Slate Roof Installation Practices
Domain 2 - Slate Roof Installation Practices
This domain tests the layout and fastening rules that let a slate roof last a century.
Double-coursing and bond
Every part of a slate field is covered by two layers of slate, called double-coursing. Each course overlaps the one below by the headlap - 3 in. minimum, 4 in. under some specifications and for reclaimed slate. Exposure = (slate length - headlap) / 2. Joints in each course must be offset at least 3 in. from joints in the course below; a continuous joint running up the roof (a ladder) is a defect.
Fastening
Use two copper or hot-dip galvanized (ASTM A153) roofing nails per slate, long enough to penetrate the deck at least 3/4 in. Nails are driven inside the covered headlap zone so at least two courses above hide them. Drive heads flush, never tight: a tight nail cracks the slate, a high nail lets it hang loose and slip.
Eaves, rakes, ridges
At the eave, the starter course sits over the under-eave slate and cant strip; its length equals exposure plus headlap, so the eave is double thickness with no thin edge. Eave slate projects 1 to 1 1/2 in. past the fascia. Rakes are flush or projecting with a rake board or metal edge. Hips take saddle (hip) slate; ridges take a comb ridge, bedded and fastened rather than merely laid.
Valleys
Valley metal (16- or 20-oz copper or a proprietary liner) is held with clips, never nailed through its center. Slate is cut evenly to the valley line and both sides are laid to a neat line.
Worked example
A 16 in. slate at 3 in. headlap gives 6.5 in. exposure. The starter course therefore measures 6.5 + 3 = 9.5 in. long. Cut starters from standard slate and put the dimension on the cut list before the crew starts, or the eave will be built with full-length slate and lose the double thickness.
Traps
Nailing through the slate below; nailing above the headlap so nails show in the exposure; skipping the bond offset; double-coursing only at the eave; leaving rake and ridge slate unbedded.