This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
98. Lead, the softest metal in general use, is extremely plastic, very malleable, flexible, and heavy, but lacking in tenacity and elasticity. It is mostly found in combination with other ores, the principal mines being at Leadville, Col.; in Derbyshire, England; the Harz Mountains, Germany; and in the south of Spain.
Lead is reduced from the ore called galena, by roasting or smelting in a reverberatory furnace furnished with long flues, to catch the particles of lead precipitated by the fumes.
Sheet lead is either cast or milled. Cast lead is thicker and heavier than milled lead, and has a harder surface. Being, however, liable to flaws, such as sand holes, etc., and irregular in thickness, it should therefore not be used if lighter than 60 pounds per square foot.
Milled lead is rolled thin, is more uniform in thickness than cast lead, bends easily, and when worked gives neater and better results, but cracks if much exposed to the sun.
Lead is always described by its weight per superficial foot, and not by gauge.
99. Roofs covered with lead should not have a pitch of more than 1 inch to the foot. Boarding under a sheet-lead roof must be more carefully laid than for any other metal; the boards should be narrow and matched, and run with the current or pitch. If the boards are not blind nailed in the joints, the nails should be set. The boards should also be well planed at the joints, to make a smooth surface, and, to prevent warping, should be thicker than is usual in such places. Planking 1 1/2- inches thick gives good results.
In sheet lead, it is best not to use sheets measuring more than 3 ft. x8 ft., or 3 1/2 ft. X l0 ft., the expansion and contraction being in such a case liable to make the roof defective. The usual size is 2 1/2 ft. X 6 ft. The sheets of lead should not be secured by nailing or by soldering, but should be left free, to allow of sufficient movement.
100. The junctions employed in lead roofs are called rolls when laid in the direction of the pitch, and drips when laid at right angles thereto. The rolls are from 2 to 3 inches in diameter, having the upper surface round and the lower corners left square. See Fig. 65. The edges are called overlap or overcloak, the latter being the edge of the sheet turned over the edge of the succeeding sheet and dressed down, forming the roll. Hollow rolls for sheet lead are constructed in the same way as for copper roofs. See Fig. 63.
The clips or tingles between the sheets of a lead roof must not be spaced more than 20 inches apart.
Nosings (see Fig. 81) are the rolls formed at the angle between the horizontal surface of the flat and the sloping sides of the roof. The flashing a on the sloping surface is turned up and nailed to the edges of the flat or deck plank, and over this is nailed a wood nosing b. The edge of the sheet of the flat is locked into and secured by clips c, c let into a rebate in the edge of the flat; the sheets d are then bent over the nosing in the form of a drip. See Fig. 81.
A lock or welt is an ordinary single lock joint used instead of a roll on flats, where there is a suf-ficent pitch; though it takes less metal, it is not recommended.
Drips are joints made across the current of the roof, and are located ac-cording to the length of the sheets used. They are constructed in two ways, as shown in Fig. 82 (a) and (b). In the first method, the sheet on the lower level a is bent up and over the step b, let into a rebate cut for it in the boarding of the higher level, and is nailed securely. The upper sheet c is lapped over it and turned down. To avoid capillary action, there should be at least 3/4 inch between the edge of this sheet and the sheet on the lower level.
In the second method (b) the lower sheet a is bent up, 1 1/2 inches above the upper level, as indicated by the dotted line b. Sheet c is bent up and over the edge of the lower one; the two sheets are then folded over and downwards, forming a lock d, as shown.
101. In covering small angular or curved surfaces of woodwork, it is necessary to screw the lead to the woodwork, to keep it in place. This is done as shown in Fig. 83. The boarding a is countersunk, and the lead b is formed into the cavity, and screwed in place with screws c, c. The cavity e in the lead is then filled with molten solder, to protect the screw heads and render it water-tight. The lead should be made to flow under the screw heads to prevent the sheet from tearing.
102. Lead flashings are constructed in nearly the same manner as other metal-roof flashings, the chief difference being the manner of securing the upper edge to the masonry or brickwork. As with other metal flashings, wall hooks may be driven in the joint over the apron or counter-flashing. Plugging with lead is, however, preferable to all other methods. The plugs are about 1 1/4 in. x 2 in., made to suit the joint in the masonry, or the raglet cut therein to receive the flashing. After the plugging, the joint or rag-let must be pointed up with mastic or elastic cement. Burning in makes the most permanent joint. The flashing is inserted in the raglet, which is then filled with molten lead and calked in. This method is, in most cases, very difficult to execute properly, on account of the position of the joint. Unless the joint is in a very exposed location, the plugging will be sufficient.
103. Lead gutters should have a current or fall of at least \ inch to the foot, and the joints between the sheets should be formed where possible with drips, the same as mentioned for flats. The sides of gutters abutting the walls should be turned up 6 or 7 inches and counterflashed, the ends of the gutters being bossed, or folded up, and soldered.
Trough gutters should have the outside edge turned over the wood or stone crown member, and left without fastening. The inner side should be carried up at least 4 inches above the outer and over a tilting fillet on the roof, or otherwise stopped off at the board line. Where the gutter is not very wide, the counterflashing may be dispensed with, and the gutter lining, as shown in Fig. 84, may then be simply flanged over the tilting fillet and covered with a counter-flashing b nailed over the tilting fillet c to the roof boards a. When a gutter comes against the back of a horizontal parapet wall, the wall should be pierced about 5 inches above the lowest part of the gutter, and an overflow should be provided to take off an occasional excessive fall of rain water, or an overflow caused by the leader being stopped at the eaves or drip. If the gutter is very long, there should be more than one outlet.
In working lead flashings around corners and in angles, great care should be taken to avoid sharp angles, otherwise the lead is likely to be cut.
104. The following weights of lead for the different parts of a roof are generally applicable: For flats and gutters, 6 to 8 pounds per square foot; for flashings, 5 pounds per square foot; for hips and ridges, 6 to 7 pounds per square foot.
Where the roofs are very much exposed, or where temperatures are extreme, heavier lead should be used.
105. For lead roofs or flats much worked over, a wooden-slat floor should be laid. In using lead for gutters with slate or tile roofing, the more the latter can cover and protect the lead from the sun, the longer the lead will endure. In using lead for apron or center flashing, on very steep gables, the stepped or skeleton-flashing method is to be preferred, as it keeps the lead in a horizontal position; otherwise, the lead is apt to stretch and crawl until it becomes too thin to be of any value.
106. The thickness and the weight per superficial foot of lead used for roofing purposes are given in the following table: