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.
For further information regarding the method of lapping and the width covered, see Fig. 12.
It is often advisable to know the strength of corrugated steel when supported at certain distances apart by supports perpendicular to the corrugations. This unsupported length determines in many cases the spacing of the purlins. The load in pounds per square foot which can be carried by a plate of span /, parallel to the corrugation, is given by the formula:
W=330Sdt / l2 in which, l = Unsupported length of sheet, in inches; t = Thickness of sheet, in inches; S = Allowable unit-stress; d = Depth of corrugation, in inches.
Table IV, giving data relative to corrugated sheets, is taken from page 172 of the Pocket Companion of the Carnegie Steel Company (edition of 1902), where also other valuable information is given.
Side Lap, and Length of:
Tiles. One of the most common sizes of plain roofing tile is 10½ inches long by 6¼ inches wide and 5/8 inch thick. Tile of this size weigh about 2¼ pounds each. They are laid with a lap equal to one-half their length. They may be laid directly upon plank sheathing in a manner similar to shingles or slate, or they may be laid directly upon purlins (see Figs. 14, 15, and 16). In the first case they are nailed directly to the sheathing, and in the second case they are connected with the purlins either with copper wire or clinch nails. Flat tiles are usually laid in cement; corrugated tiles are made so as to interlock, and consequently in most cases require no cement. One convenience of the tile roof is that the skylights may be formed by laying glass tile in place of the other.
Tile roofs are very substantial; but are very costly, in regard not only to the tiles themselves, but also in regard to the additional weight required in the trusses by reason of the great weight to be supported. Tile weigh from 700 to 1,000 pounds per 100 square feet of roof surface. They cost from $12.00 to $40.00 per 100 square feet on the roof.
Fig. 14. Method of Laying Flat Tile on Plank Sheathing.
Concrete Slabs. These are usually moulded directly in place by suspending forms from the roof trusses. They may or may not be reinforced, and in any case are usually not over 4 inches in thickness. Their weight is about 50 pounds per square foot. Their cost is from S16.00 to $30.00 per hundred square feet of roof surface. They are expensive, not only on their own account, but also from the fact that the weight of the roof trusses must be increased in order to carry the weight of the slabs. Concrete roofing may be used on roofs which are practically flat, ½ inch to 1 foot being sufficient pitch.
Fig. 15. Ludowici Tile on Steel Purlins.
Fig. 16. Ludowici Tile on Sheathing.
Felt and Asphalt. This roofing is laid upon shingles, and consists of one thickness of dry felt, three or four thicknesses of roofing felt well cemented together with asphalt cement, and laid with good laps where they join, and a coating of from 100 to 200 pounds of asphalt per 100 square feet of roof surface. Upon this asphalt, while hot, gravel screened through a 5/8-inch mesh is spread in the quantity of about 1/5 of a cubic yard per 100 square feet. This class of roofing should never be laid on roofs whose pitch is greater than 1/5, since, when heated by the rays of the sun, the asphalt will run and destroy the surface. It gives good satisfaction on roofs whose pitch is 1/12. This class of roofing can be bought in rolls, and in this case the gravel is exceedingly fine, being screened through a 1/8-inch mesh.
Felt and Gravel Roofing. This roofing is similar to the above; only, in this case, tar instead of asphalt is used for the cementing constituent. This roofing does well on roofs of flat pitch, and should never be used on roofs whose pitch exceeds 1/6. It can also be bought in rolls ready for laying, in which case the gravel is screened through a 1/8-inch mesh. The prepared roofings are cheaper than those laid by hand; but they do not give good service unless great care is taken to fasten them down securely. In economy of first cost and maintenance, they are equal to or better than tin.
Sheet Steel. This should not be laid on a pitch less than ¼, unless the ends are cemented together where they lap. It comes in sheets 28 inches wide and from 4 to 12 feet long, or it may be purchased in rolls 2G inches wide and about 50 feet long. When used in sheets, it may be had with standing crimped edges, in which case it is laid as shown in Fig. 17. In case it comes in rolls, it may be laid in the same manner as tin, with either standing seams or horizontal flat ones as shown in Figs. 10 and 11. Like corrugated steel, it comes in different gauges, No. 28 being that most commonly used. It can be laid cheaper than tin, on account of the long lengths obtainable.
Fig. 1?. Method of Laying Sheet Steel with Crimped Edges.
Fig. 18. Method of Laying Roofing on an Anti-Condensing Base.
Patent roofings of many kinds are on the market. These come in rolls usually from 2 feet to 3 feet wide, and cover about 200 square feet of roof surface. The basis of most of these covers is asbestos, felt, magnesia, or rubber; and this is treated with either asphalt, tar, or some other preparation, and in some cases is covered with fine gravel.