Installation Exposure and Headlap

Exposure

The "exposure' of a slate is the portion not covered by the next course of slate above and is thus the length of the unit exposed to the weather. The standard lap of the alternate course used on sloping roofs is 3" and is the basis upon which all roofing slate is sold and the quantity computed. The proper exposure to use is then obtained by deducting 3" from the length of the slate used and dividing by two. For instance, the exposure for a 24" slate is 24" minus 3" = 21" ÷ 2 = 10 1/2" exposure.

Sloping roofs having a rise of 8" to 20" per foot of horizontal run should be laid with the 3" lap. Buildings located in the southernmost parts of the country or on the Pacific slope may however be safely roofed with a lap of 2" providing a high standard of workmanship is otherwise maintained. For steeper roofs, such as the Mansard and others nearly vertical in plane a 2" lap will usually be found sufficient. In some sections of the country it is customary to increase the lap to 4" when the slope is from 4" to 8" per foot, while in other parts the 3" lap is considered entirely adequate. Flat roof construction should be used for slopes less than 4" per foot. For vertical walls or siding use 2" lap.

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