DURABILITY ISSUES IN ROOFING
Predicting how long a roof will last is a difficult but vital question for building owners, funders of PFI projects and their professional advisers. It is not a precise science. Formal assessments are often based on limited factual data and assumptions have to be made.
However, understanding the agents that affect the durability of a roof can lead to improvements at the design and specification stages, resulting in a longer service life. Four basic agents must be taken into account: temperature, sunlight, water and freeze-thaw action.
TEMPERATURE The interior parts of a roof are normally in a condition of long-term thermal equilibrium.
In contrast, the exterior surfaces are continuously subjected to thermal changes that can be rapid and of considerable magnitude. External air temperatures can vary between a maximum of 38ºC in summertime (recorded in Kent, August 2003) and a minimum of -27ºC in midwinter (Grampians, January 1982).
The figures in BS 7543 show how the maximum and minimum shade air temperatures vary across the UK. In the summer, the central parts of England, away from the coast, are the hottest, and in winter the Highlands of Scotland can experience the most severe cold temperatures.
On cold, clear nights with no wind, thermal radiation from external surfaces of building materials can have the effect of lowering the surface temperature. The temperature of a roofing membrane can be more than 5ºC lower than the air temperature. Thermal radiation from dark-coloured surfaces is greater than from light-coloured surfaces.
Changes in temperature can alter the following material properties:
? shape or dimensions - materials expand on heating and contract on cooling;
? strength - at low temperatures some materials become brittle;
? rheological (flow) properties - on heating some materials will tend to slump;
? resistance to solvents; and - electrical resistance.
The amount of thermal movement results from the length of the element, the coefficient of expansion and the change in temperature.
Some materials move more than others, as shown in the table. Rubber can expand by up to 60 times more than concrete when subjected to the same change in temperature.
The design and detailing of a roof needs to accommodate the predicted thermal movements, either by allowing movement at joints by sliding; by deforming in buckling or bending; or by internal restraint within rigid elements. Knowing the potential thermal movement and the Young's modulus of the material, it is possible to estimate the induced thermal stress if the element was fully restrained.
For rigid roof systems the effect of thermal changes depends largely on the rate at which they occur and the degree of restraint. Where the component parts of a building are heated or cooled at a uniform and slow rate, the structure can generally accommodate itself through gradual movements at the joints or through creep. However, with a more rapid rise or fall of temperature there will be relatively sudden movement that may cause damage.
Increases in temperature may accelerate irreversible consequences. An increase of 10ºC could double the rate of a chemical reaction, causing deterioration. Other irreversible consequences can be biological decay and creep, such as that found on sloping roof surfaces. Fluctuations of temperature may influence moisture content, crystallisation and leaching actions. As a consequence of a change in viscosity or solvency, two adjacent materials may blend together at a high temperature, a reaction that would not have occurred had lower ambient temperatures been maintained.
SUNLIGHT Astronomers tell us the sun is a ball of burning gas more than 1 million km in diameter with a surface temperature of about 6,000ºC. Part of this enormous amount of heat energy is transferred through the 150 million km of space to the surface of the earth by radiation. As well as giving us visible light that provides illumination during daytime, solar radiation provides electromagnetic energy across a wide spectrum of wavelengths. There are two particular wavebands that affect the long-term performance of roofing and cladding components: surface heating by infrared radiation and surface degradation by ultraviolet radiation.
Infrared radiation is absorbed by all forms of matter, causing an increase in surface temperature that can become markedly greater than the surrounding air temperature.
Infrared wavelengths are just longer than visible light - between 700 and 30nm. The texture, colour and opacity of the surface material have a considerable effect on the amount of infrared radiation absorbed. The absorbed radiation will raise the temperature of a material by an amount dependent upon the specific heat and thermal conductivity of the material, and the structure behind. In clear sunny weather in the UK, black mastic asphalt roofing can reach a temperature of 80ºC. Similar figures have been reported for black bituminous roofing. If a reflective coating is applied the maximum temperature rise can be reduced by 20 to 35 per cent.
For a given surface texture the colour has a significant effect on the absorption of solar radiation. In North America the building codes now recommend that new roofs are specified with white- or lightcoloured surfaces to reduce the solar gain caused by infrared radiation. The absorbancy of various colours is shown in the chart (above left).
A large proportion of the ultraviolet (UV) radiation is absorbed by the atmosphere.
The remainder reaching the earth's surface has no adverse effect on inorganic materials.
However, UV radiation is very important in its potential for deterioration of some organic materials. The penetrating power of the radiation is not great and the action is consequently confined to the exposed surface layers. UV wavelengths are just shorter than visible light - between 290 and 400nm.
Measurements of total UV radiation over the spectral range are of limited value because of the sensitivity of materials to specific wavebands.
For example, polystyrene shows a maximum sensitivity at about 318nm, whereas polypropylene peaks at about 370nm. As a general rule it has been found that radiation shorter than about 360nm tends to cause yellowing and brittleness.
Radiation of a longer wavelength tends to cause fading. Bituminous materials and some synthetic polymers are also degraded by UV radiation, and the changes to surface properties may alter the performance of the material. The combined effects of sunlight, temperature and moisture can cause the yellowing and surface denudation of glass-fibrereinforced polyester rooflights and in recent years manufacturers have taken measures to protect against this.
The intensity of solar radiation depends upon:
? cloud cover, both in type and quantity;
? season of the year;
? local topography; and - inclination and orientation of the roof slope.
Peak solar radiation is likely to occur around noon inland, and during the late morning on the coast. Solar radiation can 2 in the UK. This value ignores losses due to scattering by dust and water-vapour particles and by the earth's atmosphere, which will significantly reduce the value for a large proportion of time. In approximate terms, the radiation received on an overcast day is about one-third of that received if it had been cloudless. On very dull days the proportion would be much less.
The materials we put on the external faces of roofs and cladding need to withstand the long-term effects of infrared heating and ultraviolet degradation. Since the earth's atmosphere absorbs or reflects a significant amount of solar radiation, any climatic changes will directly affect the total amount of radiation a roof surface is exposed to over the life of a building.
Understanding these mechanisms is important for the assessment of the future performance of roof systems.
WAT ER In a typical year almost 600mm of rain will fall on London. The further west we travel, the more rain we experience; 900mm in Cardiff and Manchester, and more than 1,000mm a year in Falmouth on the Cornish coast.
Moisture is always present in the atmosphere in the form of water vapour and when surface temperatures fall sufficiently, condensation can occur. Even surfaces under cover and sheltered from the rain can become thoroughly wetted.
Conditions are particularly conducive to deterioration when moisture condenses in relatively inaccessible crevices, from where subsequent evaporation is slow.
The most significant harmful action on buildings caused by water in a solid form, such as snow or ice, is as structural loading on horizontal surfaces. Problems can sometimes occur when snow is blown into buildings through small holes, particularly into roof spaces. The impact of small hailstones of up to 10mm in diameter generally does little or no damage to most building finishes. On rare occasions hailstones over 75mm across can fall in severe local storms.
These can weigh more than 100g and can smash rooflights and patent glazing and dent metal roof coverings. On average, one of these severe hailstorms may occur in southern Britain once in five years, although they are very localised and the likelihood of one occurring in any given place is small.
Most organic materials and many inorganic materials absorb moisture to varying degrees. For example, clay roof tiles can absorb up to 14 per cent by weight and natural stone and dense concrete up to 10 per cent. The amount of moisture absorbed by wood varies according to the species.
Sapwood can absorb its own weight in water. In general terms, wood will swell as it takes up moisture.
Internal forces in an individual item may be generated as a result of differential wetting and drying or from variations in moisture content. BS 7543 advises that consideration should be given to dimensional tolerances to avoid such developments.
Man-made non-asbestos fibre-cement slates can bow or curl after laying. This can be partly explained by variations in the moisture content of the slate between the coated top face and the uncoated underside, which becomes wet in certain weather conditions, such as when dew forms on cold winter days. The problem with poor dimensional stability for double-lap centre-nailed man-made slates is overcome by the introduction of the copper rivet in the exposed margin of the slate, hooked under the slates in the course below. With some man-made slates, moisture can seep into the material at the sides and lower edges, where water may lay in contact for long periods, causing the slates to discolour around the edges and promoting the growth of moss or lichen in the damp conditions.
Clay roofing tiles made from Etruria marl clay have a good reputation for durability.
This is due to their low water absorbency, of less than 5 per cent to comply with BS EN 1304, compared with other clays that may have higher water absorbency.
Alternatively, water may be present in a gaseous form.
Water vapour's most damaging effects are seen when condensation forms on cool surfaces or within open-pored materials that are cooled on one face. Condensation promotes the growth of biological agents such as surface moulds and rot in timber. It can cause corrosion, as for example on the inside face of profiled-steel cladding, and it can accelerate chemical reactions such as the sulphate attack on mortar that can occur when flue gases condense on the inside face of an unlined brick chimney.
The presence of moisture is likely to result in physical, chemical or biological reactions that in turn may lead to deterioration of some of the material properties, such as:
? corrosion of iron and steel and non-ferrous metals;
? effect of sulphate attack on Portland cement products;
? fungal decay of wood products; and - frost damage.
Water is the principal agent that causes deterioration to roofing products, particularly when combined with other contaminants. Keeping water out of the substrate and allowing the roof construction to breathe and dry out by natural ventilation, helps to prolong the life of a roof system.
FREEZE-THAW ACTION When water turns to ice it expands. If restrained in two directions, the expansion is almost 10 per cent. For example, a 10mm-wide channel filled with water would try to expand to nearly 11mm-wide when the water freezes. This can exert a lot of pressure on the surrounding structure, causing local cracking. The cycle of wetting, freezing then thawing may be repeated many times over a winter season, causing progressive damage.
Materials that absorb water and become saturated are susceptible to freeze-thaw action. Some materials have 'pores', or small voids, between the particles of permeable solids. The term 'pore' usually applies to spaces into which water will penetrate by capillary action. Frost damage occurs when there is sufficient water entrapped in pores to disrupt the material on freezing.
The wetting may be due to rainfall or to driving rain. Also, melt water from a partial thaw of snow or ice is a common precursor to damage, since it is likely to produce saturation by water in a pure form just above freezing point with no opportunity for any subsequent evaporation to occur.
The air temperature at which frost damage occurs on inhabited buildings needs to be substantially below 0ºC, otherwise water may not freeze in small spaces. For this reason, the number of times the air temperature becomes less than 0ºC is unlikely to give an accurate indication of the number of freeze-thaw cycles in the material. 'Zero transitions' may, however, provide a useful means of ranking the frost resistance of porous materials.
One approach to predicting the potential for frost damage is to classify a site as 'severely exposed' to frost when all the following factors apply:
? average annual frost incidence is in excess of 60 days;
? average annual rainfall is in excess of 1,000 mm; and - altitude of the site is in excess of 91m above sea level.
The saturation of the material is an important factor. Frost damage can be reduced significantly by protecting the building components from saturation by detailing projecting eaves and cappings.
Materials that are at a greater risk of frost damage include roof tiles, fibre-cement sheeting and cellular-glass insulation.
Expansion of ice in the pore structure of a roof tile can cause pieces to break off.
External temperatures need to be well below freezing to cause damage, since the roof will receive some heat from inside the building. North-facing and low-pitched roof surfaces are generally at higher risk because they tend to be cooler and wetter than other areas.
Normally replacement of individual tiles can be undertaken, although the problem may become worse as the roof becomes older, until full replacement is ultimately required. Freeze-thaw tests on individual roof tiles are straightforward to carry out, although they represent unique conditions that rarely occur in practice in exactly the same circumstances.
Uncoated fibre-cement sheeting absorbs water. If the saturated sheets subsequently freeze, such as on unheated buildings in winter months, any fissures or cracks in the sheet can progressively become enlarged.
Cellular glass insulation is generally resistant to moisture.
However, if there is water in contact with the cut cells at the edges and if it subsequently freezes and expands, the insulation progressively breaks down into a black powder. Tests have found that after 25 freezethaw cycles, the wet cellular glass had less than 20 per cent of its original insulation value.
The expansion of ice in cracks, fissures and pores can induce large stresses that are repeated many times over a winter season. This is the principal mechanism for breaking down massive rocks on exposed mountain ranges over periods of millions of years. The challenge to the roofing industry is to design, manufacture, build and maintain relatively thin components that don't become saturated and, if they do, can withstand the induced mechanical stresses from freeze-thaw action over a period of three or four decades.