This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
This signifies that the slates are not trimmed with too ragged edges, but cleanly cut to fit closely at the doubling eaves course and wherever necessary at hips and ridges, or where dormers, chimneys, ventilators, etc, break into the roof.
Close Cut and Mitred Joint is sometimes formed at the hips of a roof, and is made by cutting the slates to the proper bevel for mitring, and bedding them either on oil putty or on soakers about 18 in. or 20 in. wide, cut out of 2 lb. or 3 lb. lead, and having similar lap and length to the slates. If the lead is laid on in long strips it should be at least 6 lb. lead. "When putty is used, which is of constant occurrence in common work, it is painted over to correspond in colour with the slates.
Filleted Joint is one protected by filleting. The fillets are sometimes narrow strips of slates nailed to the ridge piece of a span roof, or to brickwork in the case of the head of a lean-to roof and the sides of both varieties, if abutting thereon, and pointed in oil cement or mastic, or Portland cement, or else they consist only of Portland cement, or hair mortar, or even common mortar laid against the wall on cast iron nails every two or three inches apart. Equal parts of cement and sand, gauged with a little mortar, forms a good material, and the work being wetted, the fillet is run about 2 in. up the brickwork and 1 in. over the slates. A cool day should be chosen for the operation. These fillets are left plain or marked with trowel creases. Gables are pointed or run with cement fillets under the slates.
Heading Joint occurs in slate ridging, and is either a butt, but more properly a rebate, in which case it should be cemented with red cement. It is sometimes additionally protected by being tongued with a strip of copper.
Key Joint is made similarly to the corresponding joint of the tiler.
This involves well-nigh the whole theory and practice of slating, systematic and thorough lapping being the only safeguard against the intrusion of wet. What is technically called the bond or lap is the distance between the nail holes in the head of a slate and the tail of the next but one above it, as shown in Fig. 23, where A b indicates the lap, which should never be less than 3 in. When the nailing is effected near the centre of the slates, the lap is measured from the head, which is assumed to be 1 in. beyond the nail holes as punched for nailing near the head.
Slater's mastic is usually either putty or white lead, and is employed, amongst other purposes, for the joints of the thick pieces of slate sometimes used as coverings to hips and ridges, in which positions they are properly squared and screwed down to the boarding with copper screws. The mastic is afterwards painted to match the slating.