“Extensive green roofs increase the fire hazard of our buildings and a green roof is a potential risk under windy conditions."
Are these statements true?
How safe are green roofs?
Jörg Breuning….let’s flip that roof.
How safe are green roofs?
While in the Nineties in Germany green roofs spread faster than every bush fire the natural human fear of nature caused by green roofs grew as well. As much as fire helped us to step ahead in the evolution as much we are afraid of it and so it didn’t take long until the first fire experts spoke out loud their concerns. They pictured scenarios like the big fire of London in 1666 and fueled a hot discussion. The discussion merged down to the following questions: Can one spark ignite an extensive green roof? Can extensive green roofs provide enough fuel to sustain or even propagate fires?
Since my hometown Stuttgart is the leading green roof city in Germany it was no wonder that in 1988 all experts including city, county and state officials agreed to a large fire research program done by the Research and Material testing laboratory of Baden-Württemberg (Government).
Before they started with large scale field testing, they did the math or basically they made some burning tests in the laboratory first. It wasn’t really surprising that a bituminous waterproofing layer produces 50 kWh/m² which is 15 times more than an extensive green roof with dry grasses (3 kWh/m²). That means you can heat up your house much better with a bituminous layer as with a grass/green roofs. They also tested PS-insulation and ply wood – we know how this stuff burns. With this knowledge and knowing that it takes much less to set fire on dry plants (grasses) than on a piece of waterproofing the field research begun. During the following months (August until October) they actually tried to start fires on green roofs with a defined setup (with dry wooden wool) which was installed on the roofs and ignited.
They expected a glowing of the organic content in the growing media and they also thought that the dry plants will spread the fire immediately all over the roof.
Just keep in mind, at this time the organic content of the growing media was typically higher than the values in the actual FLL guidelines and so you also found higher plant varieties with consequently a higher amount of dead and dry plants.
After all this experiments and research just a short answer. It is nearly impossible to set an extensive green roof on fire which spreads over the roof or starts a glowing/burning of the growing media. The risk of fueling fires is 15-20 times higher on bare roofs with fully adhered bituminous waterproofing membranes as on extensive green roofs with grasses, perennials and Sedums. I guess in 1988 I installed approximately 2,000,000 sf of extensive green roofs - all on schools and kindergartens in the south of Stuttgart. Does that say something to you?
Today in Germany are at least 2 billion square foot of extensive green roofs built and there is no fire recorded related to an green roof. This research and further studies showed certain risks (i.e. sparks caused by burning grasses which can spread over the green roof by wind) which are minimized by clear definitions of green roof details and roofing details for green roof. There are considerations in building codes, German and European standards (DIN/EN). The FLL guideline always represents the latest State-Of-The-Art in addition to the above mentioned standards.
In Germany you typically get a 10-20% discount on the fire insurance when a seamless extensive green roof is installed.
In most cases the fire insurances offer a package which also covers damages caused by wind and storms. An insurance company wouldn’t give a discount if there is a higher risk in one of each; however that is only one aspect you can look at wind load topic. To backup my experience of nearly 30 years I contacted some of my friends in Germany. Since decades Dr. Gunter Mann and Dipl.Ing.Martin Henneberg are working for Optigreen and they did uncountable studies together with all major Universities in Germany and published exciting articles about green roofs as well. They maintain a close information network to more than 100 installing companies and partners. It is the biggest information resource I know and which offers a feedback of at least 20 million square foot new green roof construction every year. Speaking about this resource Dr. Gunter Mann was interviewing close to 80 installers about wind loads on green roofs. All interviewed installers look back on more than 15 years of experience and millions of square foot green roofs installed. Based on these experiences and their reports they confirmed my experience that wind erosion can happen on each building regardless the height of the roof or the height of the parapet. Most erosion on roofs below 60 foot is hardly recognized since it typically starts during the establishing phase of the plants (new installations) and the plants usually cover these areas soon. Large organic parts (wood chips etc.) and very lightweight aggregates ( less than 600g/ltr.) are blown away fairly easy and found all the time everywhere on the roof where they shouldn’t be. Most of the (hardly to see) wind damage on building below 60 ft is also caused by unique aerodynamics of the buildings itself. All installers agreed that even the smallest wind damage has to be fixed immediately with appropriate solutions to prevent further damage. It shows once more that green roof maintenance requires very high skills, well trained senses and the understanding of working with nature – not against it.
Regarding wind loads architects and installers put more emphasis on buildings over 60 ft. height and so surprisingly there are less minor problems recorded. A few big issues are well known and I like to share one true story with you:
Together with four landscape companies we installed in 1997 a 560,000 square foot extensive green roof on an over 60 foot high building. The building was also in an open field on top of a hill. I guess it was Storm “Lothar” just around Christmas with wind speeds up to 140 miles and wind gusts up to 180 miles. During this storm some large areas of the famous Black Forest where gone and 0.8% of the green roof on this big box, too (4000 sf). Sounds like a disaster? Yes and No. Yes because we knew about the exposed location of the building and that a green roof is vulnerable when plants are in the establishing phase. No, because the green roof was designed according the existing standard and the FLL guideline to withstand very high wind loads however all solutions (incl. concrete pavers etc) weren’t enough to keep things in place. It wasn’t a disaster either since the overall design was responsible that all components of the green roof remained on the roof on the roof and nothing felt down or was blown off the roof. Actually the insurance company paid for fixing the green roof since they were also happy that the force of the storm went into the green roof and not into the waterproofing or the lightweight steel deck. At the same time other wind damages on green roofs where recorded however all of them where minor (less the 1% of the area), nobody was hurt by falling Sedums but by falling trees, flying roof tiles and plywood sheets. I probably know the feedback of this article – you can’t compare the US with Germany. Regarding fire and wind load – yes you can, since with the impact of both mentioned elements a green roof works like a crash zone of a building and a green roof brings you up to a five stars crash rating – at least in the German insurance contracts. If I didn’t say it earlier, this article reflects my personal experience and nearly 30 years study of nature on green roofs. Instead of teaching being afraid of nature I help to understand, to work with nature and let nature work for me without stepping back in the Stone Age. The nature laws in physics, chemistry and biology are very complex however on this planet they are everywhere the same. I like piercing but I don’t like a nail in my belly button to hold my pants.
Since my hometown Stuttgart is the leading green roof city in Germany it was no wonder that in 1988 all experts including city, county and state officials agreed to a large fire research program done by the Research and Material testing laboratory of Baden-Württemberg (Government).
Before they started with large scale field testing, they did the math or basically they made some burning tests in the laboratory first. It wasn’t really surprising that a bituminous waterproofing layer produces 50 kWh/m² which is 15 times more than an extensive green roof with dry grasses (3 kWh/m²). That means you can heat up your house much better with a bituminous layer as with a grass/green roofs. They also tested PS-insulation and ply wood – we know how this stuff burns. With this knowledge and knowing that it takes much less to set fire on dry plants (grasses) than on a piece of waterproofing the field research begun. During the following months (August until October) they actually tried to start fires on green roofs with a defined setup (with dry wooden wool) which was installed on the roofs and ignited.
They expected a glowing of the organic content in the growing media and they also thought that the dry plants will spread the fire immediately all over the roof.
Just keep in mind, at this time the organic content of the growing media was typically higher than the values in the actual FLL guidelines and so you also found higher plant varieties with consequently a higher amount of dead and dry plants.
After all this experiments and research just a short answer. It is nearly impossible to set an extensive green roof on fire which spreads over the roof or starts a glowing/burning of the growing media. The risk of fueling fires is 15-20 times higher on bare roofs with fully adhered bituminous waterproofing membranes as on extensive green roofs with grasses, perennials and Sedums. I guess in 1988 I installed approximately 2,000,000 sf of extensive green roofs - all on schools and kindergartens in the south of Stuttgart. Does that say something to you?
Today in Germany are at least 2 billion square foot of extensive green roofs built and there is no fire recorded related to an green roof. This research and further studies showed certain risks (i.e. sparks caused by burning grasses which can spread over the green roof by wind) which are minimized by clear definitions of green roof details and roofing details for green roof. There are considerations in building codes, German and European standards (DIN/EN). The FLL guideline always represents the latest State-Of-The-Art in addition to the above mentioned standards.
In Germany you typically get a 10-20% discount on the fire insurance when a seamless extensive green roof is installed.
In most cases the fire insurances offer a package which also covers damages caused by wind and storms. An insurance company wouldn’t give a discount if there is a higher risk in one of each; however that is only one aspect you can look at wind load topic. To backup my experience of nearly 30 years I contacted some of my friends in Germany. Since decades Dr. Gunter Mann and Dipl.Ing.Martin Henneberg are working for Optigreen and they did uncountable studies together with all major Universities in Germany and published exciting articles about green roofs as well. They maintain a close information network to more than 100 installing companies and partners. It is the biggest information resource I know and which offers a feedback of at least 20 million square foot new green roof construction every year. Speaking about this resource Dr. Gunter Mann was interviewing close to 80 installers about wind loads on green roofs. All interviewed installers look back on more than 15 years of experience and millions of square foot green roofs installed. Based on these experiences and their reports they confirmed my experience that wind erosion can happen on each building regardless the height of the roof or the height of the parapet. Most erosion on roofs below 60 foot is hardly recognized since it typically starts during the establishing phase of the plants (new installations) and the plants usually cover these areas soon. Large organic parts (wood chips etc.) and very lightweight aggregates ( less than 600g/ltr.) are blown away fairly easy and found all the time everywhere on the roof where they shouldn’t be. Most of the (hardly to see) wind damage on building below 60 ft is also caused by unique aerodynamics of the buildings itself. All installers agreed that even the smallest wind damage has to be fixed immediately with appropriate solutions to prevent further damage. It shows once more that green roof maintenance requires very high skills, well trained senses and the understanding of working with nature – not against it.
Regarding wind loads architects and installers put more emphasis on buildings over 60 ft. height and so surprisingly there are less minor problems recorded. A few big issues are well known and I like to share one true story with you:
Together with four landscape companies we installed in 1997 a 560,000 square foot extensive green roof on an over 60 foot high building. The building was also in an open field on top of a hill. I guess it was Storm “Lothar” just around Christmas with wind speeds up to 140 miles and wind gusts up to 180 miles. During this storm some large areas of the famous Black Forest where gone and 0.8% of the green roof on this big box, too (4000 sf). Sounds like a disaster? Yes and No. Yes because we knew about the exposed location of the building and that a green roof is vulnerable when plants are in the establishing phase. No, because the green roof was designed according the existing standard and the FLL guideline to withstand very high wind loads however all solutions (incl. concrete pavers etc) weren’t enough to keep things in place. It wasn’t a disaster either since the overall design was responsible that all components of the green roof remained on the roof on the roof and nothing felt down or was blown off the roof. Actually the insurance company paid for fixing the green roof since they were also happy that the force of the storm went into the green roof and not into the waterproofing or the lightweight steel deck. At the same time other wind damages on green roofs where recorded however all of them where minor (less the 1% of the area), nobody was hurt by falling Sedums but by falling trees, flying roof tiles and plywood sheets. I probably know the feedback of this article – you can’t compare the US with Germany. Regarding fire and wind load – yes you can, since with the impact of both mentioned elements a green roof works like a crash zone of a building and a green roof brings you up to a five stars crash rating – at least in the German insurance contracts. If I didn’t say it earlier, this article reflects my personal experience and nearly 30 years study of nature on green roofs. Instead of teaching being afraid of nature I help to understand, to work with nature and let nature work for me without stepping back in the Stone Age. The nature laws in physics, chemistry and biology are very complex however on this planet they are everywhere the same. I like piercing but I don’t like a nail in my belly button to hold my pants.
Jörg Breuning….let’s flip that roof.
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