A slate roof is a pitched roof covered with slates: thin, flat plates fixed in overlapping courses across the roof plane. Because each slate laps those below it, the roof is watertight without the fixings ever sitting in the path of the water.
Slate was for centuries the roofing material of churches, castles, towers and town houses, and it is still the natural choice where a roof plane is swept or doubly curved: a slate can do what a tile cannot — follow a curved surface.
Natural slate and fibre cement slate
- Natural slate is riven slate rock: a metamorphic stone formed from clay that splits along its natural cleavage planes into thin plates. Colour, thickness and figure vary by quarrying region.
- Fibre cement slate is a pressed sheet of cement with fibres. Cheaper, lighter and dimensionally consistent, but with a shorter life and a flatter appearance.
On a listed or characterful building this distinction is no detail: replacing natural slate with fibre cement changes the appearance and normally requires consent.
| Natural slate | Riven metamorphic stone that splits into thin plates; colour, thickness and figure vary by quarrying region. |
|---|---|
| Fibre cement slate | Pressed sheet of cement with fibres; cheaper, lighter and dimensionally consistent, but with a shorter life and a flatter appearance. |
Laying patterns
- Double-lap rectangular slating — rectangular slates in double lap, so each slate overlaps the two courses below. Calm and regular in appearance; a major advantage is that the roof stays watertight if a single slate is lost. This pattern needs a roof pitch of at least around 30 degrees, depending on slate size, lap and exposure.
- Scallop or fish-scale slating — slates rounded at the tail, laid in a lively, fanning pattern. The German tradition of laying diminishing courses of varying size belongs in this family and is the most demanding form of slating there is.
Beyond the pattern, slate size and the required head lap determine how many slates go into a square metre — and therefore the price.
Fixings and substructure
Slates are nailed or hooked to boarding with a vapour-open underlay, or to battens. In hook slating, a slate hook grips the tail of the slate; this is the common, wind-resistant method. Use stainless hooks and nails: copper hooks from the period up to the early 1980s are found corroded through after some decades, especially on north-facing, persistently damp roof planes.
Detailing points:
- Valleys, hips, ridges and rooflights call for carefully cut slates and lead or zinc work; this is where the great majority of leaks start.
- Ventilation beneath the slates is essential: a slate roof has to be able to dry from below.
- Chimneys and penetrations need flashings in lead or zinc of the same durability class as the slates.
Lifespan and maintenance
A properly laid and maintained natural slate roof lasts a very long time — generations — and the fixings often need renewing before the slates themselves do. Fibre cement slates do not reach that lifespan.
Practical rules:
- Never simply walk on a slate roof. Slates break under point loads; use a ladder with a ridge hook or a roof access system.
- Inspect annually for slipped, cracked and missing slates, and check the flashings at abutments.
- Replace loose slates immediately. One missing slate is a small job; months of water dripping onto the boarding is not.
- Do not let moss build up in valleys and on north-facing planes; it holds moisture and blocks the drainage.
Related terms
- Roof slate
- Slate hook
- Slate hammer
- Roof pitch
- Roof boarding
- Roof tiles
- Counterbatten
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