Commercial Construction Considerations: Green Roofs

Over the last several years, the green-building movement has really taken off. For those of you who may be unfamiliar with “green” buildings, I am talking about buildings that are sustainable and environmentally conscious. The movement has touched...


Over the last several years, the green-building movement has really taken off. For those of you who may be unfamiliar with “green” buildings, I am talking about buildings that are sustainable and environmentally conscious. The movement has touched every aspect of commercial-building construction from heating, ventilating and air-conditioning systems, to lighting, roofing and even carpet and paint. Over the next few months, I am going to provide some insight on some of most popular green construction methods and the impact they may have on fire-suppression operations. This month, the focus is on one area of green construction that has a significant impact on firefighters – green roofs.

The Green Roof: What Is It?

What is a green roof? Well, the definition will be different depending on who you ask. In my opinion, you can break them down into two specific types: one that is not noticeable to the naked eye and one that is. The one that is not will look like a typical, low-slope commercial roof. What distinguishes it from a typical roof is the cap sheet material and the amount of insulation. There may be a significant amount of insulation (up to 12 inches in some locations). Building owners are investing in the additional insulation to lower utility costs.

The second type is what I would call a “green” roof; this is a roof that typically has vegetation growing on it (see Figures 1 and 2). This type of roof has most likely been encountered by many metropolitan fire departments in the past as many high-rise buildings have roofs such as this, and some that may even have a pool on the upper-most level. The difference with these green roofs is that you can find one on any typical commercial facility in your response area. Depending on the facility, there may be a waterproofing membrane, several layers of waterproof membrane, insulation, drainage mat and, anywhere from three to 10 inches of soil with significant vegetation growth (see Figures 3 and 4).

While a firefighter may see many negative aspects of these roofs, there is one advantage and that is the use of daylight to provide general lighting to the space. We will discuss how this impacts firefighters later.

Heat Build Up

While some of you may never encounter these types of roofs, vertical ventilation is still a very valuable tool when suppressing commercial fires. However, if not approached correctly, vertical ventilation of green roofs may become more difficult.

The main issue with the first type of roof described above is the increased insulation. This is valuable to building owners because the heat gain from the sun on a roof can be significant. The added insulation lowers the heat gain in the summer and the heat loss in the winter, but will also not allow the heat to escape as quickly in the event of a fire. This additional insulation will keep the heat in the fire space, making ventilation more important, but also more difficult. So, there are two issues we need to deal with, increased heat in the fire room, and how to ventilate these roofs.

The heat within the fire room is going to increase when there is more insulation. Whether it is from actual insulation (as described in the first type of roof) or soil (as in the second case), these conditions will lead to higher temperatures in the fire room. We all know what higher room temperatures mean, increased risk of flashover. While I am unaware of any studies that have been conducted to scientifically link increased insulation to increased heat, the basic laws of heat transfer tell us that with more insulation more heat will be held in the room. The heat loss from a space is directly related to the thickness of the insulation. The greater insulation value will lead to higher temperatures and increased risk of firefighter injury and flashover.

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