July 29, 2009

Letter to the Editor

Green roofs are more impervious to fire

Re: Fire safety issue raised for green roofs, JOC July 20, 2009

There are a couple of issues I would like to address having been immersed in the roof garden industry for some time now.

In Germany, there are instances where fire (and even wind) insurance is discounted for buildings which have vegetated rooftops.

In the case of fire, a roof garden is inherently more impervious to fire damage to the building because the growth media, when blended to FLL standards, is predominantly a mix of inorganic calcined materials.

In most regions of the U.S. where we sell Roof Gardens, the base of the mix is expanded clay.

This is a substance very much like common brick — raw clay is baked at 800C until it expands, leaving a lightweight product with a large percentage of small air pockets.

Not to mention its stability at high temperatures, all of these internal voids make for relatively slow thermal transfer in the case of fire at the surface.

During a recent presentation to our international group, I conducted a demonstration outdoors and during the 20-minute seminar, I aimed a 10,000 BTU propane torch downward on a small (less than one-square metre) vegetated roof garden assembly for the duration of the event.

The small Roof Garden deck was outfitted with temperature sensors at the membrane level and insulation level. After 20 minutes, the membrane temperature had only risen three to four degrees C.

While the vegetation (sedum in this case) was heavily scorched, the membrane was hardly warm to the touch at the end of the demonstration and was in fine condition.

I have heard of one case of an actual roof garden fire that occurred in Washington State. Someone threw a lit cigarette off a balcony onto the roof garden below which was vegetated with tall and dry native grasses.

Within a few minutes a fire broke out and began to envelope the roof. Once the fire department arrived, it had all but burned itself out. No damage to the roof occurred and remediation simply involved replanting.

Even in the worst-case scenario of dry grass, damage is inherently limited since the grass does not have a large amount of energy, even with a high surface to air ratio which encourages fast burn times.

Most roofs are sedum, which are far less prone to burning at a high rate since they always tend to have a high percentage of water stored away (sedums are succulents).

Zach Williams

CCM Roof Garden Manager

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