The indelible images of the scorched landscapes and structures following firestorms in California have led to some to reconsider the combustibility of green roofs. But clarity is emerging on that issue: Effective with the 2018 edition of the International Building Code, vegetative roof systems must meet the same fire classification requirements as the roof covering and roof assembly materials.

While many architects know best practices for these roofs, those with residential clients unused to the standards may want to refresh themselves and remind their clients of which green-roof planting schemes are most likely to meet the stricter fire safety requirements, compared with others.

Ways to Design a Fire-Resistant Green Roof System

Succulents are the best choice for designing fire-rated green roof.

Photo © Courtesy of Haworth Corp.

Due to many variables—such as plant type and condition, depth of the growing media, and the combustibility of the assembly materials—a classification of exterior fire exposure cannot be made with certainty just yet. However, choosing appropriate plants is key to safer roof design. For starters, some plantings are already evaluated for their fire-resistance, drought tolerance, and non-aggressive root systems which are less likely to penetrate and compromise underlying layers of waterproofing. These Class A-rated assemblies are limited to succulent-based systems. Experts recommend at least 60% of a roof’s vegetation should come from the sedum family of groundcover plantings. It is also best to avoid grasses and mosses, which can dry out and create a potential fire hazard.

Meanwhile, creating border zones that are free of vegetation can provide a fire break near and around rooftop equipment, penetrations, and structures. That also helps to reduce the potential generation of wind-borne debris at roof perimeters and corners and provide additional resistance to high-wind pressures.

When a project is complete, clients should know that maintenance also plays a role in the roof’s fire-rating. Moisture levels of growing media, for example, should be checked regularly. Irrigation schedules can also be adjusted to retain moisture and plants’ health. In the end, it’s an additional green space requiring weeding, fertilization, and removal of dead and dormant plants, since ignoring excess biomass could inadvertently fuel a fire. Architects and clients would do well to think of green roofs (and plan for them) as they would a second lawn.

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