance has useful guidance on what work is and isn't exempt.

Preventing Future Blisters

• Ensure roof deck ventilation is adequate — BRANZ has good guidance on flat roof construction details
• Keep drains and outlets clear so water doesn't pond
• Inspect your membrane annually, especially after summer heat cycles
• Address any small cracks or open laps promptly before water ingress occurs

When to Call a Professional

Call a licensed waterproofer if:

• The blister is larger than about 300 mm across
• You find multiple blisters spread across the roof
• The substrate feels soft, wet, or spongy
• The membrane is cracked, split, or heavily alligatored around the blister
• You are not comfortable working at height or using a roofing torch
• The roof is over a habitable space and any leak could cause significant damage

A qualified waterproofer can assess whether a patch repair is sufficient or whether the membrane needs to be stripped and replaced. You can find experienced local waterproofing tradespeople on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ – Flat Roof Construction and Maintenance
• MBIE Building Performance – Building Consent Exemptions
• WorkSafe NZ – Working at Height
• PlaceMakers – Roofing Products
• Bunnings – Roofing and Waterproofing
• Sika – Waterproofing Solutions NZ