Way In Building & Civil Construction

Metallic covers:

This type of roof is suitable for light roofs with low slope and for complicated shaped roofs. One of the major drawbacks is that they barely protect against temperature fluctuations so a good insulation system must be provided as well as significant amounts of condensation water.

Zinc, copper, lead, aluminium and galvanised steel sheets (pre-painted, corrugated and self-supporting) are used as materials for the metal roofs.

Zinc sheets have the characteristic that they can be plastically deformed only at temperatures of 120oC to 140oC, however they can be cold bent and folded to about -8oC, at lower temperatures they are brittle and cannot be worked.

When exposed to air, it is covered with a whitish grey layer of zinc carbonate, insoluble in water and preserving the metal from new oxidation, but if exposure to atmospheric agents is very prolonged, it modifies its structure, becoming more crystalline and fragile, reducing its useful life. Not only the acid or saline contents of the air attack the zinc plates, but also the condensation water and the mortars of lime, cement or plaster. They must be kept out of contact with other metals in order to avoid electrolytic actions. Soot crusts combined with rainwater also result in electrolytic corrosion.

It should be taken into account when using this type of material that its coefficient of expansion is very high (with a temperature difference of 50oC a strip of sheet metal of this metal lengthens or shrinks 1.47mm per linear meter, while in the same circumstances copper varies 0.86mm and iron 0.61mm. Therefore, when using this material, it should be taken into account that since the covers are exposed to continuous thermal variations if the cover has welds and nails, the metal tears when contracting, and if it is fixed at its ends, it becomes brittle due to the large number of times it is bent and unfolded. In order to avoid this obstacle, a positioning system is sought that allows the metal to expand and contract freely on the boarding with a certain play in the joints and always remaining flat.

The first precaution to be taken is to use sheets of the smallest possible width, in order to avoid warping caused by uneven expansions, and of great length, in order to reduce the number of horizontal joints.
The copper sheet has an almost unlimited duration. The copper exposed to the air is progressively oxidized and covered with a layer that is usually called patina, which at first is brownish in colour and then turns bluish green. Contact with other metals should be avoided except the bronze pieces, with copper linings. Copper sheets are too expensive to be used as a covering material (they are not used today).

Lead sheets are characterised by their high plasticity. They are manufactured as soft sheet or as hard sheet. The hard sheet is mainly used to waterproof joints or file holes. They are insensitive to contact with other metals (almost electrolytically neutral) and insensitive to atmospheric agents. It is not used to completely cover roofs. Aluminium sheets are increasingly used because of their high corrosion resistance. When aluminium is in contact with masonry, alkaline substances from cement setting combined with moisture attack aluminium. Contact with raw steel and copper-based metals should also be avoided. All contact points should be protected with zinc chromate paint or asphalt paint. The contact of aluminium alloys with galvanised steel is not dangerous. In the case of stainless steel or phosphates, a simple painting is recommended. Contact with moisture in wood (even if impregnated wood) should also be avoided by painting the aluminium.
Whichever metal sheet is used for the roof, it should be borne in mind that only one kind of metal should be used, because if there are points where two different metals come into contact, electrolytic corrosion occurs due to the action of humidity.
The following is a summary of the characteristics of some of the most common metal sheets: GALVANIZED PLATES:
Steel sheet with zinc coating that protects them from cracks or detachments.
Great resistance to climatic changes.


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