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Torch-on membrane (also known as torch-applied modified bitumen sheet waterproofing) is a system that is bonded to a surface using a gas torch. The heat from the torch melts the underside of the membrane as it’s rolled out, creating a strong, continuous bond with the substrate.
The membrane itself is a multi-layered sheet, typically 3-4mm thick, consisting of a reinforcing core (usually polyester or fibreglass) sandwiched between layers of modified bitumen. The bitumen is modified with either APP (Atactic Polypropylene) or SBS (Styrene-Butadiene-Styrene) polymers to improve flexibility, durability, and temperature resistance.
APP modified membranes are more rigid and have excellent UV resistance, making them a popular choice for exposed roof applications in the Australian climate. They perform well in high temperatures and are less likely to soften in direct sun.
SBS modified membranes are more flexible and elastic, which makes them better suited to areas that experience movement or thermal cycling. They’re often the preferred choice for podium decks, balcony substrates, and applications where the membrane will be covered by tiles, pavers, or topping slabs.
The choice between APP and SBS depends on the specific application, the expected movement in the structure, and whether the membrane will be left exposed or covered. An experienced waterproofing contractor will assess the project and recommend the right system.
The application of torch-on membrane is a skilled trade that requires trained and experienced applicators. The process involves several stages, each of which is critical to the long-term performance of the system.
The substrate (usually concrete) must be clean, dry, and free of loose material, oils, and contaminants. Any cracks, voids, or surface defects are repaired. This stage is crucial because the membrane needs a solid, consistent surface to bond to. Poor surface preparation is one of the most common causes of membrane failure.
A bituminous primer is applied to the prepared surface and left to cure. The primer improves adhesion between the substrate and the membrane. Depending on the product and conditions, curing time is typically 2-4 hours.
3. Membrane Application
The membrane rolls are positioned and aligned, then heated with a gas torch as they’re rolled out across the surface. The torch melts the underside of the membrane, and the applicator uses a roller to press it firmly into the primed surface. Overlaps between sheets (typically 75-100mm) are also torched and sealed to create a continuous, watertight layer.
The most critical aspect of any torch-on installation is the detailing at upturns, penetrations, drains, and edges. These are the areas where water is most likely to find its way through, and they require additional layers, preformed corners, and careful workmanship. The quality of detailing is often what separates a 10-year membrane from a 25-year membrane.
Depending on the application, the membrane may be left exposed (with a UV-resistant cap sheet) or covered with a protection layer such as a drainage cell, filter fabric, screed, or paving system. In trafficable areas like podium decks, a full protection and finishing system is installed over the membrane.
Torch-on membrane is versatile, but it’s best suited to specific applications where its strengths — continuous waterproof coverage, proven longevity, and resistance to hydrostatic pressure — are most valuable.
Torch-on membrane systems for external above-ground applications should be installed in accordance with AS 4654.2 Waterproofing membranes for external above-ground use — Design and installation.
Flat Roofs — This is the most common application. Torch-on systems are well suited to areas exposed to prolonged water exposure and ponding conditions when correctly designed and installed, and can accommodate thermal movement and foot traffic (with appropriate protection) better than most alternatives.
Podium Decks — Torch-on membrane, combined with a proper drainage and protection system, is one of the most reliable solutions for podiums in apartment and mixed-use developments.
Planter Boxes — The constant presence of water and soil makes waterproofing failure a common and expensive problem. Torch-on membrane, with its continuous application and resistance to root penetration (when a root-resistant membrane system specifically designed for planter box applications is specified), is an excellent choice for planter boxes on commercial and residential buildings.
Below-Grade and Basement Walls — In below-grade applications where the membrane is installed on the positive (external) side of basement walls and footings, torch-on membrane provides a durable barrier against groundwater and hydrostatic pressure.
The membrane itself is a multi-layered sheet, typically 3-4mm thick, consisting of a reinforcing core (usually polyester or fibreglass) sandwiched between layers of modified bitumen. The bitumen is modified with either APP (Atactic Polypropylene) or SBS (Styrene-Butadiene-Styrene) polymers to improve flexibility, durability, and temperature resistance.
A well-installed torch-on membrane system has a typical lifespan of 15-25 years, with some systems lasting 30 years or more depending on the quality of the membrane, the standard of installation, the level of UV exposure, and how well the system is maintained.
The key factors that affect lifespan include:
Quality of installation — this is the single biggest factor. A membrane installed by trained and experienced waterproofers with proper surface preparation and detailing will significantly outlast a poorly installed system, even if the same membrane product is used.
UV exposure — membranes left exposed to direct sunlight will degrade faster than those protected by pavers, screed, or ballast. APP modified membranes offer better UV resistance for exposed applications.
Maintenance — regular inspections (ideally annually) can catch minor damage, blocked drains, or failing details before they become major problems. Many membrane failures begin as small issues that go unnoticed.
Building movement — all buildings move to some degree due to thermal expansion, settlement, and loading. SBS modified membranes accommodate movement better than APP, which is why membrane selection should be matched to the expected movement in the structure.
Torch-on membrane is not the only waterproofing option. Depending on the application, liquid applied membranes and pre-formed sheet membranes may also be suitable. Here's how they compare.
| Factor | Torch-On | Liquid Membrane | Sheet Membrane |
|---|---|---|---|
| Best for | Flat roofs, podiums, below-grade | Complex shapes, wet areas, balconies | Roofs, below-grade walls, under-tile |
| Application | Gas torch (hot works) | Brush, roller, or spray | Peel and stick or mechanically fixed |
| Continuous waterproof layer? | Yes (continuous layer with sealed laps) | Yes (fully seamless) | Yes (continuous layer with lapped joints) |
| Typical lifespan | 15-25+ years | 10-20 years | 15-25 years |
| UV resistance | Good (APP grades) | Moderate (coating needed) | Good (UV-stable sheet options available) |
| Skill required | High (trained and experienced applicator) | Moderate | Moderate to high |
| Hot works? | Yes (fire risk management) | No | No |
| Cost | Mid to high | Low to mid | Mid |
Each system has its strengths, and in many projects, a combination of methods is used. For example, torch-on membrane on the main roof area with liquid membrane at complex junctions and penetrations. An experienced waterproofing contractor will recommend the most appropriate system, or combination of systems, for each area of the project.
Torch-on membrane installation is not a DIY job. It requires trained and experienced applicators who understand the product, the building, and the risks involved with hot works. When choosing a contractor, look for:
Registered Building Practitioner status — in NSW, waterproofing work must be carried out by appropriately registered building practitioners.
Waterproofing-specific experience — a general builder or roofer may not have the specialist knowledge required for a quality membrane installation. Look for a contractor whose core business is waterproofing.
A clear scope of works — a reputable contractor will provide a detailed scope covering surface preparation, membrane specification, number of coats, detailing methods, protection systems, and warranty terms.
Insurance and warranties — ensure the contractor carries appropriate insurance and offers a meaningful warranty on both materials and workmanship.
Understanding why torch-on membranes fail is just as important as understanding how they work. Most failures can be traced back to one or more of the following causes:
Poor substrate preparation — if the concrete surface is not properly cleaned, repaired, and primed, the membrane will not bond correctly. This is the most preventable cause of failure.
Inadequate falls and drainage — if water cannot drain away effectively, it will pond on the membrane surface. While torch-on systems can tolerate ponding, prolonged water sitting on a membrane accelerates degradation and increases the risk of leaks at any weak points.
Failed detailing at penetrations and upturns — pipes, drains, walls, and edges are the most vulnerable areas. Poorly executed detailing at these junctions is a leading cause of water ingress.
Poor lap welding — where membrane sheets overlap, the lap joint must be fully welded to create a continuous waterproof seal. Insufficient heat, contamination, or inadequate overlap widths can result in lap failure.
UV degradation — membranes left exposed to direct sunlight without UV protection or an appropriate cap sheet will degrade over time, becoming brittle and cracked.
Structural movement — buildings move due to thermal expansion, settlement, and loading. If the membrane cannot accommodate this movement, it will crack or debond, particularly at rigid junctions.
Lack of maintenance — blocked drains, accumulated debris, and minor damage left unaddressed will worsen over time and lead to premature failure.
Mechanical damage and punctures — foot traffic, dropped tools, stored equipment, and trades working over the membrane can cause punctures and tears, especially if no protection layer is in place.
A well-maintained torch-on membrane will significantly outlast one that is neglected. Regular maintenance doesn't need to be complex or expensive, but it does need to be consistent. The following maintenance practices are recommended:
Annual inspections — have the membrane system inspected at least once a year by an experienced waterproofing professional. Look for signs of cracking, blistering, delamination, and damage at details and laps.
Drain cleaning — ensure all drains, scuppers, and overflow outlets are clear of debris. Blocked drains are one of the most common contributors to membrane failure.
Checking laps and detailing — inspect all lap joints, upturns, and penetration details for signs of separation, lifting, or cracking. These are the most vulnerable areas and where leaks most commonly originate.
Identifying punctures early — any punctures, tears, or mechanical damage to the membrane should be repaired promptly. Small issues left unaddressed will allow water to enter the system and cause much larger problems over time.
Maintaining protection layers — if the membrane is covered by pavers, tiles, or ballast, ensure the protection layer remains intact and is not displaced. Exposed membrane areas should be addressed.
Monitoring movement cracks — keep an eye on areas of the building where structural movement is occurring. If new cracks are forming in the substrate near the membrane, have the system assessed to determine if remedial work is required.
Prompt repairs — do not defer repairs. The cost of a small patch repair is a fraction of the cost of a full membrane replacement, and early intervention can extend the life of the system by years.
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