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.
Slates are bedded in mortar on boarding to obtain a sound bearing if much running about over them is expected. This process is further noticed under Shouldered Joint. If rendered on the under side when laid on battens, the mortar should be haired and coloured with smiths' ashes or coal-dust. The same kind of lime and hair mortar is likewise preferable for filletings when adopted in lieu of flashings.
Open Joint occurs in ventilated or open or half-slating, which is employed for covering sheds and similar buildings where a perfectly waterproof roof is not required. It consists in laying the slates with their side or vertical joints open to the extent of from 1½ in. to 2 in., or, in other words, placing the slates that distance apart, so that the smaller the slates the greater will be the aggregate space left partially open.
Painted Joint occurs frequently in common work, being a finish given to conceal the oil putty with which the joints at the hips and ridges, in the cheeks of dormers, etc, have been stopped.
To make slating air and snow tight it is sometimes pointed on the inside with hair mortar. "When this is laid all over the slates between the laths or battens, and well tucked and pressed in over the rafters so as to completely fill up all cavities between the wood work and slating throughout, it is sometimes otherwise called rendering. The vertical joints of weather slating to walls, dormers, etc, are pointed with hair mortar and left white, or with oil putty and painted.
Rebated Joint is made between slabs by cutting out equal and rectangular grooves along both edges to be united and fitting them together, as explained under the corresponding joint in Section VIII. In roofing, the slabs are rebated and screwed down to the rafters, the joints being stopped with oil cement or putty and covered with slate half-rounds, bedded on the same material and screwed; but settlement often causes such a covering to constantly leak, and though its appearance is satisfactory, it should not be used excepting when it is properly supported by trusses. Slate rolls on hips and ridges require rebated heading joints, unless the rolls break joint on both sides over the wings, saddles, or flanges of the ridging.
A modified form of bed joint resulting from the process called shouldering, which consists in bedding the heads of slates on fillets or shoulders of haired lime or mortar about 2 in. in width to make them weather-tight and close at the tails, and less liable to rattle or to crack when stepped on. The mortar may be brought to the same colour as the slates by incorporating ashes therewith, and in some instances the fillet, when on boarding, is developed and extended into a bed for the entire slate to repose on. "Where this is the case, the process is more appropriately, though somewhat inexactly, called rendering, and the bed joint becomes a mortar one.