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How to minimise fire risk: the alternative to torch-on membrane roofing

In projects where the risk of fire is high, torch-on membrane is not always the best option, but, until now, there hasn’t been a viable alternative. This year, an innovative solution is now available that negates the risk of fire. Here’s how to specify it.


One of the most commonly specified types of torch-on bitumen membrane roofing is an incredibly effective product in terms of longevity and durability. However, the issue is that it doesn’t provide the safest option for applications where the risk of fire is high, suggests Mapei’s Jeff Perrett.

“In projects – such as commercial builds that are in close proximity to other buildings in remedial work and, in residential applications, such as old villas – the risk of fire is high, which means that torch-on membranes are not suitable.”

Why is torch-on bitumen membrane roofing a fire risk in certain applications?

“In a remedial project, for example, it’s highly likely that the old building paper could come into contact with the heat from the torch when you remove the existing roofing or when the torch-on membrane roofing has been specified as part of an extension project. We call it the ‘glancing’ heat from the torch and it’s not visible with the naked eye.

“When you are torching a roof, you can’t see how far the heat travels from the torch itself, which allows for the possibility of that high heat travelling into cavities or detailing and, ultimately, starting a fire. This is particularly dangerous because, if a part of the building ignites, especially in a cavity, the installer may not even be aware a fire has started for some time,” Jeff says.

What is the safest membrane roofing alternative for high fire-risk projects?

A new product to New Zealand, known as Spider P, is a two-layer bitumen membrane product that performs in the same way as similar torch-on membranes but has a self-adhesive ‘peel and stick’ system that mitigates all fire risk during application, Jeff says.

“This is not a new technology. In fact, Mapei has been using this technology in Europe and the United States for a couple of decades but it has only just been released in New Zealand.”

“This is an innovation that will quickly begin to supercede torch-on membranes in both residential and commercial applications.”

Similar to other two-layer bitumen membranes, Spider P has two layers however both are self stick, with a release film allowing the membrane to be rolled onto the substrate and instantly adhere – rendering the roof waterproof immediately.

“During application of Spider P, all detailing is completed using a leister hot air welding tool.”

The compound layering system incorporates Spider P, the base layer, and Spider P Mineral, the top layer, which is made from a elastomeric compound reinforced and stabilised with longitudinal glass threads. “This reinforcement allows the product an excellent dimensional stability and mechanical performance.”

When you are torching a roof, you can’t see how far the heat travels from the torch itself, which allows for the possibility of that high heat travelling into cavities or detailing and, ultimately, starting a fire…

In which applications are torch-on membranes the best option?

While torch-on membranes present a fire risk in certain applications, there are many where it offers a suitable and safe option. “In general, new builds are the best types of projects to specify torch-on bitumen membrane roofing, as well as those where there are expansive areas without any risk. This really comes down to the design of the building and if there is any complex detailing, for example, between flat roofs and other types of pitched roofing. It’s in these areas where torch-on membranes present a fire risk,” Jeff says.

How do the two main types of bitumen roofing differ?

There are two main types of bitumen membrane roofing – APP and SBS. “AAP is what Spider P is made with. AAP bitumen is harder and less flexible, which provides better heat and UV resistance and it will also perform well in very low or extreme temperatures with little or no mineral chip loss. AAP is also known in the market as elastomeric bitumen.

“In comparison, SBS bitumen is more flexible and is unable to provide the same level of performance in New Zealand conditions. SBS is also known as elastomeric bitumen. Due to its flexibility, self-adhesive ‘peel and stick’ solutions are not an option for SBS bitumen because of the lowered dimensional stability and the increased risk of the final cap layer losing its mineral chip finish.”

How do the installation times of self-adhesive and torch-on membranes compare?

“Due to the instant adhesion that is achieved with self-adhesive membranes, such as Le Fong, even in low temperatures, the on-site application time is reduced by around 40 percent, which ultimately significantly reduces the overall cost of the project,” Jeff says.