Precautions with Aluminium Roofing Material

New Zealand Journal of Agriculture, Volume 83, Issue 4, 15 October 1951, Page 300

Precautions with Aluminium Roofing Material

roofing has been employed widely for farm buildings in recent years, but ' the precautions necessary when using the material are not well known among laymen. The following article, based on a report prepared by the Dominion Laboratory, gives guidance in the handling of aluminium.

SHEET aluminium alloys vary widely in tensile strength and resistance to corrosion. Most aluminium alloys suffer pitting-type corrosion in weathering, but experience has shown that with some alloys the initial pitting slows down with time, that is the action is -stifling. Aluminium roofing must be thick enough to survive any initial pitting, and overseas experience has shown that the following alloys and gauges are suitable: — 3S (8.A.60), minimum 22-23 (standard wire gauge). AW4, minimum 22-23 s.w.g. AWIO, minimum 21-22 s.w.g. 2S, minimum 18-20 s.w.g. Even thinner gauges of alclad are superior to the above, but are not yet available commercially in New Zealand. Installation The most important factor affecting the rate of deterioration of an aluminium roof is the care taken in installation; and the greatest single factor in installation is avoidance of contact between aluminium sheets and any other metals except aluminium or hot-dip galvanised iron. Contact with metals such as lead, copper, etc., will cause perforation of the aluminium; so prone is aluminium to this form of attack that it has been known to fail merely from contact with rain-water which has flowed over copper before reaching the aluminium. Sheets should be nailed with aluminium (duralumin) nails or, if these are not available, hot-dip galvanised nails. All saddles, brackets, spoutings, rainwater fittings, etc., must be aluminium or hot-dip galvanised iron. Soft aluminium sheet only should be used for flashings. Lead and copper flashings should not be used, because even if they do not touch the roof, rain-water by running over them may be contaminated and cause pitting of the roof. It is recommended that use of leadheaded nails be avoided, but where their use is unavoidable adequate washers of neoprene rubber or bituminous roofing material should be inserted between the nail heads and the aluminium. ■ Sheets before being nailed down can be pierced with a drill or sharp steel probe, as this saves the surface from being dented by blunt nails. Drilling is especially helpful at the overlap of 4 sheets. A loose washer of soft aluminium or neoprene rubber under the head of the nail improves weather proofing. (A fabric washer impregnated with zinc chromate can also be used.) Some engineers consider the slope of an aluminium roof an important factor in removing moisture from the laps and so eliminating undue corrosion. A minimum slope of 3in. per foot is recommended. Ventilation of the space under the aluminium sheeting is also important, and if there is possibility of prolonged wet conditions without free

access of air, it is advisable to give the undersides of sheets a coat of zinc chromate primer. The sheets must not come into contact with wet cement or plaster. These materials should preferably be weathered for about a month and then protected by bituminous paint and a layer of bituminous paper before the aluminium is laid. (Where contact with wet cement cannot be avoided the use of aluminous cements should be considered, as these produce no free lime on setting.) Contact with green or resinous timber should also be prevented by a layer of tar-free bituminous paper. Alternatively, the timber could be given a coat of bituminous or aluminium paint. . Aluminium sheets should never be joined by soldering, as the joint will soon disintegrate in the weather. Jointing is best done by riveting, welding, or flame brazing. Unless a special non-corrosive flux is used all flux residues must be scrubbed off with hot water. When bending sheets along the line of corrugations try .to locate the bend to continue the curvature of a corrugation. Do not scribe the bending line or re-straighten and re-bend the sheet, as such treatment may cause cracking. Painting For aesthetic reasons many users will prefer to paint aluminium roofs, and painting, if correctly carried out, protects the metal. However, painting will remove one of the advantages claimed for aluminium, namely, that its brilliant reflection keeps the space beneath it cooler than under a painted galvanised-iron roof. Against this must be balanced. the unsightly appearance of the polished aluminium roof and the fact that in a coastal atmosphere the metal will in any case lose its lustre in time. It is advisable to paint the laps (both contact surfaces) with zinc chromate primer before laying the sheets in place. To ensure good paint adhesion the metal should be cleaned and etched. This is best done before fixing the sheets in place to avoid ' trapping chemicals in crevices. Proceed as follows: 1. Remove obvious grease by swabbing sheets with mineral turpentine. 2. Swab the sheets with a warm solution of Jib. of washing soda and Jib. of trisodium phosphate in 10 gallons of water; take care not to get this solution into crevices. 3. Wash sheets well with water, paying particular attention to places where the soda may be trapped. 4. Brush on a solution of 5 per cent, of phosphoric acid in water. 5. Wash sheets well with water. Other alkaline cleansers could be used in place of 2.

When the metal is thoroughly dry a coat of zinc chromate primer can be applied, followed by a finishing coat of the required colour. Paints containing lead or copper compounds should be avoided. Repainting should be done while the existing coat is still intact. Collection of Drinking Water Water collected from an unpainted aluminium roof is entirely suited for human consumption. If the roof has been painted, it is advisable to wait until it has been well washed by several heavy showers before collecting water for culinary purposes. Effect of Destructive Agencies Bird droppings: The Dominion Laboratory has carried out long-term exposures of various aluminium roofing materials at sites exposed to both seagull and pigeon droppings. Apart from causing a slight surface roughening of the metal the droppings have no effect on unpainted aluminium. Artificial manures: Experiments have shown that there is no risk of aluminium sheeting deteriorating seriously when used to roof sheds where superphosphate or agricultural lime is stored and handled, even when dust can rise and collect in the laps. Lack of access for air: If aluminium roofing sheets are stacked in wet conditions in such a manner that free access of air to the surface of the sheets is prevented, they may undergo rapid pitting. If aluminium roofing is freely exposed to the air as single sheets, this action does not take place. If lengthy stacking is necessary, sheets are best placed on end and preferably should be oiled. Thermal regions: The sulphuretted hydrogen present in the atmosphere at Rotorua does not affect aluminium; roofs in that area should last as long as elsewhere. ■ Effect of timber on aluminium nails: Many green sappy timbers are corrosive to aluminium, especially if these are in contact without free access of air. When using aluminium (duralumin) nails it is wise to employ more than where ordinary lead-headed nails are used, about 25 per cent, more, unless the underlying timber is thoroughly dry. Contact with rusty galvanised iron: Although contact with new hot-dipped galvanised iron will not cause corrosion of aluminium, contact with rusty iron tends to cause pitting. Aluminium sheets are therefore not recommended for repairing galvanised-iron roofs. Heat expansion: Soft aluminium sheets have a heat expansion twice that of iron, but under New Zealand conditions the movement produced by expansion will not cause serious tearing. Earthquakes: A sheet metal roof is less likely .to be damaged by earthquakes than is a tiled roof and it offers less risk to occupants than does the latter.