Tuesday, March 19, 2013


Construction details must be included to prevent natural capillary action of soils beneath foundations or below-grade floors. Capillary action is upward movement of water and vapor through voids in soil from wet lower areas to drier high areas. Capillary action is dependent upon the soil type present. Clay soils promote the most capillary action, allowing more than 10 ft of vertical capillary action. Loose coarse gravel prevents capillary action, with this type of soil promoting virtually no upward movement.

Capillary action begins by liquid water saturating lower areas adjacent to the water source. This transgresses to a mixture of liquid and vapor above the saturation layer.

Finally, only vapor exists in upper soil areas. This vapor is as damaging as water to interior building areas. Soil capillary action can add as much as 12 gal of water per day per 1000 sf of slab-on-grade area if insufficient waterproofing protection is not provided.

Microscopic capillaries and pores that naturally occur in concrete substrates create the ability for the concrete to allow water and moisture to move readily through below-grade walls and floors. This process is particularly sustainable when the interior space of the structure has lower humidity than the 100% humidity of the adjacent water-saturated soil and when the occupied space is warmer than the soil. These conditions present ideal circumstances for water to be actually drawn into the occupied space if not protected with waterproofing materials or at minimum vapor barriers where appropriate.

Water vapor penetrates pores of concrete floors, condensing into water once it reaches adjacent air-conditioned space. This condensation causes delamination of finished floor surfaces, mildew, and staining.

Therefore, it is necessary to prevent or limit capillary action, even when using waterproof membranes beneath slabs. Excavating sufficiently below finished floor elevation and installing a bed of capillary-resistant soil provides drainage of water beneath slabs on grade.

This combination of foundation drainage and soil composition directs water away from a structure and is necessary for any waterproofing and envelope installation. Refer again to Fig. 2.1 for recommended controls for proper surface and groundwater.

 Below-grade drainage detailing.
FIGURE 2.1 Below-grade drainage detailing.

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