Rule 7: Rising damp

A key principle of Damp Surveying is familiarity with Rising Damp. Rising Damp is easy to replicate with absorbent paper to understand the mechanics and recognise the profile.

Definition of Rising Damp

Water disperses through absorption and evaporation. Rising Damp is a form of absorption. Like condensation, it is a method of dispersion, not a source of dampness.

• Most moisture comes from rain falling on walls or through roofs. Most walls evaporate before it becomes a nuisance. Damp from rain that becomes a nuisance is called penetrating damp.
• Some moisture comes from leaks; waste pipes, main water pipes, downpipes, gullies, drains, even the little pipe behind your fridge.
• Condensation comes from excess humidity falling on cold surfaces.
• Floods are one-off events.
• The only remaining form of dampness is from below the ground – that is high water table ion contact with ground level bricks. This is known as Rising Damp. Rising damp is prevented with a damp proof course (PCD), a layer of slate, bitumen or plastic to stop damp from rising. 

Rising damp is defined as the deleterious vertical flow of water, derived from below the original ground level, through a property’s internal masonry wall, to above the base of the original ground floor.

All water from leaks, condensation or penetrating dampness must be dealt with at source, and are not forms of Rising Damp.

An important feature of Rising Damp, is water rises through the base of a wall, therefore both sides of the wall suffering from rising damp MUST be equally damp, when taking into account evaporation and ventilation. Furthermore the DPC must be defective (which is highly unlikely), in which case the profile will be fan like around the break. Or if the DPC is missing the profile will be horizontal, like a flood. 

Replicating rising damp

Anyone can replicate rising damp with kitchen paper or by placing the bottom of a clay brick in water. Damp will rise by about 200mm. The same effect is more rapid in a clay tile where damp can be seen to rise by as much as 400 mm. Water moves to fill the pores in the most porous substrate first. The force of attraction is equal in all directions, but gravity pushes a majority of the water downwards until all the lower pores are filled.

No one has managed to replicate rising damp to the extent that it can be peer reviewed.

Profiling rising damp

Profiling supposed rising damp is important. Like a swimming pool filling up, the rising damp profile is horizontal, not random patterns often associated with condensation. Rising damp is in constant equilibrium with evaporation. The greater the evaporation, the lower the rise of dampness. The greater the relative humidity the greater the potential rise. The effect of evaporation is to increase the rise of dampness into a corner and lower it by a door opening.

Nitrate salts

Ground water contains nitrate salts from decaying matter. The presence of mould quickly eliminates rising damp, as nitrates, found in ground and waste water, inhibit mould growth.

Rot

The damp in rising damp is insufficient to cause rot. Therefore if damp is misdiagnosed as rising damp but is condensation, leak or penetrating damp then rot will probably go undetected and become worse causing expensive structural problems at a later stage.

Constant source of water

Rising damp needs a constant source of water, such as a high water within a meter of the brick wall. It is exceptional rare in London as water is pumped out of the ground, and rarely within a meter of a building. Furthermore, London benefits from a by-law introduced in 1877 requiring a damp-proof courses (“DPC”) “beneath the level of the lowest timbers”.

1. According to Thames Water, the table water is below the deepest underground line.
2. London is full of tunnels and cuttings for the underground and overground trains, these acts a sumps, drawing surface water towards them.