Roof Coverings
When we talk about roof covering, we mean the non-load-bearing clothing of the roof. in Rural Building, we deal with three different types of roof coverings. These are:
- corrugated aluminium sheets,
- corrugated galvanized iron sheets and
- corrugated asbestos cement sheets.
All of these sheets are corrugated because that makes them stiffer so that they can go across the gap between the purlins without sagging.
CORRUGATED ALUMINIUM SHEETS
This roof covering is lighter and far more durable than the other types. Lighthess is important so that the sheets can be easily transported. Aluminium sheets are rust-proof, easily installed and have a bright reflective surface which helps to keep the building cool.
However, the thinner gauge sheets are especially vulnerable to being dented, punctured, or torn off by storms. The sheets also creak a lot when temperature changes make them expand or contract.
Aluminium roof covering is available in a variety of shapes and sizes. The sheets can be from 61 to 76 cm wide and 200 to 400 cm long. Sheets up to 18 metres in length can be ordered from the factory.
In Rural Building, we mostly deal with sheets which are 61 cm wide and 244 cm long. This size is the most commonly used and readily available.
The sheets are available in different thicknesses or gauges. The thinnest is 26 SWG (Standard Wire Gauge; 26 SWG = 0,446 mm) and the thickest is 18 SWG (= 1,22 mm).
Store the sheets in a dry place. Avoid putting them in contact with fertilizer, lime or cement.
CORRUGATED GALVANIZED IRON SHEETS
These are steel shests which are corrugated and galvanized on both sides. They are usually thicker and heavier than aluminium sheets, so they lack some of the drawbacks of the aluminium, like being easily punctured or torn away in storms.
The most common size of these sheets is 61 by 244 cm.
CORRUGATED ASBESTOS CEMENT SHEETS
This kind of roofing sheet is also widely used. The disadvantage of these is that they are brittle and break easily if they are walked on or if a heavy object falls on them. They can also break if the roof construction warps due to the humidity changes in different seasons. Another problem is that they are heavy, and so they are difficult to transport.
The standard size is again 61 by 244 cm.
HOW TO ORDER SHEETS
When ordering sheets, you must keep in mind that the overlap of two corrugations on each sheet must be subtracted to find the effective width of the sheet.
RIDGE CAPS
Ready made caps for covering the ridge of the roof are also available. They are made from aluminium, galvanized iron or asbestos cement, to fit with the roofing sheet material.
Ridge caps can be made locally out of aluminium or galvanized iron sheets as will be explained when we come to roof construction.
The caps have to overlap the sheets on both sides of the roof. in the case of a roof which has the ridge along the direction of the prevailing wind the cap must overlap the sheets by 20 cm on each side. If the ridge lies across the direction of the prevailing wind (north - south) the cap has to overlap by 30 cm on each side.
