This is the most important consideration with any form of roofing. As with interlocking tiles, slates must NEVER be fixed on a roof with less than 75mm headlap. Any slate roof fixing system with less than 75mm headlap should be rejected. Some exposed positions or roofs with flatter pitches may require a larger headlap.
Always use a softwood batten due to slate nailing considerations.
A nail with a clout like head can be used to fix slates provided that the slate is punched and not drilled as the punching from the back gives a countersink on the face of the slate for the nail head. As an alternative a stainless steel hook system can be used.
Slates should be laid with a broken bond and secured with nails or hooks to softwood battens. Before fixing, perpendicular lines should be marked giving the correct bond and spacing, Approximately 4mm should be left between the vertical joints of each slate so that is a free thoroughfare for water running down the roof. Slate should be started from the eaves and fixed diagonally across and up the roof slope.