External tiling

Most cement-based products including grouts and adhesives, are porous to some extent.

They have minute voids left by the evaporation of un-combined water or by gaps between the aggregates.

These pores allow water to permeate through the grout into the adhesive and substrate.

Water expands on freezing and in doing so in a confined space, generates enormous forces. In cold climates the cumulative effect of repetitive cycles of freeze (expand)/thaw (contract) causes rocks to fracture. This action, known as frost shattering is part of the mechanism for weathering.

For tiling this has several implications: Pores in the grout will be vulnerable to frost damage. Any voids behind the tile provide a possibility for water to gather and then on freezing, push off the tile.

Interior tiling situations tend to be maintained at more or less constant temperatures. Exterior installations, however, are exposed to much larger cyclic temperature swings.

With changes in temperature different materials expand and contract at different rates. When they are fixed rigidly together this causes stresses to build at the interface between the materials. If the stress gets high enough it can exceed the force that binds the two materials and delamination occurs.

A cement-based adhesive must be used for exterior work. Whilst standard cementbased tile adhesives are suitable for exterior use, highly polymer-modified adhesives offer enhanced bond strength, reduced porosity and therefore better resistance to frost & greater resistance to movement (thermally induced movement is inevitable particularly for walls). Tiles recommended for exterior use will often be fully vitrified and these also require a higher specification for the adhesive.