How to Re-Render After Installing a Damp-Proof Course

Why does re-rendering need its own process?

A new damp-proof course stops moisture rising up the wall, but it doesn’t remove the water and salts already there. Render applied too early, or over salt-contaminated masonry, can trap that moisture and fail within months. Following the right sequence protects both the damp course and the new finish.

Remove salt-contaminated render and plaster

Before or during damp course installation, strip back damaged finishes.

• Remove any render covering the lowest course of bricks so the damp course can be installed.
• Remove salt-contaminated plaster or render to 350 mm above the last visible damp mark.
• Rake out and replace salt-affected mortar joints.

Salt-contaminated plaster left in place will keep drawing moisture and staining new finishes.

How long should walls dry before re-rendering?

Drying takes several months and must be complete before any re-rendering, plastering, painting or pointing.

• Cement-rendered or plaster boarded walls: a minimum of 3 months, and up to 6 months for very salty walls.
• Painted or bare brick: 12 months or more, because there is no render acting as a poultice to draw salts out.

Rushing this stage is the most common reason re-rendering fails after a damp course installation.

Remove salts as they come out

As the wall dries, salts migrate to the surface. Keep removing them so they don’t redissolve back into the masonry.

• Brush away salt crystals from exposed brickwork and plaster.
• Wipe salty moisture with a slightly damp cloth, never a wet one, since water redissolves salts into the wall.
• On bare or painted brick, wipe surface salts every 2–3 weeks until no new salts appear.

When salts stop appearing, clean the exposed brickwork to remove any residue.

Carry out a site test

Masonry varies from wall to wall. Before full application, run a small-scale pilot test on site to confirm the render and admixture suit the substrate.

Mix the render with Salt Retarder

Tech-Dry Salt Retarder is a water-repellent admixture recommended for cement/sand render after a silicone damp course is installed, to prevent residual salt and water migration.

Mix ratio: Use a cement-to-sand ratio of 1:3. Other ratios may affect performance.

Dosage: 1 litre of Salt Retarder per 40 kg of cement for a general render mix. The recommended range is 0.75 to 1.5 litres per 40 kg of cement.

Mixing method:

1. Dissolve 1 litre of Salt Retarder in 20 litres of clean water.
2. Blend the cement/sand mix (for example, 40 kg of cement and 120 kg of sand) while adding the solution.
3. Add clean water only if needed to reach the right consistency.

Avoid other admixtures: Plasticisers and other admixtures may be added but are not recommended.

Apply the render

Apply a 15 mm cement render (1:3 cement/sand) containing the waterproofing admixture. Where the wall is plastered internally, apply a base coat and then hard plaster.

Batten skirting boards rather than running render down behind them.

What about cold weather?

Salt Retarder may precipitate solids below 10°C and can slow curing, particularly in low temperatures. Store and use the product between 10°C and 25°C where possible. If curing is slow, a cement accelerator may be added.

Get it right the first time with Tech-Dry

The best results come from treating damp and salt as one system: a silicone damp course to stop rising moisture, enough drying time, and Salt Retarder in the new render to limit residual salt migration. Talk to the Tech-Dry team about your project, or explore the Tech-Dry rising damp treatment and Salt Retarder product pages.