Tiling showers, bathrooms and wet rooms

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.

Voids left in the adhesive layer after bedding the tiles will allow any water that penetrates the grout or sealant to gather.

If this water has nowhere to escape to it may encourage mould growth.

Alternatively the trapped water may run along channels left in the adhesive bed and leak out at a different point to its entry.

Some substrates, particularly gypsum plaster and plasterboard, have very little strength when wet.

Continued water ingress will over time cause the substrate to fail.

Most ready-mixed adhesives are water-based. They set by allowing the water to dry out and form a strong hard bond.

However, when they are exposed to water they tend to weaken. Some are more resistant than others but almost all are affected eventually.

There are too many possible combinations of product to list so below are three main levels in a hierarchy of water resistance based on the fact that:

1. Class D1 paste adhesives are less resistant than Class D2 paste adhesives, which are less resistant than cement-based adhesives.

2.Standard grouts are more porous than modified grouts, which are more porous than epoxy grouts.

3. Plaster and plasterboard are more affected by water than render, both of which are even better when tanked.