A Treatise On The Principles And Practice Of Dock Engineering

ROOF COVERINGS. 385 As an example, take the case of a solid, cylindrical, cast-iron column, 12 inch.es diameter and 20 feet long, fixed at both ends. Then, by the fore- going formula— 40 1 + x 400 \ 1 / 20 tons per sq. in. Fidler’s formula gives 19-4 tons under the same conditions. Roof Coverings.—The roof coverings usually employed for sheds are slate, lead, zinc, galvanised iron, felt, and roofing paper. The last-named material is inferior to the others, and should only be used for temporary and unimportant purposes. Slate is the best roofing material, being unalterable in nature and exempt from decay. It has the drawback of being heavy, but this disadvantage is more than compensated for by its durable qualifies. Large sized slates form the best kind for use, as with fewer joints there is less opportunity for leakage, and with greater weight there is less chance of the slates being lifted by the wind. For the latter reason slates should be centre-nailed, and in very exposed situations they may be additionally secured by lead or copper tingles. Lead is a durable roof covering, but both heavy and expensive. More- over, it is not a suitable material for steep-pitched roofs (though, perhaps, this drawback is of little importance in the case of sheds, where the roofs are generally low-pitched), owing to its tendency to creep under the influence of expansion and gravitation. Zinc has the advantage of lightness combined with economy, but it is very subject to corrosion and decay, and is highly inflammable at a red heat. Contact with iron, copper, or lead, in the presence of moisture, produces destructive voltaic action. Lime is another deteriorating agent, as also is oak, owing to an acid which it contains. From an exhaustive examination of a great number of zinc-covered shed roofs at Liverpool, the following valuable observations were deduced :— 1. That when zinc is in free contact with the (sea) atmosphère, a slow and graduai wasting away of the zinc takes place. The metal throws off a fine flour-like substance, which forms a deposit on its surface and is washed, or blown, away or cemented by sooty matter, as the case may be. 2. That in exposed situations the wasting away is intensifled, and the surface of the zinc soon présents a roughened appearance due to close and minute pitting. Especially does this occur at the more prominent points, such as step flashings, at weather faces, at ridges and rolls, and at cappings over joints. 3. That wherever a leak occurs, and, to a greater degree, where moisture, in passing down the underside or covered upper surface of a sheet, is checked and forms into beads, as is frequently the case at the top edge of laps and joints, or where water is driven by the wind between the overlapping por- tions of sheets, the efflorescence lying there becomes enerusted and gradually hardens, biting into the zinc, and, in course of time, perforating it. 25