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.
Side Joint is one between the edges of slates, which are always left more or less ragged and bevelled after being trimmed. Some slaters prefer laying slates not quite close, but a trifling distance, say 1/8 in. to ¼ in., apart, for the purpose of leaving a channel for the easier departure of the water, and this practice is more especially regarded with favour where ton slates are used. This joint is often called vertical joint, and should always be made to range with others in alternate courses throughout the whole slope, that is, from eaves to ridge.
Torched Joint is produced by the operation of torching, which consists in back-pointing with hair mortar slates that are laid dry in the customary manner. The mortar is applied from the inside along the top edge of each batten, and is instrumental in keeping out drifting snow, but it is not very durable in its perfect state, owing to the loosening effects of alternate cold and heat.
This is synonymous with side joint.
In order that the joints may be weatherproof, the pitch of a roof for a covering of ordinary slates must not be less than about 26½°, the pitch being sometimes regarded as the ratio between the height and span, the double of which is the tangent of the angle of pitch. Thus, for example, a roof is said to be one-quarter pitch when its rise is one-fourth of the span. Equally often the amount of slope or the angle of inclination of the rafter to the horizon is denominated the pitch, which is high or steep, flat or low, according as it much exceeds or falls short of the so-called true pitch of 45°. It is quite as important that the pitch should not be so steep as to shoot off heavy rain with too much velocity for the gutters to properly discharge, as it is to avoid too much flatness, whereby the wind may drive the dripping rain under the tails of the slates or even lift them up by its violence and blow them off. As a rude, slates are most likely to keep out the weather when laid on close or open jointed sarking or rough boarding (which is better perhaps nailed diagonally), with felt between it and the slates, boards being stiffer than battens. Sometimes, however, the latter are used as well and over the former. Heavy slates are preferable for exposed situations, and these may be laid with as flat a pitch as 22°. Light sky blue ringing slates are the least absorptive. The bond or lap should not be less than 3 in., and the slates should be secured with two stout copper nails, with a double course at the eaves, no slate being laid lengthwise. Slating nails made of best quality Y. M. zinc, or of composition, iron wire, etc, are much used, but when of iron they should be painted or dipped in boiled oil. The nails vary in length from 1 in. to 2 in., with two or three intermediate sizes. It is necessary to insist that the slater makes good all damage occasioned by other workmen, especially the plumber, in order that the roof may be left sound and perfect at the conclusion or on the delivering up of the works.