Protecting a water-sensitive substrate

Plaster will lose nearly all of its cohesive strength if it gets wet for any extended period.

Plasterboard has a paper face which also loses strength when wet.

Cement-based grouts, whilst being unaffected by water once set, are porous and will therefore allow water to seep through.<br.<br /> If the joint is not completely filled with grout then of course this will allow water through.

Normal wear and tear from traffic and cleaning will erode the grout over time.

The action of various chemicals, such as cleaning liquids can gradually weaken the grout.

Either or both of these actions can reduce the ability of the grout joint to resist the passage of water.

Grout joints in corners between tiled surfaces and at junctions between dissimilar backgrounds should be filled with a flexible sealant to allow some movement between the surfaces.

However, these critical joints are often filled with the same grout that has been used for the rest of the area. The grout will almost certainly crack in time, leaving an opening for water passage.

Water-sensitive substrates such as plywood, plaster and plasterboard can be protected from damage by any water that penetrates the tiling layer, by the application of a surface waterproofing layer, known as a tanking system. The most likely places for leaks are in internal corners and around pipes, plugholes, trim etc. so these areas must be treated with care.