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RAISED FOUNDATIONS
A raised foundation typically consists of a foundation stemwall and footing supporting the exterior walls of the house, with the middle portion supported on posts and piers. Sometimes in more modern raised foundations there is a stemwall bisecting the middle of the house. The perimeter is usually concrete, but can be constructed with brick, concrete blocks, mortared rocks or even posts and piers as a perimeter, which is placed on a footing. The footing is considered to be the portion of the foundation below ground. The posts and piers consist of a poured or precast concrete pier, usually with a footing extending below grade.
The perimeter foundation carries most of the loads of the house framing, floors, roof, etc. Some foundations have a cripple wall, which is a framed wall between the floor joists and the concrete stemwall. The footings extend below grade (under the soil) from 6” deep or more. In 1949, code required concrete to have foundation bolts to connect the wood framing to the concrete in order to hold down the house in the event of seismic activity or high winds. Prior to 1949, few houses have foundation bolts. Older concrete is more likely to be in poor condition due to the substandard materials such as insufficient Portland cement, oversized aggregates, beach sand or similarly impure materials, as well as age and wear from poor drainage. Learn more…
SLAB FOUNDATIONS
A slab foundation consists of footings for the perimeter, with a concrete slab that is the floor of the house, with no access underneath. Footings depths for single story houses are a minimum of 12” wide by 12” deep, reinforced with two pieces of ½” rebar placed horizontally. The concrete slabs are typically 4” thick, and preferably reinforced with rebar or wire mesh. Depending on its age, there may be a moisture barrier between the slab and the soil, likely embedded in a sand sub-base under the slab.
The quality of the slabs and footings installed has a direct impact on the future performance of the foundation. Older slabs are often placed without adequate reinforcing steel, or with steel that is not placed in the middle of the slab, making them more likely to crack. Steel without cover by the concrete are likely to rust, causing cracks and not performing to hold the concrete together. Slabs placed on expansive soil without a sand sub base are more likely to heave when moisture expands the clay. Leaking plumbing under a slab can affect the soil, causing slabs to crack. Tree roots can extend under a slab, lifting and cracking the concrete. Poor quality concrete or excessive heat during pouring can cause shrinkage cracks as the concrete cures.
Slab foundations were used starting in the late 1940’s. In San Diego at this time, more areas were developed, and in order to find flat lots to build on the hillsides and canyons were graded. Grading a lot flat entails cutting into the hillside on the uphill side, and using that soil to push down the slope. Often, the soil was not compacted, which entails using machinery to pound the soil as it is placed in layers on the downhill side of the lot. If not properly compacted, soil will compress, allowing the structure of the house to move. Also, the soil may have been placed without benching the slope, basically cutting steps into the slope prior to placing the fill. Learn More…
