This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The question as to whether paper shall be put under shingles or not, is open to argument. If the shingles are laid without being dipped in paint or with the butts dipped only, the presence of paper will cause sweating, and, preventing any circulation of air under the shingles, will cause them to decay much sooner than if no paper is used; but on the other hand if the attic is plastered there is danger of damage from leaks and from fine snow sifting under the shingles. In the present case the attic is to be unfinished and we will use no paper on the roofs. The ridge of a shingled roof is usually finished by means of saddle boards nailed over the tops (Fig. 37), but sometimes an ornamental cresting is used. Hips are best finished by what is called "saddle board" shingling, in which a course of shingles is put over the roof shingles laid at right angles with the hip. There is little danger of roofs leaking at the ridges and hips, but the hip shingles unless well nailed are liable to be blown off. The plane surface of the roof, if reasonable care is taken, should afford no liability of leaking, but dormers, chimneys and valleys are sources of great danger. The flashing against chimneys and the vertical sides of dormers is done by means of pieces of tin or zinc about 7 inches square bent in the middle, so that one half will lay in the course of shingles and one half turn up against the vertical wall, to be covered by the shingles of the wall or by the counter-flashing of the chimney. In the Eastern States zinc is generally used for flashings, but in the West tin is the common material. In the forming of valleys, two methods are followed, that of an "open" or a "close" valley. In the former case the shingles are kept apart six or eight inches and the valley is made of zinc or tin, often in long strips locked and soldered at the joints and running under the shingles from four to six inches. The defect in this method of a continuous piece of metal is that there is no chance for the metal to expand and contract in its length, without danger of straining the joints and even of starting the shingles or slates which are laid over it. For this reason the use of separate pieces, the length of the shingles or slates and shingled in with each course, is to be preferred. These are bent, and lapped in the same way as the shingles without soldering and are free to expand and contract without damage.
Fig. 37. Ridge Finish.
In the case of the "close" valley the shingles are laid close together at the angle and narrower pieces of zinc or tin are shingled into each course. The flashing of roofs is a matter which needs a great deal of attention on the part of the superintendent, especially to see that wide enough metal is used. Counter-flashings are often omitted unless specially mentioned and should always be built into brickwork if possible. If not built in, it must be carefully wedged into joints which have been raked out, and pointed with cement. Every part of the flashing of a roof should be examined by the superintendent and it must never be left to the care of the builder or the workmen.