Natural slate roofing
| Natural slate roofing | |
| Material | Metamorphic rock, split into thin units |
|---|---|
| Standard cited here | None; see note |
| Fixing | Nailed, historically to battens |
| Common failure | Nail fatigue rather than slate failure |
Natural slate is a metamorphic rock that splits into thin, flat units and has been used as a roof covering in the United Kingdom for centuries. A slate roof of good quality outlasts most other coverings by a wide margin, and the thing that usually ends its life is not the slate.
**No standard is cited in this article.** A product standard for slate exists, but no page for it could be verified to this site's requirements — every candidate returned a placeholder rather than the standard — and citing a page that resolves without evidencing anything would be worse than citing nothing. The description below is therefore of established practice and material behaviour, unsourced, and is written to that limitation.
Nail fatigue[edit]
Slates are nailed, and on older roofs the nails corrode and fail before the slate does. A roof shedding individual slates in wind, with the slates themselves sound where they land, is usually showing nail fatigue rather than a covering at the end of its life.
This has a direct consequence for cleaning. A roof held by tired nails is a roof where disturbance dislodges slates, and both foot traffic and high-pressure water are disturbance. Slates lost during or shortly after a clean on an old roof are often attributed to the cleaning when the fixings were already at the end of their service.
Surface and lichen[edit]
Slate is dense and absorbs little water, which is why it lasts. Moss finds less purchase on it than on a weathered concrete tile, but lichen adheres to slate readily and is difficult to remove without abrading the surface.
Because a slate is a thin, split unit, material removed from its surface is a larger proportion of its thickness than the same removal from a tile. Aggressive cleaning of slate therefore costs more of the covering than the same treatment costs a thicker unit.
Delamination[edit]
Slate splits along planes, and poorer slate can delaminate along those planes as it weathers, lifting in layers at the exposed edge. A roof showing that is one where the covering itself is failing, and no cleaning method improves it.
Distinguishing a roof that is dirty from a roof that is failing is the first question on an old slate roof, and it is answered by inspection rather than by method selection.