Spalling of Natural Stone
Spalling is a term used to describe the surface damage to paving slabs caused by a number
of different reasons. On porcelain paving it may refer to damage to the edges of slabs, either
caused by being physically chipped during handling and laying, or when the slabs are laid too
close together, and any slight movement causes one edge to rub against the other. (This is the
primary reason for recommending 5 -6mm joints between slabs)
In similar fashion, bricks laid butt jointed or neighbour against neighbour with no tolerances
or jointing material, when any movement – thermal or physical – will cause the two edges to
rub together and spalling may occur.
Spalling may take place to the surface of certain natural stones, where the process of being
rived or riven (split along a natural stratification within the layers of sandstone) leave micro
flakes on the undressed surface. Larger flakes or excrescences that occur during the process of
riving may be removed by the mason at the point of quarrying, signs of which may be scratch
marks made by a combed hammer. These are not detrimental, and show that the stone has
been professionally produced.
Slate is another natural paving stone that is susceptible to spalling, with flakes becoming loose
during the lifetime of the material, and unless it becomes problematic (the flaking causing
dips and hollows within the slab) it can be ignored.
