Thermal behaviour of the slate

Temperatures reached by the slate on the roof

Once the slate cover is finished, each slate tile receives direct sunlight. Since this rock type has generally dark tone, the incidence of sunlight makes its temperature rises several degrees above the temperature of the air. When designing the roof, the effect of thermal expansion must be taken into account. The thermal expansion causes that each slate tile increases or decreases its volume depending on the temperature. Generally, the slater takes into account this effect, leaving enough space between the tiles. The variation in volume is measured by the coefficient of temperature variation, which for the slate  is estimated between 9.0×10-6 ·°C-1 and 6.5×10-6·°C-1.The linear increase in size for a slate tile can be calculated by using the formula R = X·L·t, ​​where R is the size increase in size, X the coefficient of temperature variation, L the length (in meters) and t the range of temperatures reached by the slate. For example, for a single slate tile with the following conditions:

L = 30 cm = 0.3 m

Minimum Temperature = -10 °C

Maximum temperature = 60 °C               R = 0.0000086 x 0.3 x 70 = 0.0001806 m

Temperature difference = 70 °C

X = 8.6×10-6·°C-1

Although this value is low, the sum of the total variations in size of all tiles is important for the whole cover.

This ratio should not be confused with the coefficient of thermal conductivity, defined as the heat transmitted through a body. For the slate, this thermal conductivity coefficient is estimated as 0.43 kcal/hour·°C·m-1 (1), lower than for the concrete, so in principle the slate should thermally insulate more efficiently than the concrete, considering two identical volumes of both materials.

Sunlight raises several °C the temperature of the slate. Since 2006 I have been measuring the temperature in a slate tile placed in a roof, together with the air temperature (Figure A).


During the winter months, the slate has lower temperatures than that of the air, but during the summer months the slate temperature is greater than that of the air. The measures show that when the slate does not receive sunlight (Figure B), the slate temperature is slightly below the temperature of the air, but when the slate receives direct sunlight (Figure C), its temperature raises, with a measured difference of 40 °C with the air temperature.


Installation of the thermal probe on the roof

Finally, the existence of discontinuities in the rock (microfolding, sandy levels, quartz veins) may cause tile rupture in some cases, so you have to be careful with this type of defects depending on the geographical area where the slate is going to be used.

(1) Menéndez Seigas JL. Architecture and techniques of slate roofing: Asociación Galega de Pizarristas; 2007. ISBN 84-920981-1-2