Rising damp

What is rising damp?

It’s a type of damp that develops as a result of ground water being absorbed by - and moving upwards through - the bricks and mortar of a building.

As well as causing cosmetic damage to the paint, plaster and brickwork of internal and external walls, rising damp can damage the structure of your home.

For example, it might corrode brickwork or rot the timber of floor joists.

Rising damp can also lead to mould growth on walls, which could exacerbate allergies and cause respiratory problems.

What are the signs of rising damp?

If you spot dark patches that start at the bottom of internal ground floor walls, spread upwards and have a ‘tide mark’ indicating how far water has reached, it could be rising damp. The marks may feel damp to the touch.

Other tell-tale signs include staining or discolouration to the bottom of walls, powdery salt deposits, blistering paint, crumbling plaster or peeling, loose wallpaper.

There may also be wet-rot evident on wooden floors and skirting boards and you might also notice a musty smell.

On outside walls, evidence of rising damp can include the same, wet, tide marks and white salt stains as well as crumbling brickwork and mortar.

What causes rising damp?

Rising damp is caused by water moving through the pores in your home’s walls.

Modern homes are usually built with barriers to stop this, like a damp-proof course with a damp-proof membrane.

Damp-proof course (DPC)

This is a waterproof material built into a wall in a horizontal layer between courses of brickwork a short way above ground level – at least 150mm. It may be made of thick plastic or bitumen felt, for example.

In much older properties, slate or lead would have been used for this purpose.

You can quite easily check that your property has a damp-proof course. Look for a visible line about six inches high in the brickwork of the exterior walls of your home.

Damp-proof membrane

This is a tough but flexible sheet of polyethylene placed beneath the floor of a property and connected to the damp-proof course in the walls.

Older homes (usually built before around the 1870s) that have no damp-proofing built into their fabric might be more prone to rising damp.

It can also happen in homes where the DPC or membrane installed becomes damaged or compromised.

Breaks or cracks can occur in a DPC - due to building movement or old age, for example. But, because the materials used in a DPC are so durable, this is not a common cause of rising damp.

More usually, a DPC can fail if so-called ‘bridging’ has occurred, which means the ground level outside is higher than the level of the damp course. For example, if new paths or patios are laid, or you’ve had garden landscaping done, raising soil levels.

Even piles of materials left around the walls of your house that come above the line of the DPC can let moisture into your property.

Building renovations that raise the interior ground level of your home above its original DPC can result in similar bridging.

And blockages or debris that has fallen inside the wall cavity can also allow groundwater to find its way around a DPC.

How can rising damp be treated?

Treating the cause

Rising damp is often misdiagnosed, so it’s wise to get your home examined by an independent qualified surveyor to check for the causes of the damp in your home.

You will usually have to pay for their survey, but it could save you from having costly – and what could be unnecessary – work.

Damp can be caused by plenty of other, often less complicated issues, including blocked guttering, faulty downpipes, drains or roof tiling, leaks from internal plumbing or problems with window pointing. Condensation could also be to blame.

If the problem is occurring due to suspected bridging of the DPC you already have, then you can attempt to rectify the problem.

For example, if debris in the wall cavity is to blame, that can be cleared.

Similarly, if a raised garden flowerbed or patio has been installed above the level of your DPC, then it could be lowered to below the damp barrier. It may even be possible for you to dig away excess soil yourself.

In some cases, a drainage channel can be built around your home to get rid of standing water that might accumulate on paving that lies above the level of your DPC.

If this can’t be done - or if your damp-proof course is found to be damaged - then you may need to have a replacement damp-barrier installed.

This usually takes the form of a DPC injection of a water repellent chemical cream into the brickwork (which spreads into the mortar line and fuses to create a new waterproof layer) or the fitting of a new physical damp proof membrane.

Treating the damage

Once the cause of rising damp has been treated, you can begin work on repairing any damage.

This may involve replacing wooden skirting boards that have warped or rotted. Damaged wooden floorboards or joists may also need replacing.

Contaminated plasterwork should be removed from interior walls, before re-plastering with a salt and moisture-resistant formula then decorating with a breathable paint.

Exterior walls can be re-rendered with a damp-proof formula.

Does home insurance cover rising damp?

In most cases, it won’t.

Buildings insurance policies don’t pay out for problems that are seen as the result of gradual deterioration, which is what insurers consider rising damp to be.

What’s the difference between rising damp and condensation?

Condensation is caused by moisture in your home settling on cold surfaces.

In homes which have poor ventilation, this moisture won’t evaporate away and can cause black dots of mould to form on walls and ceilings.

To minimise the amount of condensation in the home you can:

• Turn on extractor fans, or open windows, in bathrooms when you shower or take a bath
• Do the same in kitchens - and keep on pan lids - as you cook
• Let in fresh air when you can to help moist air escape
• Move furniture away from walls to allow air to circulate
• Avoid drying your washing indoors, especially on radiators

To remove black mould, carefully wipe it away with a cloth dipped in water mixed with a mild detergent or use a bleach spray (if it’s safe to use on the surface you’re cleaning) and dry thoroughly. You should wear gloves and a mask when cleaning off mould as breathing in spores could exacerbate respiratory problems.

What’s the difference between rising damp and penetrating damp?

Penetrating damp is caused by water penetrating the brickwork of your home - from exterior walls into interior walls.

Rising damp can only happen on the ground floor of a house, beginning at the bottom of walls. But penetrating damp can occur anywhere - around windows, ceilings or near the floors - both downstairs and upstairs.

Common causes of penetrating damp include:

• Overflowing gutters and leaking downpipes
• Ageing or damaged brickwork, cracks in external render or old mortar allowing rainwater to seep through
• Roof leaks
• Poorly installed windows