There are three types of felts. One is organic. Organic felts are made of paper. The other two types are fiberglass and polyester. Felts are usually saturated with bitu-men when they are made. Felts are installed in layers, called plies. The layers are held together with bitumen. The plies are mopped together in shingle-fashion. Shingle-fashion is when felts are over-lapped, just like shingles on a house Mopped means that hot asphalt is actually spread on the roof with a mop.

A roll of felt is laid in asphalt when the asphalt is hot. Then, the felt is rolled out (see Figure 23). A roofing worker follows the felt roll. He uses a broom or squeegee to press the felt so it is flat iM the asphalt. It is important not to walk on the felt while installing it.

Mechanical felt layers are often used. A felt layer spreads bitumen and rolls out the felt at the same time (see Figure 24). When the bitumen cools, the felt adheres.

Felt plies are normally 36 inches wide. Some rolls are made in metric sizes. Metric rolls are 1 meter wide. A meter 2-inch is the same as 39% inches.

Rolls have ply lines that are used to line them up (see Figure 25). The lines will let you know where to lay the next roll of felt. This system is set up to make sure that enough plies cover the whole roof. Also, if you follow the ply lines, they will overlap enough to make watertight seals.

A base sheet can be used as the first ply in the membrane. Base sheets are saturated and coated with bitumen when they are made. They are heavier than regular ply felts.

A BUR system may have a base sheet and three plies. Or it may have a four-ply memembrane without a base sheet. There are also three- and five- ply membranes.