Should I let a damp company replaster my wall in sand and cement?

The injection is the headline of a damp treatment but the replastering is the money and the mess. It is almost always specified as a cement-based backing coat containing a waterproofing additive, applied to a metre or so up the wall, with a finish coat over it. Understanding what that layer does — and what it prevents — is how you judge whether to accept the specification you have been given.

Step by step

1. Understand what the render is for.It is not there to dry the wall. It is there to hold back residual moisture and hygroscopic salts so that they cannot reach the decorated surface. It is a management layer, and it works by being impermeable.
2. Ask what the wall is built of.A modern cavity wall with a sound inner leaf can carry a dense render without consequence, because the inner leaf was never expected to evaporate. A solid brick or stone wall was.
3. Work out where the moisture will go instead.Water in a solid wall leaves by evaporating from a face. Seal a metre of the inside face and the same volume of water must now leave higher up, through the other side, or through the floor edge. The classic result is a new damp band just above the top of the render.
4. Question the height.Specifications routinely say one metre because that is the convention, not because it was measured. It should be at least 300mm above the highest sign of contamination, which on some walls is less than a metre and on others considerably more.
5. Consider lime instead in an older house.A lime backing and finish lets the wall evaporate through the plaster rather than trapping it. It is more expensive, needs a plasterer who works in it regularly, and takes far longer to dry and decorate, but it works with the building rather than against it.
6. Ask about the skirtings and the floor junction.New plaster hard down to a solid floor bridges the wall from the floor. There should be a gap at the bottom, and the skirting should be refixed clear of the new plaster, not bedded into it.
7. Agree the decoration specification.New plaster over a previously damp wall wants a breathable finish — a mineral or clay paint, or a mist-coated emulsion — not a vinyl silk or an oil-based sealer. And it wants months, not weeks, before the first full coat.
8. Read what the guarantee excludes.Most exclude condensation, penetrating damp, defective rainwater goods and anything caused by the building's own drying. Since those cover most reasons the work appears to fail, understand what you are actually buying.

Tips

• If the plaster only needs replacing because of salts, and the water source has been removed and the wall left to dry, an ordinary breathable replaster may be all that is required. Ask whether the waterproof render is genuinely necessary or simply standard.
• Never let new plaster be applied to a wall that is still actively wet. It buries the evidence and the plaster fails.
• A hacked-off wall is the one chance you will ever have to look at the masonry, the barrier and the floor junction. Go and look before it is covered up, and photograph it.

Common mistakes

• Waterproof rendering a solid wall to a metre and no further — The moisture that used to evaporate through that metre now has to leave somewhere else, and the commonest place is a new damp band immediately above the render, which then gets blamed on the original problem.
• Decorating the new plaster within a few weeks with a vinyl paint — The plaster and the wall behind it are still giving up water. A non-breathable coating traps it and the paint blisters. Mist coat only, and leave the full scheme for as long as you can bear.

Alternatives

• Lime plaster on a lime backing: The traditional and usually correct system for a pre-1919 solid wall. Breathable, sacrificial to salts, and compatible with lime mortar. Slower and needs a specialist.
• A drained membrane system: A studded plastic membrane fixed to the wall with a plasterboard or plaster finish over it, leaving an air gap. Used mainly in basements and cellars. It manages water rather than stopping it.
• Leave it bare and monitor: Hack off, let the wall dry through a full heating season, then decide. Unattractive but genuinely informative, and it prevents paying for a specification the wall may not need.

Questions people ask