Canadian Consulting Engineer

No Fuel for the Fire

It is strange to hear Keith Doucette discuss the roofing materials that can be found over his office. He is the chief fire prevention officer in Ajax, Ontario, after all, and he is based in the headqu...

August 1, 2010   By John G. Smith

It is strange to hear Keith Doucette discuss the roofing materials that can be found over his office. He is the chief fire prevention officer in Ajax, Ontario, after all, and he is based in the headquarters of the town’s fire department.

The roof over Doucette’s office is a green or vegetative roof, and it is not fire-rated. This situation is simply a reality for any structure that includes a green roofing system. Building codes may reference fire-resisting standards for everything from shingles to membranes, but they are largely silent when it comes to roofing products that actually grow. Even Toronto’s Green Roof bylaw, which now mandates the systems on buildings with a minimum gross floor area of 2,000 square metres, limits its fire safety references to the space that needs to be maintained around structures, such as referring to parapets or mechanical equipment clad in combustible materials.

The burn tests used to rate shingles and membranes simply would not apply to their organic counterparts because there are too many variables at play, explains Steven Peck, president of Green Roofs for Healthy Cities. But recognizing what he calls “overblown” questions about a potential risk — and a call by the National Roofing Contractors Association for a standard that did not exist — his trade association joined with SPRI (Single Ply Roofing Industry) to create VF-1, Fire Design Standard for Vegetative Roofs. It was accepted this February by the American National Standards Institute.

The Ajax fire hall was built before the standards existed, but Doucette is able to point out a number of the recommended features that are already common to most installations. The growing medium is set back from an adjacent wall and rimmed with stone; two elevations would keep any potential flames from spreading; and the roof deck underneath the plants does carry an appropriate fire rating. The plants themselves would hardly support a sustained fire even if they did ignite, he adds. “The BTUs you would get from burning that type of material would be relatively low.”

Peck stresses that it is not much of an issue. Popular plant choices such as sedums are better described as “moisture holding devices” that would shrivel up before they burn. “There’s just not a lot of biomass,” he says. The same is true when a roof is covered with grass.

“Data exists that supports the classification of succulent based systems as Class A fire resistance,” the new standards add. “Other systems may be tested for fire resistance as installed, but the vegetation needs to be maintained in order to continue to sustain fire resistance.” The exception may be growing media that is loaded with organic material like peat moss, but there is no available data to rate the different blends.

Hitesh Doshi, P. Eng., professor of architectural science at Ryerson University, agrees that the vegetation would present a limited fire hazard. “A lot of people have made a bigger deal out of it than it is,” he says. “When I think about a green roof, I’m thinking about something that needs little maintenance.”

Green roof designs can actually play a role in limiting the threat of a fire. For example, the design of the Ajax fire hall’s roof includes a layer of plastic cups that retain water to help keep the plants alive. The cups are fed by a trio of 10,000-gallon cisterns which capture rainwater.

Granted, such a passive system cannot be seen as an active fire suppression system. “You don’t want to leave fire safety to chance. You want to make it part of the design,” notes Doshi. He says it is unlikely that a sprinkler system similar to the one used in the interior for suppression could be engineered for the task. “Typically in an enclosed space, heat and smoke are sensed to trigger the suppression system.” But for an open roof, he’s not sure how such technology could be used in a practical way.

Demir Delen, P. Eng., director of fire protection engineering at Morrison Hershfield, suggests that the greatest fire-related risk would emerge when the green roofs contact an adjacent wall. “It’s not just [about] the top of a high-rise building. There are numerous buildings where there is a lower roof and the building continues on the other side, so there is exposure,” he says, referring to a potential issue that consulting engineers should consider. “It may introduce some other types of fire loads in other parts of the building.”

Perhaps that is why the new green roof standards focus so much attention on the potential spread of any flames. For example, recommended fire stops include walls that extend a minimum of 36 inches above the roof surface, or a 6-foot wide border in the form of a Class A roofing system. Large areas of green roof need to be divided into sections that cover no more than 1,450 square metres (15,608 sq. ft.), although wind standards will often dictate the size of any perimeter or border zones.

To Doucette, the biggest fire risk of all emerges when these green spaces are converted into rooftop patios complete with the smokers who can be careless with cigarette butts. “Ignition sources come from people,” he says. Then someone can add combus- tible materials such as deck planks or lawn furniture scattered throughout the space, but building codes wouldn’t address these risks either.

Consulting engineers could offer some added protection in these cases, and not simply by viewing the rooftop as another flooring assembly. The standards require access to a fire hydrant, but an actual standpipe next to the rooftop’s exit would be helpful, says Doucette, referring to features that he would like to see.

Building codes may not be silent on the issue forever. Delen suggests that for those who deal with formulating the codes, the whole sustain-ability issue is very much at the forefront, and says “I’m sure in the near future green roofs will be one of those important political issues.” There is a path into the building code now that Part 12 is addressing energy efficiency and sustainable design, he says.

John G. Smith owns WordSmith Media in Ajax, Ont.. Copies of the standard can be downloaded from http://greenroofs.org/resources/EFDSVR.pdf.

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“Popular plant choices such as sedums are better described as ‘moisture holding devices’ that would shrivel up before they burn.” — Peck


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