A roof membrane makes a roof system watertight. It is installed over the insulation (see picture). In some systems, a roof membrane may be installed right over a deck.

  Many.types of membranes will be described in this class. You will learn later that membrane types are very different. For example, one of the more common roof membranes is a built-up roof (BUR). BUR is a combination of parts. They are put together at a job site to form a complete roof membrane. Another common roof membrane is a single-ply sheet. It is manufactured as one sheet. It is also installed as one sheet. It can be installed in several different ways.

Built-Up Roofs A built-up roof (BUR) system is composed of three materials. The three materials are bitumen, felts and surfacing. It is called a built-up roof because it is made (or built up) at a job site.

Bitumen

The first part of a BUR membrane is bitu-men. Bitumen is what makes a membrane watertight. There are two kinds of bitumen. One kind is asphalt and the other coal tar. Coal tar is also called pitch.

Asphalt Asphalt is a petroleum product. It is refined from crude oil. There are four types of asphalt (see picture). Each type of asphalt softens at a different temperature. What this means is that each type of asphalt will soften into a thick liquid at a different temperature.

Type I (one) has the lowest softening point. Type IV (four) has the highest softening point and is called special steep asphalt. The most common type for roof systems is Type III (three). It is called steep asphalt.

The higher the softening point, the steeper the slope on which it can be used. Steep asphalts have to be used on steep-slope roofs because they will not get soft and slide when they get too hot.

Asphalt types with the lowest softening points are called dead-level asphalt. They can only be used on surfaces without much slope.

Coal Tar

The second type of bitumen is called coal tar. As you might think, it is made from coal. It is only used on roofs with very low slopes. Its softening point is even lower than Type I asphalt. There are two types of coal tar used in roofing. They are Types I and III. Type III is called coal tar bitumen. Both work well, but Type III does not give off as much coal tar fume as Type 1.

Both kinds of bitumen are heated in kettles or tankers. Kettles are trailers that get towed by trucks. Tankers are trucks that can be driven to a job site. Both are large heating tanks for melting bitumen. The material is usually pumped up to a roof through a sup-ply line. It is important that bitumen is kept at the right temperature in the kettle. If it is overheated, its quality will not be as good. Overheated bitumen can also catch fire. But if it is not hot enough, it will not flow well when it is applied. If it does not flow well, it will not adhere as it should. If that happens, a roof system will not last very long.

A BUR system is made up of layers of felts and bitumen. When the layers are installed correctly, they will create a strong, watertight membrane.