An interesting question posed to me recently by a contractor: do green roofs pose a legitimate fire hazard?

Every summer we hear about brushfires in the southwest propagated by dried vegetation resulting from perpetual drought conditions. The origins of our country’s building codes lie in the great fires that destroyed various major cities during the 1800’s. Chicago – the poster child of urban green roofs in the U.S. – nearly burnt to the ground in 1871. Could it be that green roofs, with all of their energy and environmental benefits, actually undermine concerns for health, safety, and welfare?

Maybe. It depends heavily on your growing mix.

About a year ago, Peter Kenter, a correspondent for Daily Commercial News and Construction Record, published an article highlighting the fire safety concerns with regard to green roofs raised in Toronto in wake of the city’s approved Green Roof By-Law.

Kees Govers, General Manager of Caradoc Green Roofs in Strathroy, Ontario, promptly offered a retort – which DCN also carried. Within the letter, Gover offers the following:

There are those who want to impose minimum watering requirements to solve the [fire safety] issue. That could prove impractical and difficult to police. Instead minimum standards need to be set for the type and quality of growing media and the plant selection and mixtures on green roofs as well as the implementation of fire breaks on roofs that don’t meet these standards.

Gover goes on to state that there is currently no testing protocol in place for green roof fire testing in either the UL or ULC listing. However, there is standardized growing mix offered by the independent non-profit German Landscape Research, Development and Construction Society or FLL. The FLL mixes are designed so that when bone dry, they do not contain enough vegetated matter (6-10% organic matter by dry weight) to spread a fire. Moreover, FLL mixes have been observed in Europe for over 30 years – proving that the growing media has longevity.

Do green roofs create a fire hazard during drought conditions?

Most grasses will propagate flame spread and may present a legitimate fire safety concern under perpetual drought conditions. However, there is plenty of reputable research verifying that sedums (or stonecrops) and other succulents are naturally flame resistant and won’t contribute to the spread of fire any more than a conventional low-sloped roof assembly. The City of Portland (Oregon) has recognized this point, as has the Division of Research and Advanced Studies at the University of Cincinnati, among others. In fact, some homeowners have cited the fire-retardant virtues of succulents in surviving wildfires.

This past January, after three years of development by members of the Single Ply Roofing Industry (SPRI) in cooperation with Green Roofs for Healthy Cities, a consensus-based standard for the design and construction of green roofs, ANSI/SPRI VF-1, was approved by the American National Standards Institute (ANSI). VF-1 serves as a green roof design guide for minimizing the fire risk of such systems. Within the standard, ANSI acknowledges that “data exists that supports the Classification of succulent based systems as Class A fire resistance.”

Make no mistake, ANY PLANT WILL BURN IF IT IS DRY. On the other hand, soil does not burn. The issue debated is whether or not green roofs present a unique fire safety risk due to propagating flame spread – especially when dry.

To summarize, if you have to mow it and water it, it will likely propagate flame spread when dry. Sedums and succulents are naturally fire-retardant and do not present a unique risk to propagating flame spread relative to a conventional low-sloped roof.