Unless otherwise stated, plies of felt must be laid in shingle fashion. Where a water flow direction exists, felts should be applied so that the direction of flow is not against the laps. Broom or press all plies of felt into the hot bitumen and lay without wrinkles, buckles or kinks, so that the finished roof is free of pockets, blisters or fishmouths. Each felt ply shall be completely covered with a continuous mopping of bitumen.

The number of plies are controlled by varying widths of the over laps. Four-ply (4) roofing, for example, is achieved by lapping each ply approximately twenty-seven and one-half (27-1/2) inches (699mm), which provides an eight and one-half 8-1/2) inch (216mm) exposure.

Koppers felts are marked at the factory with lap lines for this purpose. Except as specifically provided for in Koppers specification, application of felt lapping methods other than shingle fashion will not be permitted.

When beginning a membrane roofing installation, appropriate width starting strips of felt shall be used at the starting point to achieve the specification ply build-up at that point so that successive felt applications will produce the total ply requirements. Overlap ends of all connecting plies of felt (end laps) not less than 10 inches (254mm). End to end taping of these laps is not permitted.

At ridges, the felt shall be applied such that they extend up and over the ridge.

In the valleys of all non-nailable roofing systems, a minimum of one layer of Koppers felt shall be applied, just prior to the installation of the roofing system as valley reinforcement. Felt shall be embedded in a full and continuous layer of hot bitumen of the type specified for the job. Felt shall be a minimum of 36 inches (914mm) wide and extend up inclines at least 8 inches (203mm).

For nailable roofing systems, the valley reinforcement felt shall be embedded into a full and continuous layer of hot bitumen to the mechanically attached first layer of non-perforated base sheet prior to the installation of the remaining felts of the roofing specification. Felt shall be a minimum of 36 inches (941mm) wide and extend up inclines at least 8 inches (203mm).

For all specifications, backnailing is required in those areas where the slope is in excess of the Slope Guidelines.

When backnailing is required for felts applied parallel with the roof slope, fasteners shall be covered with a minimum of 2 plies of felt. For 4-ply systems, fasteners shall be located 18 inches (457mm) from the back edge (non-exposed side) of each ply. For 3-ply systems, fasteners shall be located 12 inches (305mm) from the back edge of each ply. Fasteners shall be a minimum of 7/8 inch (22mm) long, ring shanked and driven through 1 inch (25mm) minimum diameter tin disks or the equivalent for the substrate system. End laps shall always occur at a wood nailer. On non-nailable decks, fasten felts into wood nailers (Reference Wood Nailing Strips). For nailable decks with slopes 2" to 3" (16.7 t 25%), backnail at 8 foot (2.44m) centers and for slopes 3" to 6" (25 to 50%), backnail at 4 foot (1.22m) centers. Glass felts have a strong memory and will try to lay flat and straight.

It is important that during installation, the roofing applicator does not apply prolonged "point-loading" to a newly applied glass membrane such as standing, leaving equipment, mop carts, felt, rolls, etc., in place. Such action may cause bitumen displacement resulting in water leakage.