If a good quality of shingles are used and ordinary care exercised in laying them, there should be no danger of a leak on a plane roof surface. The places where leaks most frequently occur, and with which especial pains should be taken, are those around chimneys, dormers or skylights, and in the valleys.
The only way in which these places can be made tight is by the use of flashings, i. e., pieces of sheet tin, zinc, copper or lead.
For shingle roofs I. C. tin or 14-ounce zinc is generally used for flashings, and 3-pound or 4-pound sheet lead for counter flashings, although in much cheap work tin is used for the counter flashings.
The use of zinc for flashings appears to be confined to the Eastern States, where it is considered greatly superior to tin, and is very commonly used. The roofers in Colorado claim that in that State there is a certain amount of alkali in the rain water which eats away the zinc but does not affect tin. Whether this is true or not the author does not know, but no zinc is used there for flashings, and in general tin appears to be the common flashing material throughout the country. One of the best (stamped) brands of tin should be specified, and it will be more durable if painted on the under side before using.
For slate roofs 16-ounce copper should be used, as it is much more durable than tin or zinc, but for shingle roofs it would hardly be in keeping.
For all counter flashings built or let into mason work sheet lead should always be specified.
For open valleys on large roofs the author has often used galvanized iron, frequently in the shape shown in Fig. 175, the ridge at the centre being formed to prevent the possibility of nails being driven through the iron by the workmen walking in the valley. Tin valleys are often punctured in that way.
Valleys. - There are two methods in common use of forming the valleys. The first, and the one most generally used with shingle roofs, is to line the valley with long strips of tin or zinc, laid the whole length of the valley and locked and soldered together at the joints. The lining is then nailed to the roof boarding, at the edges, about every 12 inches.
The shingles are laid over the edges of the lining from 4 to 6 inches, leaving an open valley 6 or 8 inches wide, as at A, Fig. 176. On roofs having a pitch of 45 degrees or more the lining should be at least 18 inches wide, and on roofs of less pitch at least 20 inches wide.
The second method gives what is called a "close " valley. Trapezoidal pieces of tin or zinc are cut out, usually 15 inches at the top, 10 inches at the bottom, and 9 inches long for 16-inch shingles, and shingled into each course of shingles, the flashings being bent in the middle to extend up on each side of the valley, and the shingles laid close in to the valley, so that when completed the valley has the ap-
B pearance shown at B. Close valleys look rather neater than open valleys, and on a steep roof they make tight work, but open valleys arc the more common for shingle roofs, and when the pitch is less than 45 degrees are the safer to use.
Flashing against wooden dormers or any wooden wall should be done by working "tin shingles" about 7 inches square into each course; these "shingles" should be bent in the middle at right angles so that one half can be worked under the shingles on the roof and the other half under the shingles or siding on the wall, forming a sort of valley. If the pitch of the roof is less than 45 degrees the flashing should extend at least 4 inches on to the roof.*