House plans for clay soil

Expansive clay takes on water and swells, then dries and shrinks. USDA describes Vertisols, the group Houston Black clay belongs to, as soils that shrink and swell markedly with changes in moisture, and says Houston Black's very high shrink-swell potential commonly limits building site development. [1]

Around Houston the usual clay is the Lake Charles series, a Vertisol that is extensive in Southeast Texas and cracks 1/2 to 2 inches wide when dry. [2]

The movement is not the same everywhere on a lot. A Houston engineers' recommended practice asks the soil report for the probable movement of the ground surface, both up and down, and places a boring near the largest tree within 25 feet of the house. [3] A slab designed for stable soil is not designed for that, which is why the building code has separate rules for foundations on expansive soil, covered below. [6]

Texas does not require an architect to draw a single-family or two-family dwelling. The Architectural Practice Act exempts a person who prepares those plans, as long as they do not present themselves as an architect. [5]

Engineering is different. Tex. Occ. Code 1001.056 exempts private dwellings from the engineer requirement, but it carves two things back out, and one of them is engineering design for a residential slab on expansive soil, unless the work meets the International Residential Code requirements as applied in that jurisdiction. [4]

The second carve out is engineering design or inspection for windstorm certification [4], which applies in the Texas Department of Insurance's designated catastrophe area along the coast. [7]

The IRC itself sends foundations on expansive soil to the International Building Code's design rules, with an exception for foundation systems that have performed adequately in similar soil, subject to the building official's approval. [6] On Gulf Coast clay, plan on a foundation designed by a Texas licensed engineer.

Two costs come on top of the plan: the geotechnical report, which is what the engineer designs against, and the sealed foundation design itself. Prices vary with the lot, the number of borings and the engineer, so get written quotes for both before you commit to a plan.

Asking for the quotes before you buy the plan keeps them from arriving as a surprise after.

A foundation plan that names expansive soil, specifies a perimeter beam depth rather than a generic detail, and either includes post tension layout or says plainly that it needs one.

A note about which jurisdiction and code edition the plan was drawn for. Cities adopt the code with their own amendments, as Houston does, so a plan drawn for another place may need changes for yours. [8]

An honest list of what is excluded. If a plan page does not say what the set leaves out, ask before you buy.

If the plan you are looking at does not have these, that is not a reason to abandon it. It is a reason to budget the engineering before you buy rather than after.