Foundation: Slab on Grade

Stem wall foundations in areas of high rainfall, high groundwater, or low percolation rates need a perimeter drain to drain water that will collect around the foundation below grade. Install the perimeter drain so it's even and minimally sloped in the direction water should travel. The piping should slope slightly toward the main storm sewer or to an area above ground that's at an elevation lower than the stem walls. To protect the drain, make sure it's surrounded by gravel fill. The gravel fill should be continuous around the entire perimeter of the foundation. In addition, the drain pipe should have an integrated filter sock. The filter sock keeps out silt and clay soils that could clog the pipe. If the pipe has holes on only one side, orient the pipe with the holes facing downward. The discharge pipe should be large enough to properly remove groundwater around the foundation; it may need to be as large as 6". The discharge pipe should slope away from the home for adequate drainage and be screened from blockage and entry by pests. Collected water should be discharged at least 10' away from the foundation.

Before pouring the concrete for the slab, 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 foundation. The gravel layer should be thoroughly tamped with a plate compactor to create a level surface for the concrete. It's important for the excavation to be level before the gravel layer is laid out, so it will have a consistent thickness. If the gravel layer is too thin in places, it won't be able to prevent capillary action.

To prevent water vapor from entering the slab, place a 10 mil polyethylene vapor barrier on the gravel layer. The vapor barrier also protects the foundation from water intrusion if the water table rises through the gravel layer during times of heavy rainfall or snowmelt. Fully cover the foundation footprint with the vapor barrier, and overlap the vapor barrier over any tears. Overlap all seams in the vapor barrier at least 12", and seal the seams with 3" construction tape.

Properly seal the vapor barrier around penetrating plumbing and electrical stubs. To seal penetrations in the vapor barrier, cut out a 4' x 4' piece of vapor barrier material. Make an X-shaped incision in the 4' x 4' square that's just large enough to fit tightly around the plumbing or electrical stub. Place the square over the penetration; the square should lie flat on the vapor barrier. Then, tape the incision around the penetration.

Specify concrete with durable, well-graded aggregate, with a low water-to-cement ratio (less than 0.45), and with adequate compressive strength. Choose a low-shrinkage concrete mix. Upon delivery, check the concrete ticket to confirm that the concrete is the type that was ordered. If the concrete ticket indicates that the mix has been sitting for a long time, reject it, and request a new batch of concrete, rather than adding water to the mix to make it workable. Perform a slump test to determine the workability of the concrete; it should have a slump of 4” or less. Don’t add excess water to the mix to make it easier to work with or to extend the working time. Add admixtures to the mix only if they're appropriate to the environment. Don't exceed the recommended amount of calcium chloride admixtures.

When finishing a foundation slab, wait to finish the concrete until all of the bleed water has evaporated to ensure the strength of the slab. Don't add water to increase the working time when finishing concrete. Make sure the finish isn't overworked; overworking the finish weakens the slab surface, leading to cracks, spalling, and other surface issues.

In a basement slab, control joints help prevent random cracking, which can weaken the foundation slab. Place control joints no farther apart than 30 times the thickness of the slab. For instance, a 4” slab should have joints no farther apart than 10’. Control joints can be tooled or cut in after the slab has set, or they can be installed as pieces of material that create a cold joint.

A slab edge or stem wall contains millions of pores that allow water to pass through. Water can also pass through the foundation at cracks, cold joints, and other weak areas. To prevent water wicking through the slab edge or stem walls, an elastomeric asphalt coating should be applied around the foundation perimeter, from the bottom of the foundation to the final grade. Or, the vapor barrier should be extended to provide coverage for the foundation perimeter. It's important to consider what the final grade is after landscaping and make sure the coating or vapor barrier is as high as the landscaping.

On stem wall foundations, a drainage board or dimpled membrane can be installed on the coating. This practice is highly recommended if the stem wall foundation has a perimeter drain; the drainage board or dimpled membrane guides water to the perimeter drain.

To prevent cold floors and prevent heat loss and heat gain through the slab, install insulation on the slab edge. During colder months, slab insulation helps to keep homeowners comfortable and heating bills low. During warmer months, insulation helps to control air conditioning costs.

Grading has a significant impact on foundation drainage. Grading should keep water from collecting against the foundation and drain water away from the home. Slope the grade per local code. Typically, the grade should continuously fall a minimum of 6" within the first 10' and should slope to the sidewalk or street. Swales should slope per the plan, typically at least 2%. The final grade should follow the slope already established by the rough grade. The top of the foundation should be a minimum of 8" above grade. Front walks should allow water to drain over, under, or through them. Don't place front walks too high. Terminate the exterior finish a minimum of 8" above grade, so water doesn't come into continuous contact with the finish. Place plants at least 2' away from the foundation. They should also be planted low enough in the soil that they don't direct water toward the foundation.