Why DFW Homes Crack: The Clay Soil Under Your Slab

If you own a home in Dallas, Plano, Garland, Wylie or almost anywhere else in the Metroplex, you have probably heard a neighbor talk about foundation work. That is not bad luck, and it is not always bad construction. It is geology. A large part of North Texas sits on the Blackland Prairie, a belt of dark, dense clay soil that is great for growing cotton and hard on concrete slabs.

This article explains what that clay actually does, why the damage shows up where it does, and what a real repair looks like, so you can tell the difference between a house that needs watching and a house that needs piers.

What makes Blackland Prairie clay so hard on foundations

Clay is made of extremely fine, flat particles. The clays common in North Texas soak water into their structure, and when they do, they swell. When the water leaves, they shrink and crack. Engineers call this expansive or shrink-swell soil, and the Blackland Prairie clays are some of the most active in the country.

The movement is not small. A clay layer that has dried out through a long summer can open cracks wide enough to put your fingers in. A few weeks of spring rain can close those cracks and push the ground back up. Your slab is sitting on top of that, and it is only four to six inches of concrete with some steel in it.

Here is the part most homeowners do not hear: the problem is rarely that the soil moves. The problem is that it moves unevenly. The ground under the middle of the house stays fairly stable because the slab shades it. The ground at the edges dries out and rehydrates with every season. A downspout dumping at one corner or a big oak pulling water out of one side makes it worse. The slab gets lifted in one place and loses support in another, and it bends.

The wet-spring, dry-summer cycle in North Texas

DFW weather sets the clay up for trouble almost every year. A typical pattern looks like this:

• Spring: heavy rain soaks the soil. The clay swells and can lift the edges of the slab. Homeowners notice doors that suddenly stick or will not latch.
• Summer: weeks of heat near or above 100 degrees pull moisture out of the ground. The clay shrinks and pulls away from the perimeter beam. The edges of the house can drop, and cracks open in brick and drywall.
• Fall: the first real rains return. Some cracks close back up, which convinces a lot of people the problem went away.
• Drought years: this is when damage jumps. A long dry stretch shrinks the clay deeper than usual, and the slab can settle further than it has in years.

Because the cycle repeats, foundation damage in North Texas tends to be progressive. A crack that opens and closes with the seasons is telling you the structure is moving every year. That is why a single patch almost never ends the story.

Warning signs to look for around your home

Foundation movement shows up in the parts of the house that are rigid and square: brick, door frames, windows and drywall. Walk your house with this list:

Outside

• Stair-step cracks in brick. Cracks that follow the mortar joints in a zig-zag, usually starting at a corner of a window or door, or near a corner of the house.
• Gaps at window frames and trim. Brick pulling away from a window frame or garage door jamb, or caulk joints that keep splitting.
• Separation at the frieze board. A gap opening between the top of the brick and the trim board under the eaves.
• Exposed slab edge with cracks, or soil pulled away from the foundation during summer.

Inside

• Doors that stick, swing on their own or will not latch, especially if the gap above the door is wider on one side.
• Diagonal drywall cracks running from the corners of doors and windows. We cover these in detail in Drywall Cracks Above Doors: Cosmetic or a Foundation Warning?
• Sloping or uneven floors, or cracks in tile that line up across several tiles.
• Gaps where walls meet the ceiling or where cabinets meet the wall.

How a foundation evaluation should work

A good evaluation is based on measurements, not on a quick look and a price. When we evaluate a slab in Dallas-Fort Worth, the core of it is a floor elevation survey. We take readings across the house with a digital level and map the high and low points. That map shows which direction the house is moving, how much, and where the support is missing.

We also walk the outside: the grading, where the gutters discharge, trees close to the foundation and any standing water. Those are the things that feed uneven soil moisture, and if they are not addressed, a repair is fighting the same forces that caused the damage. You get the findings documented, which also matters if your insurance carrier ever asks what was found.

For more on how the process works from start to finish, read our guide, Understanding Foundation Repair.

How piers stabilize a slab on clay

You cannot stop North Texas clay from moving. What you can do is stop depending on it. That is what piers do. A pier is a support driven or installed down through the active clay to a deeper, more stable layer, then tied to the underside of the perimeter beam or interior beams. The house now rests on the piers instead of on the soil that swells and shrinks.

The two systems we use most often are:

• Steel piers: steel pipe sections pressed into the ground with a hydraulic ram, using the weight of the structure, until they reach refusal on a load-bearing layer.
• Helical piers: steel shafts with helix plates that are screwed into the soil and torqued to a measured capacity. They work well where the load is lighter or where access is tight.

Once the piers are in, the slab can be lifted back toward level in controlled stages while the readings are checked. The goal is not always a perfectly flat floor. The goal is to stabilize the structure and bring it back as far as it safely goes without creating new cracks. Most residential foundation jobs run a few days on site once work begins, depending on how many piers are needed and how hard the access is.

Protecting the repair, and the house, from here on

Piers take the soil movement out of the equation for the supported areas, but water still matters. The clay under an unpiered part of the slab, your driveway and your flatwork still reacts to moisture. A few habits make a real difference in DFW:

1. Move water away from the house. The ground should slope away from the foundation, and downspouts should discharge several feet out. Our article on standing water near your slab covers how to check this.
2. Keep soil moisture steady in drought. During long dry spells, a soaker hose placed a foot or two from the slab and run on a schedule keeps the perimeter clay from shrinking away from the beam.
3. Watch trees near the foundation. Large trees close to the house pull a lot of water from the soil in summer. Root barriers or pruning may be part of the plan.
4. Fix drainage and plumbing leaks quickly. A leak under the slab can swell the clay in one spot and lift the floor. See Slab Leaks: When Your Foundation and Your Plumbing Fail Together.

If your drainage is part of the problem, our drainage and water management team can correct the grading and install French drains or surface drains alongside the foundation work.

When to call for an evaluation

Call when you see more than one of the signs above, when cracks come back after being repaired, or when doors that used to work fine start sticking during the same season every year. Call before you patch drywall or repaint, too. Cosmetic work done on a moving house tends to crack again within a season.

Texas Structural Concepts evaluates and repairs foundations for homes and commercial buildings across Dallas, Fort Worth, Plano, Frisco, McKinney, Allen, Garland, Wylie, Rockwall, Mesquite, Arlington and the rest of the Metroplex. The evaluation is free, the findings are documented, and you get a written scope that shows the reasoning behind every pier. Learn more about our foundation repair service, or schedule your free estimate.