Concrete is a hygroscopic material, and adsorbs moisture from adjacent soils. This moisture wicks through the concrete where it will diffuse into the interior of the building, and may lead to excess humidity, chronic dampness and mold.

Concrete footings, located at the bottom of the excavation, can be exposed to water ponding, which is the severest form of moisture contact. The small particle size of cement allows the concrete to 'pull' moisture from surrounding soils with an intermediate particle size. For example, concrete has no difficulty in pulling water from damp sand. However, concrete in contact with water ponds encounters no opposing 'pull', and rising damp becomes a very significant water source inside the building.

This web page examines water ponding in excavations, how it occurs, and how it can be prevented.

SOIL TYPES

Certain soils enable water ponding:

1. Impervious Soils: These soils, for example, clay, silt and shale, allow water ponding to occur, which may lead to high levels of rising damp.

2. Pervious Soils: These soils, for example, sand and gravel, allow water to pass through to the water table below, and water ponding cannot occur.

WATER PONDING ON IMPERVIOUS SOILS

The diagram below shows water ponding making contact with the footing concrete, leading to high levels of rising damp.

The following errors were made by the contractor:

1. Fastfoot® Footing Membrane Not Installed - soil moisture is wicking into the footing concrete;

2. Drain Tile Located Too High - the top of the drain tile must be located below the bottom of the footing to ensure water ponds cannot contact the concrete;

3. Drain Tile Not Correctly Sloped - the tile must be sloped to the storm outlet;

4. No Drainage Layer - a 3" to 4" granular drainage layer should be installed over the site to ensure water ponding cannot touch footing concrete and to create pathway to drain tiles.

To prevent water ponding, please click here.

For picture of foundations suffering from rising damp, please click here.

For a printable report on Drainage & Rising Damp, please click here.