The slates are first carefully squared to size, except the heads, which may be left rough, but not concave, the edges straightened, each punched with two nail holes, and the whole sorted, if necessary, into three thicknesses.
The slates should be trimmed with the smooth face up, in order that the bottom edge next to the smooth face may lie close, and that the countersunk side of the nail hole may be uppermost to take the head of the nail.
In laying the slates the great object to be attained is that the bottom edge, or "tail," of every slate should fit as closely as possible to the backs of those immediately below it; they should therefore be laid (except the lower slates of the doubling courses) with their smooth sides downwards. The sections of the slates will therefore be as shown in Fig. 452 - the bed being a little longer than the back, and the edges ragged and splayed; but they are generally drawn square-edged, as in Fig. 454 and others. The vertical joints between the slates should be as close as possible, and each should fall on the central line of the slate below.