Two re-roofed houses on the same street can look identical and behave completely differently. One shrugs off thirty years of driving rain. The other starts leaking around the fixings within a decade. The difference is rarely the slate's colour or its quarry of origin — it is three unglamorous things: the thickness of the slate, the headlap it was laid to, and the pitch it sits on.
Get those right and a slate roof is close to maintenance-free. Get them wrong and no amount of expensive slate will save you. What follows is how to pick a sensible thickness for your roof, and where the standards and regulations actually have something to say.
It is tempting to treat thickness as a proxy for how good a slate is. It isn't. The working life of a natural slate depends mostly on what it is made of — the mineral composition that decides whether it delaminates or turns soft after decades of wetting and drying — and on how it is fixed. A well-made 5mm slate from a good bed will outlast a poorly made 8mm one.
Thickness earns its keep in three other jobs: resisting wind uplift, absorbing impacts from ladders and debris, and providing stiffness. That last one matters more than people expect. A stiffer slate can be laid at a longer gauge and in larger sizes without flexing and chattering in a gale.
These compound. A small slate, steep pitch and sheltered inland site will happily take the thin end of the range. Change any two and you should be moving up.
Natural slate for UK roofs generally runs from about 4mm to 9mm. As a rough starting point:
Treat the bands as a starting point, not a rule. The supplier's technical data, the fixing standard and the site's real exposure decide the answer.
Below roughly 30°, headlap has to increase, and slates generally get bigger and thicker. In exposed locations the same thing happens. If your roof is both low-pitched and exposed, you are firmly in the thicker band, and it is not worth arguing with it.
On minimum pitch: many natural slate systems start at around 20°, and some can be taken down to about 15° with a fully boarded, sealed underlay and increased headlap. Below that you are into specialist territory. If someone proposes 17.5° with thin slates, ask questions before the battens go on.
In the UK, the two documents that matter are BS 5534 for slating and tiling, and BS EN 12326 for the slate itself. Building regulations do not hand you a thickness in millimetres; they require the roof to be weathertight, structurally adequate and able to resist the wind loads for the site. Those requirements are met by following BS 5534 and the manufacturer's guidance.
In practice that means:
Where structure, pitch or a listed building is involved, take professional advice — a structural engineer or the local conservation officer — before ordering. It is far cheaper than rectifying a roof that was never going to work.
Thin slate with too little headlap is the classic leak. If you reduce thickness to save weight, you may need to increase headlap, which shortens the gauge, which means more courses and more slates than you budgeted for. For a double-lap roof the gauge is the slate length minus the headlap, divided by two. Do that sum before you order, not on the morning the delivery arrives.
Nails must pass through the slate and still hold firmly in the batten without being overdriven. Thin slates crack at the fixing more easily; thick slates need longer nails. Use corrosion-resistant nails — copper or stainless steel are the usual choices with natural slate — and make sure the batten size and nailing pattern match the slate size and exposure.
If the building is listed or in a conservation area, the size and thickness of the slate may be part of what gives the roof its character. Thin, small, weathered slates on a steep pitch look nothing like a uniform modern roof, and swapping them can attract a refusal. Speak to the conservation officer first. If you are matching existing slates, measure a proper sample across several courses rather than one or two.
Reclaimed slate is a route some owners take, and it can work well. Check thickness consistency and nail-hole positions: a slate already holed near the head may not accept a new fixing in the right place.
Start with the roof you actually have: its pitch, its exposure and the slate size you want. Let those three set a band, then confirm it against BS 5534 and the supplier's data. When two factors push you towards thicker slate, go thicker. The extra cost is small next to re-roofing a decade early.
And if there is any doubt about the structure, the pitch or the building's status, resolve it before you order. Slate is a long-term material, and the decision you make in the merchant's yard will sit above your head for the next fifty years.
Photo: Didgeman / Pixabay
