Due to many variables—such as plant type, condition, depth of the growing media, and 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, pick plantings 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. Class A-rated assemblies are limited to succulent-based systems. Experts recommend at least 60% of a roof’s vegetation 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, border zones free of vegetation can provide a fire break at rooftop equipment, penetrations, and structures. They also work to reduce the potential generation of wind-borne debris at roof perimeters and corners and provide additional resistance to high-wind pressures.

Clients should know that maintenance also plays a role in the roof’s fire-rating. Moisture levels of growing media should be checked regularly and the irrigation schedule adjusted to retain moisture and plants’ health. Weeding, fertilization, and removal of dead or dormant plants is just as important here as on the lawn, since ignoring excess biomass could inadvertently fuel a fire.