On a low slope roof, the uppermost layer is the roof membrane. It is this layer that functions as the waterproofing layer and is most directly subjected to the weathering elements. The most commonly used roof membranes are as follows:
- Built-up roof membrane
- Modified bitumen roof membrane
- Single-ply roof membrane
A built-up roof membrane consists of several individual layers laminated into one membrane. A single ply membrane consists of a single layer. A modified bitumen membrane is between a built-up and a single ply membrane and usually consists of 2 to 3 layers. A built-up roof is generally the most labor-intensive roof, followed by the modified bitumen roof and the single ply roof.
Anatomy of a built-up roof membrane
A built-up roof membrane consists of several layers of roofing felt adhered together by bitumen (asphalt or coal tar). In a typical built-up roof, they felt is laid over a mopping up pitching, followed by a second mopping of bitumen, and then the second felt, and so on. Thus, a number of felt layers (called plies), separated by inter-ply moppings of bitumen, are necessary to build a built-up roof membrane-hence, the name “build up” roof.
A built-up roof normally consists of 3 to 5 plies. The greater the number of plies, the thicker and, hence, stronger and more durable the membrane. The last felt is typically covered with a surfacing material to protect the membrane from the effects of weather and external fire.
The most common surfacing material is gravel laid over a floor coat of bitumen. Because the quantity of bitumen for the flood code is much greater than that required for inter-ply mopping, it is poured over the roof, while the inter-ply bitumen is simply mopped on. The following images show some of the tasks involved in the process.
In the alternate layers of felt and bitumen in a built-up roof, the bitumen is the waterproofing material. However, bitumen alone cannot be used, because it is a thermoplastic material. It becomes soft at high temperatures and begins to flow. At low temperatures, it becomes hard and brittle, and cracks. Thus, bitumen does not have the requisite tensile strength to withstand stresses imposed by the changes in temperature, deck movement, foot traffic, hail storm, etc.
The felts work as reinforcing material, giving the required tensile strength to a built-up roof membrane. Thus, the function of felts in a built-up roof membrane is similar to that of steel bars in a reinforced concrete member.
The felt also stabilizes the bitumen against flow, since the interwoven felt fibers form mini-receptacles within which the bitumen is held, preventing its flow. A heavy mopping of bitumen without the felt simply cracks in cold weather due to the lack of tensile strength and flows like a thick paste during hot weather, due to the lack of containment. In other words, the felt allow for a more significant buildup of bitumen, which increases the weatherproofing and waterproofing of the membrane.
A felt consists of fibers (or strands) pressed into a sheet. It may be either an organic felt or an inorganic felt, depending on whether the fibers are of organic or inorganic type. The organic felts are made from paper or wood fibers, or a combination of both. In organic felts are made from glass fibers. Asbestos fibers were once used but have been discontinued due to health concerns.
The manufacturing process for both organic and fiberglass felts is similar to papermaking, in that a mass of fibers are pressed under rollers to give a thin, flat sheet. In fact, an untreated organic felt (without bitumen treatment) is virtually indistinguishable from a thick handmade paper. An untreated fiberglass felt looks like a woven mat. The weave in a fiberglass felt is so sparse that light and air can easily pass through it. By comparison, the organic felt is static, nonporous, and opaque to light.
After the rolling process, the felts are treated with bitumen. Since both coal tar and asphalt are black in color, a treated felt is black in color. Treated felt is used in a built-up roof. Therefore, the term “felt” in our discussion generally implies a bitumen-treated felt. The treatment consists of simply covering the felt with bitumen.
There is a fine distinction between how the bitumen is held in a treated organic or fiberglass felt. An organic felt soaks the bitumen like a blotting paper. Therefore, an organic felt is considered to be saturated with bitumen. A fiberglass felt, on the other hand, is impregnated with bitumen.
The impregnation of bitumen in a fiberglass felt makes it more opaque, but despite the treatment, a fiberglass felt is highly porous. The treated organic felt is nonporous and, hence, water resistant. Only an organic felt is specified as an underlayment for roof shingles and tiles or as a water resistant layer over wall sheathing. Because of its porosity, a fiberglass felt cannot be used in such applications. However, as we shall see later, the porosity of fiberglass felts is an advantage in built-up roof applications.