The Effects of Rising Damp on Masonry, Plaster and Timber

Most homeowners first notice rising damp as stained walls and peeling paint. Those are frustrating on their own, but they’re also the visible end of a slow process that can affect masonry, plaster and timber if it’s left untreated. The good news is that it’s usually gradual, and it can be treated once the source is correctly identified.

What is rising damp, and why is it more than a cosmetic problem?

Rising damp is groundwater drawn upward through permeable building materials by capillary action. Brickwork, mortar and natural stone all contain tiny capillaries that allow moisture to move. It usually stays in the lower part of a wall, typically within about a metre.

The moisture carries dissolved salts with it. Some come from the ground, and some are dissolved out of the bricks and mortar themselves. When the water evaporates, the salts stay behind and crystallise.

This is why rising damp is more than cosmetic. Rising damp and salt attack are two separate but interrelated processes. Salts can keep damaging masonry as they crystallise in its pores, and many of them attract moisture from humid air, so a wall can stay damp even after the original source has been dealt with.

How does rising damp affect brickwork and mortar?

Salts crystallising inside brick and mortar can gradually break down the material. Bricks perform best when kept reasonably dry, and years of repeated wetting, evaporation and salt crystallisation take their toll. Common consequences include:

• crumbling mortar joints
• surface flaking
• salt deposits on walls
• detachment of render
• ongoing masonry deterioration

As the condition progresses, repairs tend to become more extensive and costly, which is why early attention matters.

What does rising damp do inside the home?

The internal effects are usually the first signs people notice. Moisture and salts moving through a wall can cause:

• paint blistering
• wallpaper lifting
• plaster deterioration
• persistent staining
• musty odours

Repainting on its own does not solve the problem. Unless the source of moisture is dealt with and salt-contaminated plaster is replaced, the damage is likely to return.

Can rising damp damage timber?

Yes, where timber is in contact with damp masonry or sits over a poorly ventilated subfloor. Skirting boards, door linings and the ends of floor joists are the usual places. Decay fungi generally need timber moisture content to stay above about 20%, and prolonged dampness can allow that to happen.

Over time, affected timber may:

• lose strength
• become soft or brittle
• warp or distort
• need partial or complete replacement

Rising damp isn’t the only cause of timber decay. Leaks and poor subfloor ventilation can do the same, so it’s worth looking at all the possible sources.

Is it really rising damp?

Not always, and this is the question to answer before spending money on treatment. Condensation, penetrating damp, high external ground levels and leaks can all look similar, and heritage guidance stresses that not all dampness in buildings is rising damp.

Some pointers:

• Rising damp tends to show as tide marks and salt deposits low on a wall, usually within about a metre. Damp well above that suggests another cause.
• Condensation tends to appear on cold surfaces such as windows and corners, often with mould.
• Penetrating damp often worsens after rain and relates to a specific defect, such as a leaking gutter, cracked render or a crack in the wall.

A professional assessment can identify the source of moisture, the extent of salt contamination, any concealed damage and the correct treatment options.

How is rising damp treated?

Treatment starts by cutting off the source. A damp course, such as a liquid damp course injected into the mortar, stops further moisture rising. Then:

1. Address contributing factors such as raised ground levels and poor drainage.
2. Let the wall dry out. This can take time.
3. Remove and replace salt-contaminated plaster.
4. Re-render using a salt-retarding admixture, such as Tech-Dry Salt Retarder, to help stop residual salts migrating through the new finish.

Heritage buildings may call for more nuanced approaches, so get specialist advice for older or listed properties.

Does rising damp affect property value?

Rising damp may affect a sale as a damp house deters buyers. Rising damp is among the defects covered in pre-purchase building inspections, and visible signs may raise questions about repair costs. Inspections can’t always detect it, though, because weather and furnishings can hide the evidence. In addition some vendors cover up rising damp with waterproof plaster meaning a new owner may need to install a damp course and replaster. For owners planning to sell, finding and treating the problem early, and keeping a record of the work, can help buyers feel more confident.

Can rising damp damage your home’s structure?

It can damage the materials the building is made from, particularly masonry, mortar, plaster and timber in contact with damp walls, especially when it’s left untreated for many years. The damage is usually gradual rather than sudden. Recognising the warning signs early, confirming the cause and acting on it is the best way to protect the home and its value.

Talk to the Tech-Dry team about damp course and salt-retarding products for your situation.