Residential Foundation Design
Residential Foundation Design
How do residential foundations built today differ from homes built in the past?
What is the evolution that has led to today's building practices in the Houston area?
Is new house construction adequate to eliminate the need for foundation repairs and house leveling?
Foundation design plays a key role in minimizing the need for foundation repair. Sadly,
building codes lag far behind what is feasible to prevent foundation problems. Because
of the weak building codes, most houses built this year will not be immune to the need
for foundation repairs and house leveling.
Pre 1960 homes were predominantly pier and beam.
A raised wooden sub-floor containing wooden cross members known as beams was supported every 6-10 feet by either a wooden post or a concrete pier. Typically the posts and piers were buried 1-4 feet into the ground. Sometimes the wooden posts rested on a concrete slab that was placed on the surface of the ground. Typically the perime ter of the house was supported by a 2 - 4 foot high continuous concrete beam, but in so me cases there was no concrete beam.
The exterior concrete beam was a blessing because it kept rain water out from under the pier and beam house. Over the decades though, soil built up around the concrete
perimeter, and the today the under floor interior area may hold rain water.
Following a heavy rain, in a pier and beam house, doors and windows may become stuck, and walls develop minor cracks.
Why will a pier and beam house move after a rain?
Because poor drainage allows water to accumulate or pass under the crawl space, causingthe clay soil to expand. A soil moisture increase under a pier will cause it to hea
ve. If clay soil becomes saturated, it loses the ability to support one or more piers and
the effected piers will sink.
Some of the other problems that can arise in a pier and beam foundation include: woodrot in beams of posts, warped beams, broken beams or joists, and termite damage.
Occasionally we have found that the spacing between piers was too great, leading to sagging beams. Or perhaps a second story was added some years later, and the existing beams cannot support the additional load.
To this day pier and beam houses have proven to be reliable structures, with occasional foundation repair requiring shimming, post replacement, extra beam support, replacing damaged beams, joists and sill plates, or adding additional support.
Cheap concrete slabs evolved in the late 40's, in a movement to slash construction costs by replacing the wooden floor and beams with a cheaper concrete slab.This technique really gained momentum in the late '50s.
Early concrete slab foundations consisted of a four inch concrete pad with no reinforcing steel. In later construction, the reinforcing steel was ro
lled out, and looked much like fence with 4 inch squares. There was a single concrete reinforcing beam 10-20 inches thick traversing the middle of the house. The theory was that the slab and the whole house would float upon the Texas clay soils.
The flaw with a weak floating slab was that as the house aged, localized moisture induced either: swelling, settling and / or soil consolidation, which lead to cracks and foundation failure. The concrete simply was not thick enough to support the enormous
load of the house and resist the forces of expanding and contracting clay soil.
Foundation repair and house leveling is sometimes hard to accomplish on concrete slab homes built in the 50s and 60s due to the weak nature of the poorly reinforced concrete.
FHA Standards for residential concrete slab foundations:
Universal design standards began in 1968 as a result of the establishment of a Federal Housing Authority standard for the "Criteria for Selection and Design of Residential Slab on Grade" This standard attempted to encompass soil plasticity (sensitivity to moisture), climate changes, and house loading. Several types of slabs were established. All except Type IV foundations often require foundation repair.
As a word of caution and truth in advertising, the FHAhas not established "approved foundation repair methods".
Type I slabs are 4 inches thick with a perimeter beam 10 inches thick and 6 inches wide, with no requirement for wire or steel reinforce ment.
Type II slabs require welded wire across the 4 inch slab, with perimeter beams increased to a depth of 16 inches and a width of 8 inches.
Type III slabs require rebar reinforcement in both the 4 inch slab and beams, with perimeter beams plus internal beams every 15 feet, beams being 20 inches deep and 8
inches wide. A variation to the Type III slab has been the addition of post tensioning cables, and a reduction in beam spacing. Much of the rebar was replaced with cable.
Houston houses since '68 have typically been built to the Type III standard. The need for foundation repair in Type III construction arises with homes as little as 18 months old.
Type IV slabs are the new standard of excellence for Houston soil conditions. Theslab is elevated by concrete beams supported by bell bottom piers extending to load bearing strata or at least to a depth of 7-20 feet.
Type IV concrete foundation construction is the more expensive than Type III. Given two homes side by side, most buyers choose the one that is $10000 cheaper. Logic says that the cheaper foundation must be good because "itwas built to code". Code is woefully inadequate in most Houston and surrounding cities.
Type IV concrete foundation construction is the most effective method for avoiding foundation problems. It is a high tech throw back to the old pier and beam construction,with the benefit that piers extend deeply to more stable load bearing strata. Today less than 1% of the homes being built utilize Type IV construction methods.
What is the solution to avoid foundation repair?
Much of the time, the need for foundation repair can be attributed to poor drainage, improper watering, and plumbing leaks. Problems may also arise from improper soil
compaction and soil preparation prior to slab pouring.