This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Valleys in slate roofs should be open valleys of metal at least eighteen inches wide, often laid, like a tin roof, with the end joints locked and soldered and the edges securely nailed to the roof hoarding. Tin, zinc, or copper maybe used, but copper is to be preferred for valleys, as well as for all flashings. Where the slope of the roof changes or where dormers, chimneys, or other vertical parts cut the roof, wide aprons of metal should be set and turned up against the rising wall to be covered over by. the wall covering, or with lead counter-flashings built into the brickwork. Gables are finished by continuous metal members, run at least eight inches under the slates, and tacked over the edge of the wooden finish, or cemented into grooves cut in stone copings, called "reglets."
A good practice is to cap all flashings; that is, the metal which runs under the slates is simply turned up against the wall, and a separate piece of metal is wedged and cemented into the reglet and turned down over the other flashing to within half an inch of the roof, allowing free movement to each, which will be necessary on account of expansion and shrinkage. For the same reason it is much better to form the valleys and hips of sheets which are laid in with each course, lapping, and not locked and soldered.
Slates are sometimes laid on strips of wood or battens without boarding, but do not make so tight a roof; for fireproof roofs, however, it has been common to secure the slates to small T bars by bolts and then plaster the under side, but a better way is to lay porous terra cotta blocks between the irons to which the slates may be nailed or cemented. A method of slating which may be em-ployed where absolute tightness is not required is called half-slating, and consists in leaving a space between the sloping edges of each slate, not exceeding half the width of the slate, as shown in Fig. 156.
In appearance, good slates should have an even color without spots, and present a hard straight grain, which shines in certain lights with a metallic, silken luster. They should be square and true, and free from warped or nicked edges, and neither too brittle or too soft.
Fig. 156. Half-Slating.