Inclination of Sides of Roof to Horizon.
Height of Roof in parts of Span.
Ordinary slates .
The more severe the climate, and the smaller and lighter the slates, the steeper should be the roof, otherwise the wind will lift the slates and blow the rain up under them. A high roof, however, is of course more expensive, as it contains for the same span more timber and more surface to cover than one of flatter pitch.
Slates are laid either upon boarding or battens.
Boarding costs more than battens, but keeps out the wet and heat better, and is almost necessary for light slates.
Battens may be used for heavy slates, and are nailed upon the rafters at a distance apart equal to the "gauge." (See p. 208.)
The scantling of the battens used with rafters 12 inches apart varies from 3 inches by 1 inch for large slates to 2 1/2 inches by 3/4 of an inch for the smaller sizes.
The " back " of a slate is its upper surface.
The "bed " is its under surface.
The " head " is the upper edge of a slate.
The "tail" is the lower edge.
The " margin " is the part of each course exposed to view on the outer surface of the roof.
The "lap)"1 is the distance by which each slate overlaps the next slate but one below it. This should never be less than 2 1/2 inches or 3 inches. The flatter the roof the greater should be the lap.
1 Sc. Cover.
The "gauge" is the depth of the margin.