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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 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.

One of the issues we need to address to lower the heat load in the space is ventilation. We need to ventilate in order to give the combustion gases a point of escape, so they can replaced with cooler air from the building. Based on the roofs described above, firefighters may need to think outside the box. If you encounter a roof with and increased amount insulation, this may not pose too much of an issue. Most ventilation saws will easily cut through the insulation material and into the roof deck. Some roofs may have insulation embedded in concrete, which is more difficult, but nothing firefighters are not trained and equipped to handle, since poured-in-place concrete roof decks have existed for years. Where ventilation begins to become a little different is when dealing with the green roof with vegetation growing on it in soil.

A roof with a small amount of soil does not present great challenges. You can simply use a hand tool to clear the soil away in the areas where you want to use a ventilation saw and cut through the roof material. The full-depth green roofs will be more difficult because you literally have to dig a hole to get to the roof decking to gain access to the space. This will be labor intensive and time consuming, but not unachievable.

Vertical Ventilation

Earlier, when I discussed the use of day lighting in green buildings, I indicated that they might help us. In order to provide a significant amount of daylight in a space, most “green” buildings have a large number of skylights and a significant amount of glass. Thinking outside the box, firefighters could use the skylights for vertical ventilation. Additionally, the large amount of glass may be used for horizontal insulation.

One additional consideration with roofs in general is weight. While the roofs described above (and any roof with a load on it) have additional weight, this is not an issue under normal conditions because the structure should have been designed to accommodate these loads. The additional weight becomes an issue when the building structure becomes compromised due to fire. When the structure becomes compromised we need to remember that whatever the structure is holding up will be coming down with it, and in the case of green roofs that could be a significant amount of soil.

Green roofs can be broken into two types, roofs with added insulation, and roofs that are truly green and have vegetation growing on them. Both will lead to increased heat load in the fire room, and the latter will require quick thinking when attempting to ventilate. This article is intended to provide some background information on green roofs and current construction methods. It is important that firefighters find out what types of roofs they have in their jurisdictions and determine the safest and most efficient way to ventilate.

MATTHEW STIENE, a Contributing Editor, is the Senior Facility Manager for the Mecklenburg County Real Estate Services Department, in Charlotte NC. Matt possesses a Master of Engineering degree in Fire Protection Engineering from the University of Maryland, and a Bachelor of Science degree in Mechanical Engineering from Clarkson University. He has been in the volunteer fire service for 16 years in both New York and North Carolina, serving three years as chief and has held numerous leadership positions. Matt has worked in the facilities construction and management industry for over 10 years as a consulting engineer, project manager, and facility manager. He is a licensed professional engineer in North Carolina, and is a certified facility manager and certified facility management professional. Matt can be reached at