Slab-on-grade foundations are a building
engineering 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, the potential for large
heat losses where ground temperatures fall significantly below the interior temperature, and a
very low 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; proper soil compaction prior to pour can minimize this. The slab can
be decoupled from ground temperatures by insulation, with the concrete poured directly over
insulation (for example, Styrofoam panels), or heating provisions (such as hydronic heating)
can be built into the slab (an expensive installation, with associated running expenses).
Slab-on-grade foundations are commonly used in areas with expansive clay soil, particularly
in California and Texas. While elevated structural slabs actually perform better on expansive
clays, it is generally accepted by the engineering community that slab-on-grade foundations
offer the greatest cost-to-performance ratio for tract and semi-custom homes. Elevated
structural slabs are generally only found on large custom homes or homes with basements.
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 degrading
until the pipe fails. Copper pipes must be lagged, run through a conduit, or plumbed into the
building above the slab. Electrical conduits through the slab need to be water-tight, as they
extend below ground level and can potentially expose the wiring to groundwater.
|© Bell Inspection Service 2007