The minimum pitch required by modern building codes is 4:12. This would mean that the roof must rise four inches for every twelve inches measured toward the peak of the roof. Slate is measured in thousands of pounds per square inch and not hundred per inch like normal roofing materials.
Slate is mined in quarries and about five percent is actually used for slate tiles. The larger slate blocks are jack hammered into smaller piece which are then cut into rectangular pieces. Then individual tiles are split from the block by a person using a hammer and chisel. If the slate is frozen or too cold the slate can break into many small and useless pieces. Then holes are punched out from behind where nails will be used to hold the tile in place. The holes are punched from the back leaving a blow out area where the punch came through, so the nails can be counter-sunk when being placed on the roof.
In the past, slate tiles were fabricated so that the mineral grains were oriented along the length of the tiles or what is known as "on grain." It has become more efficient and economical to fabricate tiles of varying grain orientation. Though, slate tiles with varying grain orientation must be thicker to offer comparable resistance to breaking.