Diagnosing damp and tracing the causes

Make sure you know what the cause of dampness is before trying to cure it, otherwise you may be dealing only with part of the problem, or even adopting the wrong remedy for the sort of damp involved.

In an old house with a slate damp-proof course (DPC), slight movement of the building can crack the slates, allowing damp from the ground to rise into the masonry above the crack. A single point failure will cause a semicircular patch of damp up to 1 m (3 ft 3 in) or so across, while multiple cracks will lead to an almost continuous band across the affected wall.

If you’re not sure of the cause of a damp patch on a wall, try the foil test. Dry the wall surface with a fan heater, then tape some kitchen foil tightly over the damp area. If the surface of the foil is wet after 24 hours, you have condensation. If the foil is dry but the wall surface beneath it is damp, you have rising or penetrating damp. Discount rising damp if the moisture is more than 1 m above outside ground level. 

Patches of dampness on walls around windows and doors are usually caused by rain getting through gaps between their frames and the surrounding masonry. Where the damp is below the opening, it may be because there is no drip groove to stop the water creeping under a projecting sill or threshold. If there is a drip groove, make sure the rain is not crossing it because it is blocked with paint or mortar.

Discovering exactly where a pitched roof is leaking can be difficult. Rain can trickle down the roofing felt and then along the sides of rafters before it drips onto the loft floor.

Look for clues such as dampness on a party wall or chimney stack in the loft, which might indicate that flashings are defective or missing. Getting someone to play a hose on the roof, area by area, while you remain inside the loft can also help to reveal where the water is getting in.

At the same time as checking lofts, check underfloor spaces and built-in cupboards for signs of dampness, inspect structural timbers and joinery for evidence of woodworm, which thrives in slightly damp environments. Look for the small flight holes made by the beetles when they emerge from the wood and the Ïne wood dust created by the pest.

Check the untreated backs of all freestanding chests and wardrobes, and the unpainted top and bottom edges of doors at the same time.

Use a bradawl to test the soundness of skirting boards if there are signs of damp in downstairs walls or the underfloor space.

The backs of skirting boards are usually left unpainted, so they readily absorb moisture from the masonry. However, severe deterioration of the boards is often not apparent because of paint applied to their face sides.

Leaks in plumbing and central heating pipework can cause damp patches which could be misinterpreted as rising or penetrating damp. This is especially common where the pipes are run beneath a floor or are buried in wall plaster.

Here a pinhole leak or a weeping fitting can release surprisingly large volumes of water as time goes by, especially if it has no chance to dry out naturally. If this is the cause of the problem, you have two possible courses of action. You can either expose the fault and then replace the affected pipes, which will cause a lot of disruption.

Alternatively, you can simply leave them where they are and bypass them by installing new ones.

If the roof isn’t leaking but the timbers and loft insulation are damp, the likely culprit is condensation. This is caused by warm moisture-laden air rising into the loft from the rooms below and condensing on cold surfaces within the loft space. In serious cases, roof timbers can start to rot and saturated insulation materials can stain ceilings.

The kitchen and bathroom are the main sources of condensation in the home. Bathing, cooking and washing up, and washing and drying clothes all pour large volumes of steam into the air.

Portable gas space heaters and paraffin stoves also create a lot of moisture. The problem is made worse by poor room ventilation and over-efficient draught-proofing, both of which stop warm moisture-laden air inside the house being replaced with cooler drier air from outside.

Unventilated fitted cupboards built against outside walls can suffer badly from condensation. This can lead to unsightly mould growth, which will quickly spoil clothes stored there.

If you think you have rising damp in your house, locate the damp-proof course (DPC) and make sure it isn’t covered by a flower bed, path, drive or patio. Look for rendering that has been applied over the DPC. Check whether there is a vertical DPC sandwiched between the house wall and the end of a garden wall built up against it.

Curing these common causes of rising damp will solve the problem for little or no cost, saving you from incurring an expensive bill from a professional damp-proofing firm.

Take a look at these ways to combat damp