What are hygroscopic salts?

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.

How high should I remove contaminated plaster?

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.

What is salt-resistant render?

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.

How long must I wait before replastering?

After removing contaminated plaster and treating the DPC, the bare masonry must dry to acceptable moisture levels (below 5% WME) before replastering. This typically takes 4-12 weeks depending on wall thickness, heating and ventilation. Do not rush this stage — replastering too early traps moisture.

Do I need to treat the wall before replastering?

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.

TradeCalculator

Salt Contamination Calculator — Removal Height & Render Spec

Calculate salt-contaminated plaster removal area (1m above tide mark), salt-resistant render quantities and drying time before replastering.

Wall Length

Length of affected wall

Tide Mark Height

Height of visible salt line

Salt Source

Price Per m² (optional)

Treatment cost per m²

How We Calculate This

This calculator determines the area of contaminated plaster to remove and renovation render quantities.

The formula

Removal Area = Wall Length × (Tide Mark Height + 1m above)

Render kg = Removal Area × 1.1 × 20 kg/m²

Treatment process

Step 1: Remove plaster to 1m above tide mark
Step 2: Allow masonry to dry (4-12 weeks)
Step 3: Apply salt-inhibiting primer
Step 4: Apply salt-resistant renovation render

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.