Galvanised corrugated sheet iron has an extensive application in roofing work. It is comparatively cheap, and when properly galvanised fairly durable. There is much dispute as to the length of time it will last. No definite "life," however, can be assigned to a galvanised iron roof, except all the conditions are fully known, and these are most difficult to determine, the length of time it will last depending upon the quality of iron and galvanising, thickness of sheet, and the kind of atmosphere the roof is placed in. In the sulphurous atmosphere of a manufacturing town it is probable that galvanised iron will not last more than one-quarter the time that it will in a pure country air. And, again, it will last longer in a dry atmosphere than in a moist one.

Galvanised iron is iron coated with zinc, and this latter metal has the distinct advantage of forming an oxide on its surface that is not dissolved by ordinary rainwater. If the water, however, becomes by any means acid, as it does in the neighbourhood of towns by dissolving the acid fumes, then this protecting film of oxide is eaten away, the coating of zinc soon disappearing and the sheet iron rusting into holes. When the galvanised iron begins to show signs of deterioration, it is a good plan to at once paint it, and to follow this up periodically. A good paint to use is one of a metallic oxide character. Common tar should not be used, as this is not a good medium for protecting sheet iron.

In fixing corrugated sheets it is usual to have a side lap of one corrugation, as shown in Fig. 175 {a), and this should be arranged the same way up as in the sketch, and not upside down, as one occasionally finds sheets erected. In the latter case, the joints are almost sure to leak. A safer joint is (b), in which two corrugations are lapped over. This makes a much better job, but adds somewhat to the cost, both in labour and material. The end laps of sheets run about 6 in., sometimes less, and occasionally more. The longer lap is always preferable, especially if the roof is flat. Where much snow is likely to lodge a large end lap is the safest, so as to avoid as much as possible the back-ing-up of the water. It is usual to fix a washer (made out of about 16 gauge iron) on the rivet before hammering down and snapping. This is to avoid leakage around the rivet. Sometimes rubber washers are used in addition, and on very exceptional occasions canvas packing is placed between the joints. It should be remembered that, however carefully galvanised iron is fastened to timber, the holes in the sheets are bound to pull a little and get loose on account of the difference in expansion and contraction, due to changes of temperature, between metal and wood. On a wholly iron structure this is not so bad; but even in this the intensity of the sun's heat-rays will cause a greater expansion in the galvanised sheet iron than in the framework underneath.

Roofing Work Galvanised Sheets And Gutter Angles 190

Fig. 175.

Tiles are sometimes formed out of galvanised sheet iron, and also out of sheet zinc, as shown in Fig. 175 (c). They are lapped over and nailed or riveted the same as corrugated iron. Sheet zinc for roofing purposes unfortunately has a high degree of expansion and contraction for changes of temperature, and, therefore, should never be fastened together in long lengths. A method that can be followed to overcome this difficulty is explained by Fig. 175 (d). The timber section represents a rafter or roll, to the bottom of which a flat plate is secured. The sheets have the edges turned up on each side, and dropping in between the rolls rest on the flat plate. The caps can be made up in short lengths (say, about 3 ft.), and fitted over roll and edge of sheet, and lapped over each other. No nails or screws should pass through the joints, so as to allow perfect freedom for expansion and contraction.

In the case of curved sheets of either corrugated iron or zinc there is not so much trouble with expansion and contraction, as the change of length is taken up in increased or decreased curvature.