Slate Roofing - Importance of Grain Direction
It is well-documented that for maximum strength certain strong-grained roofing slates must be produced "on the grain". This means the long dimension of a piece of roofing slate must run parallel to the grain. "Cross-grained" or "diagonally-grained" roofing slate is significantly weaker than properly fabricated shingles. Slate is like wood in this respect. Imagine the weakness of a cross-grained wooden rafter.
All of our quarrying and fabrication techniques have been designed to produce slate that is "on the grain". Although this centuries-old method causes us to have higher labor costs and waste factors than companies that produce roofing slate with no regard to grain direction, it results in a better product and significantly less breakage on the jobsite.
As shown below, our slate splits in two directions: 1) along its cleave into layers, and, 2) along its grain. The grain cannot be seen (as opposed to the cleft pattern) and runs in a single direction perpendicular to the cleave. Once a shingle has been produced, it is only possible to determine the grain direction by breaking the piece.
When the blocks of slate are brought to the splitting shed, they're first crosscut against the grain with a diamond wetsaw. The crosscut blocks are then sculpted, or split along the grain, with hammer and chisel. These blocks are split into smaller blocks which are then halved again and again to produce rough shingles. The rough shingles are then mechanically trimmed and punched or drilled to produce the finished product.