Slab-on-grade foundations
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Slab-on-grade foundations are a building practice whereby the concrete slab that is to serve as the foundation for the structure is formed from a mold set into the ground. The concrete is then poured into the mold, leaving no space between the ground and the structure. This type of construction is most often seen in warmer climates, where ground freezing and thawing is less of a concern and where there is no need for heat ducting underneath the floor.
The advantages of the slab technique are that it is relatively cheap and sturdy, and is considered less vulnerable to termite infestation because there are no hollow spaces or wood channels leading from the ground to the structure (assuming wood siding, etc., is not carried all the way to the ground on the outer walls).
The disadvantages are the lack of access from below for utility lines, a tendency to transmit cold upward in areas where ground temperatures fall significantly, and a very low grade elevation that may expose the building to flood damage in even moderate rains. Remodeling or extending such a structure may also be more difficult. Over the long term, ground settling (or subsidence) may be a problem, as a slab foundation cannot be readily jacked up to compensate.
Care must be taken with the provision of services through the slab. Copper piping, commonly used to carry natural gas and water, reacts with concrete over a long period, slowly regrading until the pipe fails. Copper pipes must be lagged, run through a conduit, or plumbed into the building above the slab. Electrical conduit through the slab needs to be water-tight, as it extends below ground level and potentially exposes the wiring to groundwater.
External links
- This Old House - Slab Foundations (http://www.thisoldhouse.com/toh/knowhow/design/article/0,16417,559048,00.html)
- Repair-home.com - Slab Foundations page (http://www.repair-home.com/how_to/home_construction_foundations.htm)
- Concrete Network - Building a High-Quality Slab (http://www.concretenetwork.com/concrete/slabs/)
- Negative opinion on slab construction (http://www.hammerzone.com/archives/letters/foundation/slab_vs_crawl.htm)