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Rising damp

What exactly is rising damp?

Rising damp is, as the name suggests, damp that is moving or progressing upwards from below the ground and it differs from other causes of damp such as condensation or rain penetration, which require a totally different approach to resolving the problem.

The height to which damp can rise depends on the following factors: the water table, the type of construction material/masonry attached and the evaporation rate. Typically it can rise to 900mm or more in affected walls. Deposits of salt can be noted by a horizontal line appearing like a tide mark with an area of discolouration below it.

My property is old so does that make it more susceptible to rising damp?

Rising damp appears more frequently in older building as opposed to newer ones. Building technology today uses various barrier methods to keep the water out such as DPC (Damp Proof Course) or DPM (Damp Proof Membrane). It is interesting to note that DPCs have been a requirement for use in walls since 1875, while DPMs applied to floors, have only been a requirement since the1960s.

Typically, older buildings ‘need to breathe’ naturally and allow for evaporation, which may not exist in dry or overly centrally heated buildings which need humidifying.

How do you diagnose rising damp?

Rising damp may appear accompanied by a tide mark, but this is not a diagnosis in itself! It can be diagnosed via electrical moisture readings but these can be misleading as older buildings which appear virtually dry can/may have salt deposits from evaporation or water penetration by rain splashing, and these will lead to a high reading.

Rain penetration should not be confused with rising damp. Green staining is indicative of rain penetration on external walls. It is worth pointing out that tests to determine moisture levels contained within the wall thickness can help to rule out condensation which is on the surface.

The surveyor’s report for the older property I am buying stated it was affected by rising damp. How can I be sure the diagnosis is correct?

If you believe the diagnosis is not correct, you may choose to challenge it and/or seek a second opinion in writing from an independent consultant or chartered surveyor. Do not, however just seek a second opinion from a contractor.

How can rising damp be controlled or managed?

Measures to control rising damp should include building work that allows the building to breathe. This may include replacing hard cement render or pointing with a lime pointing or mortar. Measures to improve drainage externally and/or improved groundwork may also assist.

You might also consider replacing a DPM (Damp Proof Membrane) with a breathable alternative. The application of waterproof coatings or waterproof renders should be carefully considered, however, as they could exacerbate the existing damp problem.

The installation of a retrofit damp proof course has been recommended. Will it resolve the problem?

A retrofit DPC, if not installed correctly, can be damaging to the building and possibly prove expensive and ineffective.

Can plaster repairs be used on my previously damp walls?

Using lime plaster for normal repairs can be considered. Paints such as lime wash or a soft distemper are suitable media for re-decoration, as they will allow the building to breathe. A salt resistant plaster will temporarily disguise or hide the problem but it will not resolve it.

Note that the use of a salt resistant plaster may require listed building consent before it can be applied. Traditionally a poultice of whiting and water can be used to remove residual patches of salts from plasterwork.

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