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
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
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.
This standard is designed to enable the Specifier/purchaser to compare the key physical properties of natural slates to ensure the best fit for the project in mind. A broad range of attributes are tested, but the three most critical areas are:
- Thermal cycle
- Sulphur Dioxide Exposure
- Water Absorption
BS EN 12326 replaced the previous slate standard, BS 680 in 2004.
Thermal Cycle
Essentially, this process assesses the levels of rust or oxidisation within the slate. The test involves repeatedly soaking six pieces of slate in water and then drying them out over a period of up to three weeks. The samples are then inspected for corrosion. The results are classified as follows:
T1: No apparent change or some surface rust or other changes that neither effect the structure, nor form runs of discolouration.
T2: Oxidation or appearance changes of the metallic inclusions with runs of discolouration but without structural changes
T3: Oxidation or appearance changes of metallic minerals that penetrate the slate and risk forming holes. Classic Natural Slate does not supply T3 slates.
All Classic Natural Slates have been independently tested to T1 or T2 standard
Sulphur Dioxide Exposure
This test examines the ability of the slate to resist atmospheric pollutants. The slate samples are placed in a hermetically sealed container for up to three weeks and subjected to a standardised mechanical scraping test to measure any softness caused by the chemical disintegration of any carbonate content. Results are shown as S1 (no change), S2 (the slate must be split at least 5% thicker) or S3 (slates must be at least 8mm).
Slates containing more than 20% carbonate content are not suitable for roofing or external cladding.
All Classic Natural Slates have been independently tested to S1 standard.
Water Absorption
Excessive water absorption will result in natural slate being vulnerable to frost damage. If the slate absorbs 0.6% or less of its mass in water, it is classified as A1, the highest grade, and needs no further testing. If the absorption is greater than 0.6% it is classified as A2 and must be subjected to a separate freeze-thaw test, showing no deterioration in mechanical strength.
All Classic Natural Slates have been tested to A1 standard.