This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
The terms applied to the different parts of roofing slate are: the gauge, or weather, which is that part of the slate which is exposed when laid, as shown at a, Fig. 88; the lap, which is the distance that each slate overlaps the second one below it; the head, or upper end of each slate, as at c; and the tail, or lower end of each slate, as at d. The bed is the under surface of each slate when laid. The back is the upper surface when laid.
Rendering consists in covering the under side of the slate with haired mortar, especially when the slates are laid on battens, though it is sometimes done when the roof is boarded, by laying the slate on the mortar, which has been applied directly to the boards. Rendering prevents the wind from blowing the rain or snow through the crevices when felt is not used over the boards. It is not effective when the slate is laid on lath or battens, as they are likely to settle and crack the rendering.
Shouldering consists in applying to the head of each slate, to a depth of 2 inches, a thin bed of haired mortar or slater's cement. Shouldering is resorted to only in very exposed situations.
Torching consists in pointing the joints between the heads and tails of the slates from the under side with haired mortar or slater's cement, and is done after the slates are laid; it is of little value, as it soon falls out, leaving the joints open.