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.
Flanched Joint is one protected by a weathering or covering, consisting of one or more tiles bedded slantwise over it in mortar or cement. Flanching answers the same purpose as the filleting just described in the previous paragraph, and, in fact, by many the terms are viewed as synonymous.
This involves the principle of tiling, and has been already sufficiently alluded to.
If there are no hip tiles, what are called skew tiles are sometimes used and bedded in cement and mitred at the hip. The tiler rarely cuts his tiles.
Mortar Joint is formed when tiles are laid in mortar, which often happens to a 3-in. lap. In iron roofs tiles are laid on wood laths with or without mortar. This joint is further described in the last paragraph of this section.
Pointed Joint occurs when plain tiling is back-pointed on the inside when the pitch is low, or as noticed under Weather Joint. Pan tiling is likewise made as air-tight as possible when it is not intentionally left open for ventilation, by pointing both horizontal and vertical joints on the inside with hair mortar composed of ordinary mortar and ox hair. Sometimes, however, the side joints of both pan and plain tiling are pointed on the outside, but the tails of the latter should not be thus sealed, otherwise the water that elsewhere finds its way through the covering cannot escape.
To keep out the weather with a covering of plain or crown tiles, these should be of the best quality, and glazed if possible, the roof pitch being not less than 30°. The tiles must be laid on strong oak or fir laths to a gauge of not more than 4 in. at the most, or to a 2 in. lap or bond. In all exposed situations, or in any with a pitch less than what is called a true pitch, which is 45°, the tiles ought to be bedded and pointed with hair mortar, prepared with hydraulic lime and ox hair. There can be no doubt, however, that with the object of keeping out the snow it is best in all cases to point the inside, taking care to use mortar that will stick. So long as the water is thrown off and the framing made sufficiently strong, there is not much advantage gained in having a high pitch beyond lessening the strain on the members of the principals, but in many instances after a high pitch has been decided on, the increased height thereby obtained for providing additional accommodation has not been utilised, and consequently the extra expenditure has been thrown away. The pitch of a roof is the angle having for its tangent the ratio between the height of the roof and the half span, this determining both the length of the rafter and its inclination to the horizon.