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The upper side of the slate when laid.
The underside of the slate when laid.
The lower edge of the width of the slate when laid.
The upper edge of the width of the slate when laid.
The angle of the spread of water beneath a butt joint.
The angle of the roof slope.
The area of exposed slate when laid on the roof.
The distance by which the tails of slates in one course overlap the heads of the slates in the next course but one below.
The horizontal distance between the side of a slate and the side of the slate immediately above it. The bond should never be less than the lap.
The vertical distance between the tail of one slate and the tail of the slate immediately above it. This can be calculated as follows:
Gauge= length of slate – lap
2
The distance between the tail of the slate and the nail hole. This can be calculated as follows:
Holing gauge= Gauge + lap + 15mm