Rising Damp in Walls and Damp-Proof Courses

How to tell rising damp from other moisture problems, mechanical and injected damp-proof courses, and how to restore the wall afterwards.

Bubbling plaster, flaking paint and a wavy tide mark at a certain height on the lower walls of an older house’s ground floor are a familiar sight. People often call it just “damp” and repaint, only to see everything return a few months later. The cause is the wall drawing water from the ground, and the fix is not paint but cutting off the route the moisture takes.

How rising damp develops

Porous materials such as brick, stone and lime or cement mortar contain very fine channels. Water climbs through them against gravity, much as a dry sponge soaks up water from the end that touches it. In a well-built wall, a horizontal damp-proof layer laid between the foundation and the masonry blocks this rising moisture. If it was never installed, has broken down over time or the moisture has found a way around it, the wall starts wicking water from the soil continuously.

The water reaches the wall surface and evaporates, leaving dissolved salts behind. Salt crystals grow in volume and push plaster and paint off from within. Some salts also attract moisture from the air, so the wall can look damp even in dry weather.

Signs, and how it differs from other moisture

Rising damp is easily confused with other problems, so diagnosis matters:

rising damp — starts at the base of the wall, climbs to a certain height, has a salt band at the upper edge and may show both inside and outside;
condensation — mould in cold corners, around windows and behind furniture, usually in winter and where ventilation is poor;
penetrating damp — a localised stain that worsens after rain, coming from the roof, gutters, window junctions or a cracked facade;
a burst pipe — a stain that appears suddenly and spreads regardless of the weather.

For an accurate decision, moisture is measured at different heights, and sometimes samples are taken to test salt content. Moisture decreasing from bottom to top points to capillary rise.

Deal with external causes first

The problem often begins with later changes. If the ground or paving around the house has been raised above the old damp-proof course, water bypasses it. If downpipes discharge at the base of the wall, there is no apron, or the ground slopes towards the house, a permanently wet zone forms along the wall. If an old stone or brick wall has been sealed with non-breathable cement render or tiles, moisture cannot escape outwards and climbs higher instead.

Without fixing these causes, even the most expensive damp-proof course may not fully work.

Ways to install a damp-proof course

There are two main ways to create a new horizontal barrier in an existing wall.

Mechanical cutting. The wall is cut section by section along a line near floor level, a waterproof membrane or plastic sheet is inserted into the slot, and the gap is packed with special mortar or wedges. The method is reliable but must be carried out by a specialist who takes the wall’s load-bearing capacity into account; sections are kept short so the wall does not settle. It is difficult in thick stone walls and irregular masonry.

Chemical injection. Holes are drilled at set spacing along a line, and a product that blocks pores or makes them water-repellent is injected. It spreads through the masonry to form a zone that stops moisture rising. Hole spacing, depth and quantity depend on wall thickness and masonry type, following the manufacturer’s instructions. In walls with voids, holes are first filled with grout so the product does not run away into cavities.

Restoring the wall afterwards

The wall does not dry immediately once the barrier is in; in thick walls this can take a long time. Old salt-saturated plaster is removed to a point somewhat above the upper edge of the damp zone, and mortar joints are cleaned out. The new coat is not ordinary cement plaster but a renovation (salt-retaining) plaster, whose pores hold salts within the layer while letting the wall dry. Non-breathable paint, vinyl wallpaper or tiles should not go onto a wall that has not fully dried.

A short roadmap

Confirm the moisture source with measurements.
Correct the ground level, apron and rainwater discharge.
Choose the damp-proofing method with a specialist to suit the wall.
Remove salty plaster and restore with a renovation plaster system.
Carry out decorative finishes only once the wall has dried.

Zaferoğlu İnşaat plans renovation of apartments and private houses from uncovering hidden problems through to finishing work. Measure how high the damp band rises above the floor and tell us; it helps separate rising damp from other causes.

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