Foundations and North Texas clay: the most important line in your budget

If you have lived in the metroplex for any length of time you have heard someone talk about their foundation. There is a reason it comes up here more than almost anywhere else, and it is not the concrete.

The problem is the dirt

Much of North Texas sits on expansive clay. It is a soil that behaves less like ground and more like a sponge with opinions. When it takes on water it swells. When it dries out — and August in Fort Worth dries things out — it shrinks and pulls away from whatever is sitting on it.

A slab can handle being lifted. What it cannot handle is being lifted unevenly. And uneven is the normal case: the north side of a house stays shaded and damp while the south side bakes, a downspout dumps into one corner, a large tree drinks out of the clay along one wall, a sewer line leaks under one room. The soil under one part of the house goes up while the soil under another goes down, and the slab is asked to bridge the difference.

That differential movement is the source of nearly every foundation complaint in DFW. Not bad concrete. Soil that moves at different rates in different places.

What a soil report actually tells you

Before a foundation is designed, a geotechnical engineer takes borings on the lot and reports what is down there — soil types, moisture, and critically the plasticity index, which is a measure of how much the clay is inclined to swell and shrink. From that, he calculates the potential vertical rise for the site: how much the soil could realistically move.

Your structural engineer designs the foundation to that number. Without it, he is guessing, and guessing on the one element of the house you cannot go back and fix.

Post-tension and conventional slabs

Most new homes in North Texas sit on a post-tensioned slab. Steel tendons are laid in the slab in sheathed ducts, the concrete is poured around them, and once it has reached strength the tendons are tensioned and anchored. That puts the whole slab into compression.

Concrete is strong in compression and weak in tension, so keeping it compressed is exactly the right idea. A post-tensioned slab can accept more differential movement before it cracks, and small shrinkage cracks tend to stay tight rather than open up.

A conventional slab uses reinforcing steel and, on a stiffened design, deeper beams in a grid to resist bending. Done to a proper engineered design it works too — you see it more often on smaller footprints and on sites where the soil is less troublesome.

The honest caveat on both: a foundation system does not change the soil. Post-tensioning makes a slab better at tolerating movement. It does not reduce how much the clay swells. That is why the work that happens before and around the pour matters as much as the pour itself.

The work that actually protects a foundation

Site preparation

Stripping the organics, bringing the subgrade to the engineer's specification, and compacting fill in controlled lifts rather than dumping it and driving over it. Where the report calls for it, select fill or lime treatment to reduce how reactive the clay at the surface is. This is unglamorous, invisible once the slab is poured, and one of the biggest differences between builders.

Moisture conditioning

Getting the subgrade to a consistent moisture level before the pour matters. Pouring on clay that is bone dry after three weeks of heat means the soil will take on water afterward and swell under a house that was set on it dry.

Drainage

Positive grade away from the house on every side. Gutters that discharge well clear of the slab, not at the corner of it. Downspout extensions or buried drains. Where a lot needs it, area drains and a French drain. This is the single most effective and most neglected foundation protection there is, and it is a fraction of the cost of anything else on this page.

Vapor retarder and details

A proper vapor retarder under the slab, sealed at laps and penetrations. It keeps ground moisture out of the house and helps the concrete cure at a consistent rate.

What you should do after you move in

A new slab is not finished being cared for on closing day. The goal, forever, is even moisture around the perimeter.

• Keep the grade sloped away. Beds settle over time and mulch piles up. Check it once a year.
• Keep gutters working and discharging clear of the house. A clogged gutter dumps a concentrated column of water into one spot, which is exactly the differential you are trying to avoid.
• Water the perimeter in dry spells. Soaker hoses set a foot or two out from the slab, run modestly and consistently, do more good than a heavy soak once a month. The aim is steady, not saturated.
• Do not over-water either. A sprinkler head aimed at the foundation is its own problem.
• Be careful with large trees near the house. A mature tree can pull a remarkable amount of water out of the clay along one wall.
• Fix plumbing leaks immediately. A slow leak under a slab is a moisture source in exactly the wrong place.

Hairline cracks are normal

Concrete shrinks as it cures, and small hairline cracks in a slab or in drywall during the first year are ordinary. What is worth a phone call is a crack that is growing, a crack wide enough to catch a fingernail, doors that begin to stick or swing on their own, or gaps opening between trim and wall.

We would always rather look at something and tell you it is nothing than hear about it two years later.

Why we go on about this

We started out remodeling old houses around Fort Worth, which means we spent years walking into the aftermath of foundations that were not designed for what was under them. It is the one part of a house where doing it right is invisible and doing it wrong is permanent.

If you have a lot you are considering, we will tell you what we think the soil is going to ask of the budget. Send us the address.