Poor Drainage and Foundation Movement

The soil under a house is not negotiable. Its moisture content is. Rain, irrigation, where a roof sheds, and the shape of the ground in the first few feet out from the wall all move it, and unlike the clay itself, every one of those can be altered. That is the whole reason drainage takes up so much of the discussion about foundations in this region — it is the one input a homeowner genuinely controls.

It is worth being precise about what that control buys. Correcting drainage does not repair a foundation that has already moved, and no arrangement of gutters and grading holds soil perfectly still through a North Texas year. What it does is reduce how sharply, and how unevenly, moisture swings at the perimeter — and the perimeter is where the movement that damages a house usually starts.

For water to change the soil's volume it first has to get into the soil, and heavy clay is unusually slow at accepting it. The USDA's official description of Houston Black — the dominant clay across the Blackland Prairie under the eastern Metroplex — classes the series as moderately well drained with a permeability class of very slow, and rates runoff as high on slopes of less than 1 percent and very high on slopes of 1 to 8 percent[1].

Very slow permeability and high runoff, stated together, describe ground that sheds water sideways rather than taking it down evenly. Rain landing on a lawn over that clay does not distribute itself politely through the profile. A large share of it travels across the surface until it finds somewhere low to sit — and it sits there long enough to soak in at that one spot while the ground a few feet away stays comparatively dry.

A house makes that tendency worse before anyone does anything wrong. The US Geological Survey notes that impervious surfaces — roofs, roads, driveways, sidewalks, parking lots — do not allow water to pass through them[2], and a house is a large impervious surface with a system bolted to its edge for collecting everything that lands on it and discharging it somewhere specific. Across land generally, roughly a third of precipitation becomes runoff, with the remainder evaporating, transpiring, or infiltrating[2]. Around a building, the roof has already decided where a good part of that third is going.

The arrangements worth looking for are not exotic. They are the ordinary ones that take water which would otherwise have spread out and deliver it to a single part of the perimeter instead:

• Downspouts that discharge at the wall. A downspout emptying onto the ground beside the slab concentrates the runoff from a large area of roof into a few square feet of soil, repeatedly, every time it rains.
• Ground that falls back toward the house. Grade settles over the years, and mulch and topsoil get added and re-added. A slope that once ran away from the wall can end up running toward it without anyone having decided to change it.
• Beds held in by solid edging. A raised planting bed with a continuous border along the wall is a basin. It holds irrigation and rainfall against the foundation rather than letting it drain off.
• Flatwork that has tilted. A patio, walkway, or driveway that has settled toward the house turns a large paved area into a funnel pointed at the slab edge.
• Irrigation that reaches only part of the perimeter. A sprinkler zone covering the front and none of the back keeps one side of the house at a moisture level the other side never sees.
• Mature trees and large shrubs close in. Roots withdraw moisture from the soil they occupy, which is a localized drying effect on whichever side of the house they happen to be on.

Most houses have one or two of these. Having them does not mean a foundation is failing. It means there is a known mechanism concentrating moisture change in a specific place, which is worth knowing about before something starts moving rather than afterwards.

A perimeter that is uniformly wet in March and uniformly dry in August is being raised and lowered as a unit, and a house largely tolerates that. What racks a door frame and steps a crack through brickwork is one part of the slab sitting on soil at a different moisture than another part at the same time.

That is why the list above matters more than the rainfall total does. A downspout emptying at one corner, a bed holding water along one wall, a tree drawing down one side — each of those produces exactly the condition that causes trouble, which is a moisture difference across a single structure.

The underlying volume change is the same one described on the clay soil page. Drainage simply decides where it happens, and how abruptly.

This is something you can observe yourself, and the observation is worth considerably more than a guess. Do it during and just after real rain rather than on a dry afternoon — a dry lot tells you almost nothing about where the water goes.

Walk the full perimeter and note three things: where each downspout actually discharges and how far from the wall, where water is still standing an hour after the rain stops, and whether the ground within the first few feet falls away from the wall or back toward it. Photograph each spot and keep the date on the photograph. One pass is a snapshot; the same pass repeated in the opposite season is evidence.

Grade does not need instruments. A straight length of timber laid on the soil with a level on top will give you the direction of fall, and measuring the drop over the first several feet out from the wall turns "it looks flat" into a number you can compare against next year's number.

It is also worth establishing, once, whether high-shrink-swell clay is even part of your situation. The NRCS Web Soil Survey lets anyone define an area of interest and pull the soil map unit and its properties for that area[3], which replaces a regional assumption about your lot with the mapped record for it.

Standing water is an observation, not a diagnosis. And a persistently wet area with no rain behind it is a plumbing question before it is a drainage one — worth ruling in or out early, because it changes what everything else means.