Article and photos by Gregory Havel
Dead and live loads on roofs are naturally a concern to firefighters, whether the loads were part of the structural design or added later. We usually think of air conditioners and ventilators when we think of this type of load. Other common loads include rooftop gardens; solar energy collectors; and antennas for TV, data, and communications.
Photo 1 shows a communications tower that was designed and built as part of a building in an urban area. This type of installation, including the masts, is often located on top of high-rise buildings in urban areas, but is of less concern than other types of antenna, although the presence of these is still worth a note on a preincident plan for the building.
Photo 2 shows a common type of simple base for a satellite dish often used by professional installers and sold in do-it-yourself kits. The base consists of a simple steel frame weighing about 50 pounds (22.68 kg) that is designed to rest on the ridge of a roof. These are usually not anchored to the roof, but are ballasted with concrete blocks weighing about 40 pounds (18.14 kg) each. Larger antennas use more concrete blocks as ballast. The antenna mount and mast weigh an additional 50 pounds, with another 50 pounds for the antenna itself. The total weight of the installation in photo 2 is about 470 pounds (213.2 kg), and is supported by two or three roof trusses. In many parts of North America, this weight is equal to or greater than the live (snow) load that was calculated for that area of the roof. If the antenna was planned for that location, and if the roof is reinforced at that location, that is still a significant amount of weight above, and deserves a note on the preincident plan for this building.
Not all rooftop antennas are as easily visible as the one in photo 2. These antennas are often located near the center of the building so that they will not be visible from the ground. Photo 3 shows a 10-foot (3.048 m) diameter antenna that has been added to a building that is more than 100 years old. Because of the high parapet wall on three sides of the roof and the antenna’s location at the center of the roof, this installation is barely noticeable from the street, and is clearly visible only from the crosswalk at a nearby street intersection. This antenna uses a base similar to the one in photo 2, except that it is designed for use on a flat roof. It is larger because the antenna is larger, and it uses more concrete block for ballast.
Even though engineering calculations may show that by today’s standards the roof structure of this 100-year-old building is over-designed for its required live load, we must keep in mind the age of the structure, and the simple fact that most buildings deteriorate and weaken with age. This antenna on this roof will be significant to the firefighters on whom it falls during a fire, and deserves a note on the preincident plan.
Many buildings, including older buildings, do not have interior roof access ladders or stairways. Since the only access to the roof is by portable ladder, roofs are often omitted during fire inspections and building surveys.
An easy way to view the roofs in a neighborhood is to set up an aerial apparatus in the street in the middle of the block at a time of day with little traffic, and to take digital photos of the rooftops in all directions. These photos can be enlarged and analyzed back at the station, and notes made on preincident plans regarding concerns that are noted. In addition, this can be a valuable exercise for the aerial apparatus crew in preparing the apparatus for use, and in traffic control. If the exercise is repeated every couple of years, photos can be compared to show what has been added to rooftops.
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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor’s degree from St. Norbert College; has more than 30 years of experience in facilities management and building construction; and has presented classes at FDIC.