Tin roofs, while under construction, should be kept clean, and rubbish never be allowed to collect, as nails and other hard substances are liable to cause perforations of the tin if stepped on by careless workmen. If tin is used for covering of a fireproof roof, the top filling should be of porous terra cotta which will receive the nails, and an even surface should be secured by smoothly plastering the surface of the terra cotta with hard cement. Steep fireproof roofs are often made by using hollow terra cotta tiles between T-irons, which may be treated in the same way.
Copper Roofing. The methods of laying copper roofing differ very little from those described for tin, except that the sheets of copper are much larger and therefore lend themselves more easily to moulded forms such as ridge rolls, hips, finials, etc. Copper roofing is often "crimped," that is, the surface is covered by fine corrugations which present a softer and more even appearance than the plain metal, and conceal the wavy appearance which plain sheets will acquire.
Slate Roofing. On roofs which pitch more than five inches to the foot, slates may he used. Ln this case the boarding should be matched and covered with tarred paper. On this surface the slates are laid in layers beginning at the eaves with a double course, and working upwards to the ridge. The ganging of the courses is determined by the " Bead cover," that is, the distance which the top of the slate is overlapped by. the second slate above. (Fig. 1:t5>.) This lap should not he less than three inches.
The usual sizes of slates are eight by sixteen and ten by twenty inches, and they run about ; ,3 to + inchin thickness. Slates should be put on with tinned, galvanized, or copper nails, two to each slate, and care must be taken that the nails are not driven in hard enough to crack the slates, and vet hard enough to prevent them from rattling. The nails should have large heads and be 3-penny or 4-penny nails. Where slates arc cut, as against hips or valleys, care should be taken that each slate receives two nails. For thick slates the holes should be drilled and countersunk. For first-class work the slates, for a dis tance of two feet each side of valleys and above gutters, should be "ren dered," i.e., bedded in elastic cement, and the same precaution taken for a foot in width against hips, ridges and all vertical parts. If slating
is required for very flat slopes, the whole should be laid in cement.
Flashings. Valleys in slate roofs should be open valleys of metal at least eighteen inches wide, often laid, like a tin roof, with the end joints locked and soldered and the edges securely Hailed to the roof hoarding. Tin, zinc, or copper may be used, but copper is to be preferred for valleys, as well as for all fleshings. AVhere the slope of the roof changes or where dormers, chimneys, or other vertical parts cut the roof, wide aprons of metal should be set and turned up against the rising wall to be covered over by. the wall covering, or with lead counter-flashings built into the brickwork. Gables are finished by continuous metal members, run at least eight inches under the slates, and tacked over the edge of the wooden finish, or cemented into grooves cut in stone copings, called reglets. ' A good practice is to cap all fiashings; that is, the metal which under the slates is simply turned up against the wall, and a separate piece of metal is wedged and cemented into the reglet and turned down over the other flashing to within half an inch of the roof, allowing free movement to each, which will be necessary on account of expansion and shrinkage. For the same reason it is much better to form the valleys and hips of sheets which arc laid in with each couse, lapping, and not locked and soldered.
Slates are sometimes laid on strips of wood or battens without boarding, but do not make so tight a roof; for fireproof roofs, however, it has been common to secure the slates to small T bars by bolts and then plaster the under side, but a better way is to lay porous terra cotta blocks between the irons to which the slates may lie nailed or cemented. A method of slating which may he em ployed where absolute tightness is not required is called hul f slating, and consists in leaving a space between the sloping edges of each slate, not exceeding half the width of the slate, as shown in . 156.
In appearance, good slates should have an even color without spots, and present a hard straight grain, which shines in certain lights with a metallic, silken luster. They should be square and trite, and free from warped or nicked edges, and neither too brittle or too soft.