A white ‘bloom’ has appeared on my roof tiles. What is it, and will it harm the tiles?

This effect is called efflorescence and occurs mainly in concrete tiles. Efflorescence is a naturally occurring phenomenon caused by water in the form of rain, condensation or dew penetrating into the pores of the concrete and dissolving lime. The solution diffuses to the surface of the product; the water then evaporates and leaves behind a white film of lime. The lime naturally occurs in the cement which is used in the manufacture of all concrete products.

Since the lime content of the concrete can vary and the weather conditions obviously differ, the level of efflorescence can also fluctuate considerably. The same chemical process which brings the lime to the surface of a tile carries on, enabling it to be degraded and washed away by the rain, so that eventually the efflorescence disappears by itself – usually in a matter of months. Once the lime has disappeared from the surface of the tiles it rarely reoccurs.

If you want to be sure to avoid the possibility of any such problems, use a manufacturer that applies surface treatments on their concrete tiles during manufacture, this generally eliminates any efflorescence.