Restoration Guide: Design and Engineering of Foundations

Roger Diez

Editor's Note: This is article 2 of 9 in the Foundations Chapter of the Old House Web Restoration Guide. This guide was developed and edited for old homes from original materials in the U.S. Housing and Urban Development (HUD) Rehab Guide.


Section 1--Existing Foundation Overview

Residential foundations in the United States are largely concrete or concrete block, although some old houses in rural areas may have stone foundations. When a home restoration project requires replacing old foundations with new construction, you must follow regional, state, and municipal building codes. Because building codes vary from region to region and inspector to inspector, it is best to verify requirements early in your project. Additionally, environmental conditions such as soil conditions can impact how your foundation settles and may require you to adjust the amount of reinforcement you use.

Many production builders have historically built thicker foundation walls with minimal reinforcement. In this case, the footings and foundations may be subject to settlement and cracking problems and care should be taken when replacing existing concrete walls.

Refer to American Concrete Institute (ACI) standards, local building codes, and publications by the National Association of Home Builders, the National Concrete Masonry Association, and the Portland Cement Association for foundation design parameters.

Section 2--Basement Floors

Basement floors are typically concrete slabs with a thickness of four inches. They can be damaged by deterioration, and settlement due to soil heaving or poor compaction. Severely cracked or damaged basement floors should be replaced if structurally unsound. Otherwise, a bonded or unbonded slab poured over the existing floor may be a suitable repair.

2.1: Providing a New Floor Slab or Replacing a Portion of the Existing Slab

In the case of an old house with an earthen basement floor, excavate no deeper than the bottom of the foundation wall footing. To provide sufficient support, use a minimum of four inches of gravel, crushed stone, or coarse sand under the concrete that covers the top of the footing by several inches. This can help provide uniform support and avoid cracking due to soil shrinking and swelling.

In the case of high water tables or poor drainage, a protective layer--usually polyethylene sheeting--must be used to prevent moisture migration through the slab. Use the same techniques when replacing a portion of an existing slab.

2.2: Pouring a New Fully Bonded Floor Slab over the Existing Slab

If your basement floor is clean and structurally sound, but you want to level it off before finishing it, you can pour a one to two inch overlay on top of the existing slab. You can use either conventional, low-slump concrete with plenty of sand or a fast-drying, specially formulated thin topping or underlayment referred to as "self-leveling."

Toppings provide a finished floor surface, while underlayments require a floor-covering material. This relatively inexpensive home restoration technique reduces the chance of shrinking and cracking, but should not be used if there is danger of a severe water problem.

2.3: Providing a New Unbounded Floor Slab over the Existing Slab

If the surface of the existing slab is not suitable for a bonded overlay, use a new unbonded four-inch slab. Follow these tips:

  1. Clean the existing surface and fill pitted and badly worn areas with a mortar
  2. Place a layer of polyethylene sheeting over the existing slab to prevent moisture from traveling between layers and keep bonds from forming between slabs
  3. Use nonstructural welded wire reinforcement to minimize shrinkage and cracking

This home renovation technique reduces expense by eliminating the need to remove the existing slab. However, it raises the floor height considerably, reducing head room.

Section 3--Crawl Space Floors

The typical crawl space in an old house has an exposed-dirt floor. Pouring a concrete floor can help with the resulting moisture, vermin, and insect problems. Conventional concrete can be difficult and expensive to use in these odd spaces, but new concrete mixtures using lightweight aggregates have been developed just for his purpose.

To seal the dirt floor of a crawlspace, use a specialty concrete product using Zonolite of Vermiculite aggregate. This fast-drying mixture is pumped into the crawlspace using lightweight mobile mixing equipment and adheres to almost any surface. It should be poured to a depth of three inches over a polyethylene moisture retarder and trowel-finished. While ideal for uneven surfaces, this concrete mixture typically cannot bear heavy loads.

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