You've got your pitched roof, but how do you cover it? Peter Mayer of Building Performance Group considers the whole-life costs of a variety of roofing options, from bitumen to concrete to metal to tiling …
Roof coverings have a tough job. They have to keep rain, wind and snow out; resist frost action; withstand high daytime and low night-time temperatures; and cope with pollution, ultraviolet radiation and the adverse effects of plants, lichens and mosses.
Furthermore, there is compelling evidence from meteorological records that the weather is getting worse. There has been a gradual increase in the annual number of severe storms, and about 33 tornadoes are now reported in the UK every year.
<B>Factors affecting durability</b>
In view of the uncertain climatic effects of global warming, a case can be made to overspecify the nailing frequency and pattern for tiles. This would reduce the risk of roof-covering displacement due to severe wind uplift. In any case, tiles should always be nailed at edges such as verges, eaves, ridges, hips and valleys.
Use non-ferrous fixings such as copper, aluminium or stainless steel for enhanced durability. The zinc galvanizing on nails may fail during installation or in use as the wind moves the slates or tiles and rubs the galvanizing off.
Ensure the diameter of the nail shank and the nail length is suitable for the exposure of the site. The British Standard for Slating and Tiling, BS 5534, gives detailed guidance. More resistance to wind uplift is achieved by using improved nails such as ring-shanked or helical-threaded shank nails.
Most types of fibre cement slates are held by a copper rivet at the tail of the slate, while bituminous shingles are nailed and adhered with a mastic or roofing adhesive.
<B><I>Holding down ridge and hip tiles</I></b>
Mortar correctly mixed can provide adequate bond (the tensile bond strength) to hold down concrete or clay ridge or hip tiles for low-rise domestic buildings in a sheltered environment. However, experience following damage to roofs during gales suggests tensile bond strength deteriorates over time or the mixes are not gauged to provide sufficient tensile bond strength. Mortar used for holding down ridge or hip tiles should be tested or designed from established tables of mortar tensile adhesion strength. Alternative approaches include using ridge or hip tiles with mechanical fixing as well as mortar bedding, or using a dry-fix system.
<B><I>Resistance to chemical deterioration</I></b>
Roofing products containing lime or limestone may be eroded in industrial areas with high concentrations of sulphur or nitrogen acid gases, or in marine areas that have high salt concentrations and high humidity. If using natural slates in industrial or marine conditions ensure the slates conform to the highest acid resistance class to BS 680 as well as the wetting and drying test.
<B><I>Resistance to frost attack</I></b>
Vulnerability to frost attack depends on the capacity of the roof covering to absorb water, its pore structure, the frequency of freeze–thaw cycles and rainfall regime. Specify roof coverings that are covered by tests assuring resistance to frost attack, such as the BS EN 12326–2 freeze-thaw test.
<B><I>Resistance to ultraviolet light</I></b>
The ultraviolet light component of sunlight can adversely affect the resins in artificial slates, although this typically results in changes in the slates' appearance rather than a physical deterioration.
Bituminous products become brittle when exposed to ultraviolet light. Thermally induced movement will result in cracks and failure. The deleterious effects of ultraviolet light can be minimised by using the bitumen shingles modified with Atactic polypropylene (APP) or Acrylonitrile butadiene styrene (ABS).
A coating of mineral granules resistant to ultraviolet light will protect bituminous shingles for as long as the coating remains. Shingles to BS EN 544 are tested for adhesion of mineral granules.
Lichens and mosses tend to become established on roofs where the pitch is low or where the roof face is sheltered or north-facing. Plants, lichens and mosses are part of the process that erodes and converts rock to soil, and the same processes can take place on roof coverings. Lichens and mosses retain moisture in contact with the roof covering, increasing the risk of freeze–thaw damage. The plants feed on the minerals in the roof covering, leading to surface deterioration and ultimately spalling, cracking and disintegration.
A toxic liquid may be used provided it has been tested to ensure it will not damage the covering. Alternatively, lichens and mosses may be scraped off with a long pole scraper, taking care not to damage the roof coverings and to safeguard those carrying out the work. A more permanent solution is to fix a copper wire across the roof, which releases copper salts during rain.
<B>Modes of failure</b>
<B><I>Displacement or slipping of slate and tiles</b></I>
- This may be caused by corrosion of the nails that secure the slates or tiles to their tiling battens and is known as "nail sickness".
- Clay or concrete nibs failure will result in tiles slipping.
- A common reason for tiles to become dislodged is the suction effects of strong gusts of wind.
- <B><I>Surface deterioration</I></b>
- In industrial or marine environments, concrete tiles and artificial slates may suffer surface etching to expose aggregates or fibres.
- Steel tiles will corrode at cut edges unless treated.
- <B><I>Cupping and curling of artificial slates</I></B>
- This may be due to differential moisture absorption or thermal response of the slates. Cementitious slates may undergo carbonation, which changes the crystalline structure of the cement and distorts the slate's shape. Depending on the degree of cupping or curling, this may affect appearance or, if severe, could compromise the weatherproofing function of the covering.
- <B><I>Delamination and spalling of concrete or clay tiles</I></B>
- This is commonly the result of frost action, particularly on low-pitched roofs where rainwater run-off is slow, allowing water to be absorbed by the tiles. Delamination may also be the result of the under-firing of clay tiles during manufacture, resulting in excessive soluble salts, which lowers the frost resistance of the units.
- <B><I>Disintegration and delamination of natural slates</I></B>
- Premature disintegration may occur where the slates include iron pyrites, which oxidise, or sulphuric acid in the atmosphere, which attacks calcium carbonate in the slates. Visible damage to the slates includes powdering, flaking and blistering. If clay minerals are present in the slates, delamination can occur.