Water intrusion through the slab is the movement of groundwater into the slab.
The slab foundation is made of concrete, which is porous. Concrete has the ability to wick up water through capillary action. Capillary action is the tendency of water, because of its molecular structure, to defy gravity and move upward through small spaces (capillaries). Water is able to rise through the soil, moving upward between soil particles and into the foundation slab. Water entering the home through the slab can wet carpeting, vinyl, and tile grout, potentially leading to mold growth.
A layer of gravel
To prevent groundwater from rising through the soil and wicking into the foundation slab, a layer of gravel should be laid out in the foundation footprint before the concrete is placed. The spaces between granules of gravel are too large for capillary action to take place, so groundwater cannot move upward through them.
A vapor barrier
A layer of gravel alone isn't enough to prevent water intrusion, however. Water can also rise in vapor form through the gravel layer and into the slab. To prevent water vapor from entering the slab, a 10 mil polyethylene vapor barrier should be placed on the gravel layer. The vapor barrier also protects the slab from water intrusion if the water table rises through the gravel bed during times of heavy rainfall or snowmelt.
Cause 1:
Lack of gravel layer below slab
If the foundation crew doesn't install a gravel layer in the foundation area before placing concrete, water will be able rise through the soil and into the porous foundation slab.
Solution
Before pouring concrete, install a layer of gravel in the foundation footprint. Because the spaces between granules of gravel are too large for capillary action to take place, the gravel keeps water from reaching the underside of the slab. The gravel layer should be at least 4" thick and be thoroughly tamped with a plate compactor to create a level surface for the concrete.
Cause 2:
Lack of vapor barrier below slab
If the foundation crew doesn't install a vapor barrier in the foundation area before placing concrete, moisture can rise in vapor form through the gravel layer and into the porous slab. Also, if the water table rises through the gravel layer during times of heavy rainfall or snowmelt, water will enter the slab if there is no vapor barrier in place.
Solution
To prevent water vapor from entering the slab, place a 10 mil vapor barrier on the gravel layer. The vapor barrier also protects the slab from water intrusion if the water table rises through the gravel layer during times of heavy rainfall or snowmelt.
Cause 3:
Improper vapor barrier installation
If the foundation crew doesn't install the vapor barrier correctly, it will be ineffective. Any gaps or tears in the vapor barrier create pathways for water vapor to enter the home through the porous slab.
Solution
Before the concrete is poured, make sure the vapor barrier fully covers the foundation footprint. The vapor barrier should drape into all grade beam trenches to provide a capillary break around the perimeter. Overlap all seams in the vapor barrier at least 12", and seal the seams with 4" construction tape.
