If the slates vary greatly in size they should be assorted in lots, and the breadth of the courses decreased gradually from the eaves upwards.

The thickest slates should be in the lowest courses.

The lowest course of all, called the "doubling eaves course," is laid with a double layer of slates, the lower one being cut so as to be about one inch longer than half the length of the uncut slates.1 The highest or "ridge course " is also a double one. The slates in these courses are nailed near the head.

The eaves course is supported and the tails of the slates kept well up by a wedge-shaped board called a "tilting fillet "2 (tf, Fig. 452), or "eaves board." This prevents any open space occurring under the tails of the slates into which the wind could penetrate so as to loosen the slates and make them rattle. When battens are used the effect of the tilting fillet is produced by a " tilting batten," thicker than the others. (See tb, Fig. 454.)

In the hips and valleys the slates have to be cut off obliquely to fit the angles; in the angles of valleys, and also where walls, chimneys, or windows cut into the roof, they have their sides slightly raised by means of tilting fillets running parallel to the valley. (See Fig. 480.)

1 Sometimes the under course is formed of uncut slates laid lengthways, but this should not be allowed. 2 Sc. Doubling.