For Natural Slates, tiles and artificial slates, the shank diameter and length should be determined by the exposure of the site, the nails withdrawal resistance and the manufacturer’s recommendations, For Natural slates the head diameter should be not less than 10mm (3.0mm shanks with 10mm heads are available)
Hooks intended for slating should be formed from stainless steel wire conforming to BS 1554:1990 grades 316 S11 or 316 S19.
Typically, slate hooks attach to the batten by a spike at the upper end, which is driven into the batten or a hook that wraps around the batten. The use of wrap hooks is not recommended. The opposite end should have the form of a curved hook into which the slate fits. The shank may be straight or crimped, the crimps being in the same plane as the roof slope. Hooks with crimped shanks reduce the capillary rise of water at the perpendicular joints between slates. The peak to trough dimension of the crimp should not exceed 5mm.
The slate grip gap in the hook should be small enough to hold the slate securely but should not be so small that it would damage the slate. When laid, it should be adequately tight fitting around the slate, due account being taken of different slate thicknesses and variations in thickness for different slate types, to avoid wind rattle and lateral drift.
The hook shank diameter should be not less than 2.70mm and should not exceed the minimum thickness of the slates being fixed. The hook shank diameter should be of a sufficient size to resist the calculated wind uplift forces.
Centre nailed slates
Centre-nailed, double-lap slates are fixed with two nails close to the side-edge of the slate and positioned immediately above the slates below.
In centre-nailed slating, the gauge is the spacing at which the battens are fixed to the rafters, trusses or counter-battens to provide the required head-lap. The margin is the exposed length of the slate measured in the same direction as the gauge. Except in random slating with diminishing courses, gauge and margin are the same.
Centre-nailed slates usually require longer nails near the eaves, due to the greater thickness of slate. Where there is a gap between slate and batten, such as when tilt is provided at the eaves, longer nails are required unless thicker battens are used to take up the gap between slate and batten. Nail lengths in such cases should generally be equal to the sum of the calculated batten penetration, twice the slate thickness and any gap between the slates and the batten. The use of thin packaging laths is not recommended to fill the gap between the slates and the battens because they might split and, as a result, reduce the nail withdrawal resistance.
Hook fixed slates
Hooked double-lap slates are fixed with a metal hook that engages the tail of the slate in the course above and overlaps the head of the slate in the course below. When hook fixing, additional nail fixings should be used at eaves, ridges and top abutments and, to prevent lateral drift, at verges, hips, valleys and side abutments.
Double-lap slates should have minimum fixings of two nails to every slate when centre-nailed. Nails should be of a diameter and length appropriate to the thickness of the slates held down and the position of the slates on the roof. For natural slates and stone slates the head diameter should be not less than 10mm.