This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
Flat roofs are now being constructed for board schools, blocks of workmen's dwellings, lodging-houses, and other buildings where space for recreation and other purposes is desired; but for small houses, especially in the country, they are seldom used. Fre-quently the only Hat roof in a house is that of the bay-window.
The framework of a lead flat consists usually of wood joists and purlins covered with boards one inch or more in thickness. The boards should be tongued and grooved (in order to prevent warping), of uniform thickness, and laid with their length in the direction of the fall of the Hat; where uneven, straight-jointed boards, laid transversely, are used, the lead may eventually be cracked by the edges of the boards. As lead expands and contracts considerably with the rise and fall of temperature, it cannot be simply nailed to the boards like felt, but must be allowed free play; otherwise cracks are sure to occur sooner or later. To allow the necessary freedom, wood-rolls are generally used, as shown in figs. 74 and 75, and the lead is cut into somewhat narrow strips, one edge of the strip being nailed to one of the wood-rolls, as at F, Fig. 75, the other edge being dressed over the next roll, as at G, but not nailed or fixed in any way.
Fig. 71 - Diagram showing the smallest Inclination for various roof -coverings.
Fig. 74 Half plan of Lead Flat over Bay window, cc. rolls, D. gutter . E. drip In gutter; v v, 9 x7 tapering joists ; GG, 3½ x 3 purling.
Fig. 75. - Transverse Section through Lead flat (on line a a in fig 74) F, undercloak nailed to wood-roll C; G, overcloak; H, splash lap. I, 1-inch tongued and grooved boarding; J, wood purlin.
The overcloak should extend an inch or more along the flat of the sheet below, as at H.
Wood rolls are usually 2 or 2 ½ inches in diameter, and fixed at distances varying from 1 foot 6 inches to about 3 feet from centre to centre. Certainly no greater distance than 3 feet should be allowed, and smaller distances are preferable. The sheet-lead is manufactured in various widths up to 9 feet, and the spacing of the rolls should be regulated by the width of the sheet, the size of the rolls, and the overlap of the lead. Thus, sheet-lead 8 feet wide may be cut into three strips 2 feet 8 inches wide. If 4 inches along one edge of each strip be dressed up to form the undercloak on a 2½-inch roll, and 7 inches along the other edge be dressed up to form the overcloak and splash-lap, the flat portion of the strip will be 1 foot 9 inches wide, and this gives the distance apart of the rolls, which will therefore be 1 foot 11 inches from centre to centre.
Fig. 76. - Longitudinal Section through Lead Flat (on line b b In Fig. 74).
Seam-rolls are sometimes used. These do not require wood cores, and are therefore adapted for curved surfaces, where the formation of curved wood rolls would be somewhat costly, but they cannot be used where likely to be trampled on. They are formed, as shown in Fig. 77, by dressing up one edge, a, of one of the sheets of lead to be united. Tacks or tingles of lead or thin copper, B, are then nailed at intervals of 3 or 4 feet to the boarding alongside the lead, and turned down over it. The edge of the adjacent sheet of lead, c, is set up and turned over till it reaches about half-way down the undercloak. The two edges are then dressed together, and finally bent into the form of a roll, as shown at D, a temporary wood core being sometimes used in the operation. If lead tacks are used, small sinkings should be cut in the boards to receive them.
Fig. 77 - Lead seam-roll.
It is usually said that the fall or inclination of lead flats and gutters should be not less than 1½ inches in 10 feet; wherever possible, however, a greater fall should be given, say 2 or 3 inches in 10 feet A fall of 3 inches in 10 is only 1 in 40, not by any means an excessive slope for the conveyance of dirtv niin-water and melting snow. Lead is sometimes used for covering roofs of ordinary pitch, bat cannot be recommended, as, in consequence of expansion aided by gravity, and contraction opposed by gravity, it gradually "crawls"
Fig. 78.- plan of Lead Gutter 30 feet long.
a 1. A 2, A 3, lead "flats" sloping 1½ in In l0 feet; Bl, B2, drips 3 in deep; C. roll; D, tilting fillet for ; G, cesspooL down the slope. The lead on the roof of Bristol Cathedral crawled 18 inches in 2 years.
The evil effects of the transverse expansion and contraction of the lead sheets may be avoided by means of the longitudinal joints just described, but the lead may be cracked by longitudinal expansion and contraction if the end or transverse joints are not properly made. Particular care must be observed in the construction of long gutters, that the transverse joints are sufficiently numerous, and are not nailed or soldered or fixed in any way. Fig. 78 is the plan of a lead gutter 30 feet long, and Fig. 79 the transverse section of the lowest "flat". The transverse joints, Bl and b2, are made by means of drips, which must never be more than 10 feet apart, or less than 2½ inches deep. A good form of drip is shown in Fig. 80. Care should be taken that there are no sharp angles to cut the lead, and that the boarding is cut out at a to receive the edge of the lower sheet, otherwise a slight ridge will occur, which may result in the cracking of the upper sheet The upper sheet should extend an inch or more on to the lower flat. In order to economize lead, the drips and fall in long gutters are often reduced to such an extent that solder has to be used in order to make the joints water-tight This practice is most reprehensible.