Rising damp is the common term for the slow upward movement of water in the lower sections of walls and other ground-supported structures by capillary action. Although rising damp of up to 5 meters in height has been observed the height of rise is typically much lower and is rarely above 1.5m.

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In simple terms rising damp occurs when groundwater travels upwards through porous building materials such as brick, sandstone, or mortar, much in the same way that oil travels upwards through the wick of a lamp. The effect can easily be seen by simply placing a piece of porous brick, stone, or mortar in a shallow tray of water and observing how the water is absorbed into the porous material and is transported above the waterline.

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Rising damp can be identified by a characteristic “tide mark” on the lower section of the affected walls. This tide mark is caused by soluble salts (particularly nitrates and chlorides) contained in the groundwater. Due to the effects of evaporation, these salts accumulate at the “peak” of the rising damp.

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The plaster becomes hydroscopic due to the effervescent salt contamination. Besides treating the cause of the rising damp, the contaminated plaster also needs to be removed and replaced with a salt inhibiting render.

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Rising damp in historical buildings is particularly challenging needing specialized treatment as the substrate is vastly different from that of modern homes.