A common way to install roof insulation board is to put it in several layers. The joints of the different layers of insulation board should be staggered (see Figure 19). This is true for the joints on all four sides of the board.

When roof board insulation is attached to nailable roof decks, it is usually done with screws or other mechanical fasteners (see Figure 20).

Tapered Insulation

Tapered insulation boards have already been mentioned as one way to create slope in a roof system. Rigid boards are tapered at a factory. The tapered boards are then used to put together a sloped insu-lation system. Tapered systems are made of many layers of roof insulation boards. In this system, flat boards are used for fill

Tapered boards are available in many sizes. Usually they are 4 feet (1.2 m) wide and have lengths of 2 feet (0.6 m), 4 feet (1.2 m) or 8 feet (2.5 m). The most common tapered slopes are 1/8 inch per foot, 1/4 inch per foot and 1/2 inch per foot.

Tapered roof board insulation is made of several materials. It is made of cellular glass, perlite, polyioscyanurate foam or polystyrene.

A complete roof layout must be planned before installation begins with tapered boards. This includes deciding where valleys and crickets will be to create drainage. Sometimes, there have to be many differ-ent patterns.