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.
Back Joint is the same as that formed by torching or back-pointing, and is described under Torched Joint.
Slates are bedded on close boarding either rough or wrought on one side, or else on battens about 2½ in. x ¾ in., the former being preferred, as it keeps the roof cooler and drier, and forms a stiffer covering. Asphalted felt, which is both damp proof and a non-conductor of heat, is sometimes laid over boarding, and battens occasionally, in good work, again over this. The distance between the battens is the same as the gauge, which is equal to half the difference between the length of the slate and the lap. If the slates are nailed near the centre the full length of the slate is taken as the measurement, but if near the head then one inch is deducted therefrom for the space between the nail holes and the head. Slates are usually kept down on their beds by double nailing without straining or bending them through holes punched either near the heads or centres, the nails used having large flat heads, and being either of copper, zinc, galvanised iron, composition, or iron dipped in boiled oil. Slates are cut and mitred at the hips (over strips of 6 lb. lead lapped and nailed), and cut at the valleys. At the eaves, and where breaks or rakes occur, they are bedded on tilting fillets as a wind protection. Thicker pieces or slate fillets are often used for ridges and hips, being nailed and bedded in slate mastic. In other cases ridging, either of sawn slate, hewn stone, earthenware, 01 glazed stoneware, is used, and bedded and close jointed in cement so as to drop dry, and such ridging is tied occasionally together by an iron rod passing through the perforated rolls. There is no variation of any moment when bedding slates on iron roofs, the boards and battens being easily secured to wood packing fastened to the iron bearers. Slates can also be bedded in mortar directly upon sloping brick work.
This is similar in principle to that of the Bricklayer, and is obtained by making the side joints of each course alternate with those of the courses above and below, a double course being laid at the eaves.
This is explained under Bond. It is always necessary in slating, and usually it is enjoined that the joints should be properly broken.
Cement Joint is formed when fixing slate ridging, which is set in oil cement or mastic, and secured with screws, the cement being painted to match the slate.
Close Joint signifies as close a joint as possible, as, for instance, when hewn ridge stones form the ridging they are close jointed and set with oil mastic so as to be perfectly watertight. Some particulars pertinent to this paragraph will be found under Side Joint.