Most slate roofs use copper nails as they typically last longer than stainless steel and compatible with copper, lead and lead-coated copper flashings. A standard ¼” thick slate will require an 11 gauge nail where a thicker slate will require a 10 gauge copper nail. The nails are available in smooth shank and ring shank. Most roofers prefer smooth shank as it makes for easier repairs.
Selecting the length of the nail will be based on ¾” minimum wood deck, the underlayment and two times the thickness of the slate. The most common nail lengths are 1 ½” and 1 ¾” with a 3/8” diameter head.
In most cases, nails are sold by the pound. A 20” x 10” x ¼” slate will require 2.32 LBS of 1 ½” nails for one square of material. Each piece of slate will have two nail holes which are drilled or punched depending on the slate supplier. The holes should always be punched on the back of the slate and the thinner side of the slate. Punching the slate will produce a spalled countersink on the slate face so the nail head sits flat. Slates that are ¾” or thicker will require four nails per slate.