Slates. Slates are of various sizes (see division on Materials), and the various parts of a slate are designated as follows : - the upper part of a slate, or that part which is seen when the roof is finished, is called the "back;" the lower or under side the "bed;" the lowest edge of a slate, as a, fig. 425, the "tail;" the upper edge b, the "head." To prevent the rain, etc., from gaining access under the slates they are laid so as to overlap each other, and made to " break joint," as shown in the drawing; the solid part, as d, of one covering the joint of the two next, as c c. The part of the slate in this arrangement thus exposed to view is called the "margin," as the part e e, fig. 425; and its depth, which varies according to circumstances, from top to bottom, as from e to e, is called the "gauge." The depth which an upper slate covers the slate below it is called the "bond" or "lap," and is measured from the line c d or e f, fig.426 (which line running through the centre of the nail holes is called the "nail line"), to the tail or lower edge a or b. Before fixing, the slates are trimmed at the edge, and the holes punched as near to the head as possible without incurring the danger of breaking the slate. The slates are fixed either to "boarding or to "battens," these being secured at intervals to the upper edges of the rafters of the roof. Fig. 427 illustrates the mode of fixing slates to boarding - a a a a, the board nailed to the rafters b b b b; c c c the " slates;" d d the " margin."
If the slates are fixed to battens, which are small timbers 2 to 3 inches wide and three-fourths of an inch in thickness, the distance from centre to centre of the battens is determined by the "gauge" or depth of the margin e e, fig. 427. This is found by halving the distance from the "nail line" c d, fig. 426, to the tail a, fig. 425, of the slate, deducting from this the width or depth of the "bond" or lap, and dividing the result by 2. Thus, in fig. 425, the slate a b is a duchess slate, 2 feet long by one foot wide. The nail line is 1 inch from head b; this gives 23 inches as the distance from this to the tail a of the slate; the "bond" or "lap" is fixed at, say, 3 inches, which deducted from 23 gives 20, and this divided by 2 gives 10 inches as the "margin" e e, fig. 425, and the distance from centre to centre of the battens f f. Fig. 428 gives the section of the arrangement, where a a is the rafter; b b the battens; c c the slates. In fig. 429, the "tilting piece "or "eaves board" is shown at a. This is feather edged, thicker at one edge than at the other, and its office is to tilt up the lower or eaves course of slates; the width of the tilt-ing-piece a is 6 inches; the thickness of lower edge 1 J; and of its upper edge three-fourths of an inch in thickness. In this fig. b b b the battens, c the rafters, d d e e the slates.