Salt Contamination Calculator — Removal Height & Render Spec

Hygroscopic salts (chlorides, nitrates and sulphates) are drawn up walls by rising damp and deposited in the plaster as the water evaporates. These salts absorb moisture from the air, causing persistent dampness even after the rising damp source is treated. The contaminated plaster must be removed and replaced.

Remove all plaster to at least 1 metre above the visible tide mark (the line where salt deposits and staining stop). This ensures all salt-contaminated material is removed. The entire width of the wall should be exposed — do not just hack off a thin layer.

Salt-resistant renovation plaster (such as Dryzone Damp-Resistant Plaster or Limelite Renovating) is formulated to resist the effects of residual salts in masonry. It has high porosity, allowing moisture to evaporate, but resists salt crystallisation damage. Standard gypsum plaster should NOT be used on salt-contaminated walls.

Bare masonry dries roughly 1 month for every 25mm (1 inch) of wall thickness (BRE Digest 163 “Drying out buildings”). A 230mm/9-inch solid wall — the type most prone to rising damp — therefore needs around 9 months, and Safeguard Europe advises delaying replastering for six months or more after the DPC is installed. Only thin or very well-ventilated walls dry in 4–12 weeks. Replaster once the wall has stabilised to a near-equilibrium, acceptable moisture level (follow the maximum substrate moisture stated by your renovation-plaster manufacturer, rather than a single fixed meter figure). Do not rush this stage — replastering too early traps residual moisture and the treatment fails.

Yes. After the wall is dry, apply a salt-inhibiting primer or slurry coat to the bare masonry. This provides an additional barrier against any residual salts migrating into the new plaster. Follow the specific manufacturer's instructions for the renovation plaster system being used.