Hello slate people
The roof is probably the most exposed part of any house. Rain, wind, hail, snow, pollution, sunlight… are continuously exerting great pressure on the roof. Roofing materials must be then reliable and durable. However, even the most durable materials are finally modified by the environmental conditions. Today we are going to take a look to these weathering processes, how can affect the integrity of the roof, and their effect on roofing slates.
Sunlight and temperature variations
Usually, the energy provided by direct sunlight is about 1 kW h (Kilowatt per hour) per square meter. Roofing slate, a dark material, can reach up to 80 °C (or 176 °F) in full sunlight during the summer. During night, the temperature can drop down to 20 °C (or 68 °F). This daily temperature variation creates a continuous stress due to thermal expansion-contraction, which can lead to fracture in artificial materials such as plastic, asphalt, steel or fibrocement. Luckily, this is not the case of roofing slate. The internal structure of roofing slate is a dense, highly organized fabric of small minerals forming a homogeneous rock, which adapts uniformly to thermal stress. Roofing slates do not break by this reason, unlike other materials.
UV rays degradation
Photons, which sunlight is composed of, provide energy to the roof. About 5% of this energy is in the form of ultraviolet photons, with energies larger than 3 eV. These photons can destroy chemical bonds, especially in organic materials such as plastic, wood and asphalt. The result is the cracking and eventually collapsing of the roof. Another effect of the ultraviolet radiation is oxidation. Roofing slates resist UV radiation without problems; their mineralogical composition is not affected by this radiation. Only in some cases, when iron sulphides (pyrites) are present, we can have some oxidation processes, but never damage of the structural integrity of the slate.
Wind exerts forces on roofing. The taller the structure, the higher the force experienced by the wind. In high-wind areas this can be an important problem. Besides the danger of catastrophic failure, the intermittent vibrations caused by the wind can lead to material fatigue. This problem is usually avoided by a correct installation, but again roofing slate has an advantage over the rest of roofing materials, which is the weight and strength. Slate shingles are solid, and when properly installed they can stand the wind without any problem. Just think on the historical slate roofs of the Highlands, which have resisted the fierce winds of Scotland for generations.
Moisture: rain, hail, snow, frost and dew
Rain is not usually a big issue for roofs, it tends to clean the surface. Hail is something different. Depending on the location, it can hardly hit the roof, even breaking some parts of the cover. As for the rest of roofing materials, slate can be pierced by large size hail. However, a slate roof is designed to keep working even with some broken pieces. Failures due to hail are very rare, this is not a serious threat for a slate roof. The effect of the snow load is the weight, which can stand on the roof for many months. One meter of wet snow roughly creates a load of 4 MPa on a roof. A normal roofing slate can usually resists more than 50 MPa, so we would only start worrying when the snow height reaches more than 12 meters. Ice and other types of snow are denser, but again the accumulation on the roof should be very high before any problem occurs. Also occurring in cold areas, freeze-thaw is a process that can threaten the integrity of the roof. In porous roofing materials, such as fiber-cement, clay tiles or wood, water fills in the pores. When this water freezes, increases its volume, putting pressure on the material, until it thaws again. This process repeated several times usually leads to the failure of the material. Roofing slate is also a porous material, but these pores are very small and scarce, so it is very unlikely that the slates can absorb water and hence be affected by this process. Average water absorption values for roofing slates are between 0.2 and 0.4%. Finally, dew can be a serious threat for roofing materials such as wood. As moisture increases, wood swells, while drying means shrinkage. Generally speaking, volume changes are not desirable in any construction material. Slate is, by far, the most reliable cover you can use on a roof.
Atmospheric gases/acid rain
Since the industrial revolution, the chemical balance of the atmosphere has changed. Chemical components coming from the combustion of fossil fuels are reactive with some components of roofing materials. Some roofing slates may have small amounts of iron sulphides and/or carbonates in their composition. These minerals can be affected by atmospheric pollutants and develop some alterations such as oxidation and/or gypsification. Anyway, this is just an aesthetical effect; it is very unlikely that these alterations jeopardize the waterproofness of the cover. On the other hand, acid rain attacks not just the roofing material but also other parts of the cover, such as gutters, nails and metallic components. Atmospheric contamination is the dark hand lying behind most of the problems we have nowadays with roofing materials.
Biological colonization can be a problem for some materials. The accumulation of dead colonies of organisms is visible as dark stains on a light-colored roof. Not for slate, as I posted a couple of months ago, roofing slate is the less prone material susceptible to this biological colonization.
I am not going to say that roofing slate is the best material you can use on your cover, because as a scientist, I know there is not absolute truth. But I wouldn´t accept any other roofing material in the house my wife and I are planning to build one of these days.
Berdahl, P., Akbari, H., Levinson, R. and Miller, W.A., 2008, Weathering of roofing materials – An overview. Construction and Building Materials, v. 22, n°. 4, pp. 423-433.