Slate Roof Installation

Natural slate is laid as a double-lap covering: each slate is overlapped by the course above and also by the course two above, so that at any point there are at least two thicknesses of slate and the joints are always covered. The key measurement is the headlap, the distance by which a slate overlaps the slate two courses below it.

Headlap and exposure

Canadian guidance sets a headlap of 75 mm (3 in) on slopes from 8:12 to 20:12 and 100 mm (4 in) on slopes below 8:12. The exposure (gauge) follows from the slate length and headlap: subtract the headlap from the length and divide by two. A 24 in (600 mm) slate with a 3 in headlap therefore has an exposure of 10 1/2 in (about 265 mm). Joints in each course are offset at least 38 mm (1 1/2 in) from those in the course below.

Fixing with nails

• Slates are usually fixed with two nails through holes punched near the head, about 1 1/2 in (38 mm) in from the side edges, so the next course covers the nail heads.
• Copper nails are the traditional choice, with stainless steel also common; galvanized steel is less durable and can rust before the slate wears out.
• A common rule sets nail length at twice the slate thickness plus 1 in (25 mm). US federal preservation guidance gives 3d nails for standard slates up to 18 in long, 4d for longer slates and 6d at hips and ridges.
• The nail is driven so the head sits just clear of the surface and the slate hangs on it. Driving it tight can crack the slate; leaving it proud lifts the slate above.

Starting and finishing

A cant strip at the eaves, matching the slate thickness, tilts the first course so it lies at the same angle as the rest. An undercourse of shorter slates is laid first, followed by the first full course with joints offset. Hips and ridges are finished with slate, metal or lead depending on regional tradition.

Regional practice

In North America slate is normally nailed to solid wood decking over underlayment. In the UK and much of Europe slates are nailed or hooked to timber battens over an underlay, and UK work is governed by BS 5534 and BS 8000-6. In parts of Europe, slates are commonly fixed with stainless steel hooks instead of nails. Standard slates are about 4.8 mm (3/16 in) thick, with thicker grades used on heritage and heavy roofs.