Application Data Exposure of slate � The orthodox method of application provides for a double head lap of 3", the exposure being determined by deducting the lap from the length of the slate and dividing the difference by 2. Thus slate 18" in length should show an exposure of 7 1/2". Rising & Nelson Slate Co., LLC generally recommends a 3" headlap should be used. On some very steep pitch roofs, a 2" headlap may be adequate except in the most rigorous climates. A slate roof should have a minimum pitch of 4" to 12". Workmanship � The slate, as provided by Rising & Nelson Slate Co. should be properly applied by competent and reliable workmen. The most important detail is the maintenance of the specified head lap throughout. Nails for Use with Slate Slate nails may be copper weld, copper or stainless. However, numerous slate roofs applied with ordinary wire slate nails many years ago, have given a long period of service and are still intact. Freedom from moisture keeps the nails dry and rust free. For best work copper, stainless or aluminum alloy slate nails are recommended. Sizes of nails � 1 1/2" nails are suitable for ordinary condition for commercial standard slate on main roof and 2" nails for hips and ridges. Thicker slates require longer nails (add 1" to twice the thickness of the slate). Nail holes are machine punched in accordance with Federal Specifications. Sizes of Slate A �square� is the standard measurement and is the number of pieces of and size required to cover one hundred square feet of roof surface, at 3" head lap, not including waste in cutting for hips, valleys, dormer, etc. Where a 2" double head lap is used, fewer slates are needed. The standard lengths are 24", 22", 20", 18", 16", 14", 12", 10". For some years, architects have continued to favor the use of one length and random widths or graduated lengths and random widths. This avoids the multiplicity of uniform rectangular exposures and improves perspective.