This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Slating. In measuring slating, the method of determining the number of slates required per square is similar to that given for shingling; but in slating, each course overlaps but two of the course below, instead of three, as in shingling. The usual lap, or cover of the lowest course of slate by the uppermost of the three overlapping courses, is 3 inches; hence, to find the exposed length, deduct the lap from the length of the slate, and divide the remainder by 2.
The exposed area is the width of the slate multiplied by this exposed length, and the number of slates required per square is found by dividing 14,400 by the exposed area of 1 slate.
Thus, if 14"x20" slate are to be used, the exposed length will be 20-3/2 = 8 1/2 inches; the exposed area will be 14 X 8 1/2
= 119 square inches, and the number per square will be 14,400÷119 = 121 slates.
The following rules should be observed in measuring slating: Eaves, hips, valleys, and cuttings against walls are measured extra, 1 foot wide by their whole length-the extra charge being made for waste of material and the increased labor required in cutting and fitting. Openings less than 3 square feet are not deducted, and all cuttings around them are measured extra. Extra charges are also made for borders, figures, and any change in color of the work, and for steeples, towers, and perpendicular surfaces.
The following table, based on 3 inches lap, gives the sizes of the American slates, and the number of pieces required per square: