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Construction Concerns: Roofing Materials

Updated: May 22, 2023

Over the centuries, people have opted for roofing materials that are less easily ignitable. If it ignites, it is less likely to spread fire by flying brands. This is in response to conflagrations, defined as fires that are not easily extinguished and that spread from the building of fire origin to other buildings in the neighborhood. Sometimes, the change in preferred roofing materials was voluntary. Other times, it was the result of legislation or by royal decree.

Among the earliest known roofing materials are thatch; turf; and large, leafy branches. Photo 1 shows a residence in Jamaica in the late 1800s with a roof of palm branches. This type of construction was the norm in most parts of the world for thousands of years; it is still common in some parts of the world.

In “green” or sustainable construction, sod (turf) roofs are considered desirable since they absorb and use water for growth of plants rather than letting it run off. They also help cool the building by evaporation of the water in the soil and vegetation.

As villages and towns developed, these materials were replaced with less-ignitable materials like wood shingles and shakes. Photo 2 shows a roof of wood shingles on a modern home. You can purchase these wood shingles in their natural form and impregnated with a fire-retardant chemical, which is the only type permitted in many areas that are prone to wildland fires.

The effects of the fire-retardant chemicals in the wood are often reduced by the stain or paint with which the shingles are sprayed or coated to achieve the desired color. Although these are more difficult to ignite than thatch, palm leaves, or dry grass, they still produce embers that can be carried for a distance on air currents, spreading the fire from the building of origin to its neighbors. Wood shingles are still in common use in some parts of North America and other parts of the world.

During the times of the Roman empire and the medieval period, public buildings and other large buildings with pitched roofs were built using roofs of tile or terra-cotta, a manufactured, fired clay product; and slate, layers of sedimentary rock that are split to thickness and attached to the roof sheathing, usually pitched. Photo 3 shows a modern building with a roof of terra-cotta tiles. Photo 4 shows a modern building that will have a roof of slate. Both of these roofing materials are still in use today despite their high cost compared to other types of roofing materials, because of their long life when properly installed, and their resistance to ignition by flying brands.

In the late 19th century, the “built-up roof” of overlapping layers of tarred felt mopped with hot asphalt or coal-tar pitch was developed. This type of roof was designed for use on large buildings with flat roofs and was usually covered with a layer of pea gravel, which served multiple purposes. The pea gravel was a lighter color than the asphalt or pitch and reflected heat and ultraviolet light away from the roof, keeping the building cooler and extending the life of the roof. The pea gravel also added weight to the roof, reducing the possibility of wind damage. The noncombustible gravel also separated flying brands from the combustible roofing felts and asphalt or pitch, reducing the possibility of conflagration. Roofs of this type were also installed without the gravel ballast, but were also less common. The built-up roof is still in use in the 21st century, although it is rapidly being replaced by other types of roofing materials.

“Roll roofing” was developed in the early 20th century. This was a heavier felt, impregnated with asphalt, coated with colored sand or granules of crushed stone, and nailed to the roof deck with seams sealed with roofing cement. This was less expensive than a built-up roof with a gravel surface, but it provided similar protection against ignition from flying brands.

A short step beyond roll roofing was the development of the “three-tab shingle” made of materials similar to roll roofing but which could be cut and fitted to a building more easily. These shingles are attached with nails or staples to the roof deck in overlapping rows, starting at the eaves and working up toward the ridge. Photo 5 shows a single-family residence with a steeply-pitched roof of three-tab shingles.

A step beyond the three-tab shingle is the “dimensional shingle” of asphalt-impregnated fiberglass felt coated with a surface of colored sand or granules of crushed stone, shown in photo 6. These shingles are installed in much the same way as the three-tab shingle; they have similar characteristics, except for a longer life expectancy since the fiberglass felt does not rot from moisture like the natural fiber felt used in the three-tab shingle.

During the latter part of the 20th century, the rubber membrane roof became common. Photo 7 shows a rubber membrane roof being installed on a noncombustible (Type II) building.

The foreground shows the painted steel roof deck that is attached to steel bar joists. Beyond this, two layers of fire-retardant-treated polystyrene insulating board are being installed with the synthetic rubber roof membrane on top. Depending on the roofing material manufacturer and specification, the seams in a roof like this can be made with adhesives, solvent welding, or heat welding. This roof membrane is attached to the building only at the edges and flashings and is held in place by large gravel ballast. This method of construction uses the gravel ballast for ultraviolet resistance, resistance against wind damage, and for resistance to ignition from flying brands. This type of roof can also be installed as a fully-adhered system with the insulating board glued to the roof deck and the roof membrane glued to the insulating board with no gravel ballast needed.

Metal roofs of aluminum or steel panels with baked-on enamel finishes were also developed in the late 20th century. These can be installed as pitched roofs on new construction and as replacement roofs on older buildings. Photo 8 shows a roof of steel panels that is attached to the roof deck with exposed screws.

Photo 9 shows a roof of steel panels with concealed fasteners that are interlocked at the standing seam and screwed to the roof deck at the opposite edge.

In addition to these samples, these metal roofs are available as interlocking panels that imitate the appearance of terra-cotta tile, slate, wood shingles, and other patterns. These metal panels are noncombustible by nature, use paint that is resistant to ultraviolet fading, and use enough fasteners so that they will not detach easily from the building.

The most recent development in roofing materials is the system of interlocking plastic roof panels with concealed fasteners designed for use on pitched roofs (photo 10). These imitate the appearance of any of the other types of roofing systems including slate, terra-cotta tile, and wood shingles but are combustible although the plastic from which they are made includes a fire-retardant chemical.

During previous generations and centuries, it was easy to tell the type of roof on a building simply by its appearance. In our 21st century, we can no longer be sure of the building’s roof type simply by its appearance. Following are some considerations:

  • From the ground, a roof that appears to be of terra cotta tile or slate could be the real thing, complete with its great weight, noncombustible features, and natural resistance to ignition from flying brands.

  • From the ground, a roof that appears to be of terra cotta tile or slate could be interlocking metal panels, with reduced weight, noncombustible features, and natural resistance to ignition from flying brands.

  • From the ground, a roof that appears to be of terra cotta tile or slate could be interlocking plastic panels, with reduced weight, combustible features, and less resistance to ignition from flying brands that, once ignited, will burn readily with clouds of dense black smoke.

The only way to tell these materials apart is from inspections before the fire incident and from the results of these inspections being included in preincident plans for the building.

Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 35-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 35 years of experience in facilities management and building construction; and has presented classes at FDIC.

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