Concrete slab foundations have become very popular, especially in the south and west where soil freezing is not a problem and conditions often make basement foundations impractical. A typical slab-on-grade foundation is concrete poured over compacted soil and gravel basically at ground level. A raised floor foundation uses strong wood beams set atop concrete piers or perimeter walls to elevate the floor from a few inches to several feet.
Comparisons Are Difficult
Comparing slab and raised floor foundations is sort of an apples vs. oranges situation. The material and labor for a slab may be less expensive than raised flooring in some areas but this can be offset by differences in heating and cooling and maintenance expense. Cost also will vary with area and how high the house floor has to be above any potential for water to get in the house.
Difference Can Be Minimal
The Southern Pine Council says the difference in initial cost between a slab foundation and a wood raised floor system is minimal and a raised floor is the clear winner where floors must be elevated to protect against water. The American Institute of Architects says adding fill to get a slab above a flood elevation makes slab homes more expensive, while costs with raised floor systems do not change significantly. All slabs must be elevated enough so that rainwater will not get in the house.
Maintenance Is an Issue
Maintenance can be a cost consideration. Plumbing is buried under or in the concrete of a slab foundation. A leak or problem can require tearing up a significant area of flooring. Raised floor systems, set on beams with space underneath, allow repairs more easily. Raised floor systems also permit insulation under the flooring to reduce heating and cooling costs.
Settligng Can Cause Cracks
Concrete slab foundations are more susceptible to ground settling, especially if elevated with fill dirt. Uneven settling can lead to cracks in the slab. Raised floor foundations are less likely to experience differential settling causing cracks and breaks in concrete stem walls or piers. Site preparation, including grading and hauling, spreading and compacting gravel also are higher with slabs.
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