Rising damp is the upward movement of ground moisture through the pores and capillaries of masonry walls. It occurs where a damp proof course is absent, has failed, or has been bridged by raised external ground levels or internal plaster. The moisture carries soluble salts from the ground which crystallise in the wall as it dries, leaving characteristic tide marks and white salt deposits.

How high can rising damp travel up a wall?

Rising damp rarely rises more than 1-1.5m above ground level. The height is limited by the balance between capillary suction drawing moisture upwards and evaporation from the wall surface releasing it. The 'tide mark' of salt deposits and staining at this height is one of the key diagnostic signs of rising damp.

How is rising damp treated?

The most common treatment is chemical DPC injection - a silicone-based cream or fluid is injected into a series of closely-spaced holes drilled at low level in the mortar bed joint, creating a water-repellent barrier. The wall is then re-plastered using a salt-resistant renovation plaster. Physical DPC insertion (saw-cutting a slot and inserting sheet DPC) is more reliable for thick walls but more disruptive.

How do I know if I have rising damp or condensation?

Rising damp typically shows a tide mark at a consistent height (usually below 1m), salt crystallisation on the wall surface, and damage concentrated at the base of walls. Condensation tends to be more widespread, appears on cold surfaces and in corners, and is associated with mould growth. A professional damp survey using a moisture meter and salt analysis can confirm the source.

Rising Damp - Construction Glossary

Definition

Rising damp is the upward movement of ground moisture through the pores and capillaries of a masonry wall by capillary action. It occurs where a damp proof course (DPC) is absent, has failed with age, or has been bypassed by raised external ground levels or paving bridging over it. As the moisture evaporates from the wall face it deposits soluble salts from the ground, leaving characteristic tide marks, white salt deposits, and deteriorating plaster and decoration.

In practice

Rising damp is most common in pre-1900 properties built before DPCs were standard, and in buildings where the original DPC has failed or been bridged. It rarely rises more than 1-1.5m above ground level - the height at which evaporation balances capillary suction - creating a characteristic damp zone at the base of ground floor walls.

The most widely used treatment is chemical DPC injection - a silicone cream is injected into closely-spaced holes drilled along the mortar bed joint at low level, forming a water-repellent barrier. The affected plaster must be fully stripped back, the wall allowed to dry, and then replastered using a salt-resistant renovation plaster to prevent hygroscopic salts contaminating the new finish. Physical DPC insertion (saw-cutting and inserting sheet DPC) is more reliable for very thick solid walls but significantly more disruptive.

Building Regulations

Relevant PartPart C - Site preparation and resistance to moisture

Approved DocumentAD C (2004, amended 2013)

PreventionDPC at minimum 150 mm above finished ground level

Common standardBS 6576 - Chemical injection DPC

Approved Document C requires all new masonry walls to have a DPC to prevent rising damp. Remedial DPC treatment in existing buildings is not notifiable under Building Regulations, but where the works form part of a larger project (e.g. a conversion) the moisture resistance of the walls will be assessed as part of Building Control sign-off.

Full Building Regulations guidance

Related Calculators

Damp Proof Membrane Calculator

Rising Damp

Just so you know...