Torch-on membrane is one of the most durable and widely used waterproofing systems for decks in New Zealand, particularly on low-slope or flat decks where water pooling is a real risk. When installed correctly, it forms a seamless, long-lasting barrier that protects the structure beneath from moisture damage. This guide walks you through the process, but be aware that torch-on work involves open flame and hot bitumen — it carries genuine safety risks and is best left to experienced waterproofers in most cases.
What Is Torch-On Membrane?
Torch-on membrane (also called torch-applied bituminous membrane) is a modified bitumen sheet product that is bonded to a substrate using a gas torch. It comes in rolls and is available in base-sheet and cap-sheet layers. The cap sheet is typically finished with a mineral or granule surface for UV resistance and foot traffic. It is commonly used on concrete decks, plywood substrates, and flat or low-pitch roofs across NZ.
NZ Building Code Considerations
Before you start, check whether your deck project requires a building consent. Under the NZ Building Code, waterproofing a deck that is part of a habitable space (e.g., over a garage or living area) is likely to require consent. Even if the deck itself was built under a Schedule 1 exemption, applying a new waterproofing system may still need council sign-off — always confirm with your local council before proceeding.
The Building Code clause E2 (External Moisture) is the key standard here. Your waterproofing system must meet E2 requirements to prevent moisture ingress into the building structure. BRANZ publishes guidance on acceptable solutions for deck waterproofing that are worth reviewing.
Tools and Materials You'll Need
- Torch-on base sheet and cap sheet membrane (from suppliers such as Mitre 10, PlaceMakers, or specialist waterproofing suppliers)
- LPG gas torch and regulator
- Primer (bituminous or as specified by the membrane manufacturer)
- Stiff broom and vacuum
- Utility knife and straight edge
- Seam roller
- Safety gear: fire extinguisher, heat-resistant gloves, safety glasses, steel-capped boots
- Flashings (aluminium or lead) for edges and penetrations
Step-by-Step: Applying Torch-On Membrane
1. Prepare the Substrate
The substrate must be clean, dry, and structurally sound. For plywood decks, use a minimum 17 mm marine-grade or structural plywood (check the manufacturer's specification). Fill any gaps, fix loose boards, and ensure the deck has adequate fall — typically a minimum of 1:60 slope toward the drain to comply with E2 requirements. Remove all dirt, dust, and any old coatings.
2. Apply Primer
Roll or brush a compatible bituminous primer over the entire surface and allow it to dry fully. Priming improves adhesion and is a critical step — do not skip it. Follow the manufacturer's drying time, which can vary depending on temperature and humidity.
3. Install the Base Sheet
Unroll the base sheet and cut it to length before torching. Starting at the lowest point (near the drain), heat the underside of the membrane with the gas torch using a slow, sweeping motion until the bitumen becomes glossy and begins to flow slightly. Press the sheet firmly into the substrate as you go, working out air bubbles with a roller. Overlap each sheet by at least 100 mm at side laps and 150 mm at end laps, torching all overlaps thoroughly to create a watertight bond.
4. Install the Cap Sheet
Apply the cap sheet in the same manner, staggering the joints from the base sheet by at least 300 mm so no two seams align. The cap sheet provides the finished, UV-resistant surface. Pay particular attention to corners and upstands — these are the most common failure points. Membrane should turn up at least 150 mm onto any vertical surface (wall, balustrade post, or parapet).
5. Flash All Edges and Penetrations
All deck edges, drains, pipes, and post penetrations must be properly flashed. Use compatible metal flashings and ensure they are integrated into the membrane system with additional torch-on strips. Poorly flashed penetrations are the number-one cause of deck waterproofing failures in NZ.
6. Inspect and Test
Once cool, inspect all seams and laps by pressing firmly — there should be no voids or lifting edges. A flood test (ponding water for 24–48 hours) is the best way to confirm there are no leaks before applying any topping or decking boards.
Safety First
Torch-on work involves an open LPG flame near combustible materials. Always have a fire extinguisher on site, keep a bucket of water nearby, and never torch near timber framing, insulation, or other flammable materials without a heat shield. Check WorkSafe NZ guidelines for working with gas torches. Ensure adequate ventilation — bitumen fumes can be harmful in enclosed spaces.
When to Call a Professional
While a competent DIYer can tackle torch-on membrane on a simple, accessible flat deck, there are situations where you should always engage a licensed waterproofer or roofing contractor:
- The deck is over a habitable space (legal liability if it leaks)
- The substrate is damaged or complex (multiple penetrations, unusual geometry)
- A building consent is required — inspectors will want to see the work done to a professional standard
- You are not confident using an LPG torch safely
- The manufacturer's warranty requires professional installation
A qualified waterproofer will also be familiar with NZ-specific products and can ensure your system meets MBIE Building Performance requirements. You can find experienced local waterproofers on tradedirectory.co.nz.