Are the small white-centred chips on my clay tiles harmful to the tile’s durability?

The small pits which are occasionally visible on the surface of clay products are created when pockets of lime immediately below the surface expand, causing the surface above the lime to be pushed up or ‘blown’. This expansion takes place as soon as the tiles leave the kiln as the tiles absorb moisture from the atmosphere.

Lime occurs naturally in most clay and can usually be neutralised, ie prevented from expanding, by submerging the tiles in water. Manufacturers make every effort to prevent lime blows, although occasionally the process can still occur before the tiles have been fully soaked.  The expansion action of the lime only occurs immediately after the tiles leave the kiln. The process stops once the tiles have absorbed moisture and can not re-occur. Therefore there is no risk of further ‘pitting’ to the tile surface after the tiles have been laid on the roof.

It is a common misconception that clay products can be attacked by frost action due irregularities within the surface finish. But there is no possibility that the small pits, or ‘lime blows’, will affect the future durability of the tiles. 

The European Standard for clay roofing tiles, BS EN 1304, does not regard small pits or chips of 7 mm or less in size in the surface of the tiles as faults.