In depth
Drainage Correction in San Antonio: how the job actually runs
Drainage correction is the foundation repair that does not involve the foundation. On expansive clay, a slab moves because the soil under one side is wetter than the other, and on the limestone north side a slab settles because water gets into the fill it sits on. Getting the water off the foundation and keeping the moisture even around it is the first step on almost every San Antonio evaluation, the whole fix on a good share of them, and the reason a pier plan holds after the lift. It is also the cheapest thing on the price list, which is why an honest evaluation mentions it before it mentions piers.
Why water moves San Antonio houses
About 32 inches of rain a year fall on the metro, in bursts: a wet May and June, a wet September and October, and a long dry summer between them. Blackland clay south and east of the Balcones fault, under Cibolo, Schertz, Converse, Live Oak and the south side, swells when the spring rain soaks it and shrinks when the summer bakes it, and a slab on top rises and falls at the edges. If the water were even all the way around, the whole house would rise and fall together and nobody would notice. It is never even. A downspout puts a hundred gallons at one corner in a storm. A sprinkler soaks the bed on the front while the back stays bare. A live oak on the east side pulls water out of the clay faster than the west side can lose it. A neighbor’s lot drains onto yours. The slab gets a wet edge and a dry edge, tilts toward the dry one, and the brick opens above the garage door. On the limestone side, in Stone Oak, Helotes and Boerne, the same downspout puts water into the fill under a post-tension slab, and the fill settles where it is wet. The full mechanism is in why San Antonio homes have foundation problems.
The water survey
The evaluation follows the water before it looks at piers. Where does the roof drain, and are there gutters. Where do the downspouts end. Which way does the ground slope in the first ten feet from the foundation, and where does water stand after a storm. Which beds are watered, how often, and on which sides. Where are the trees, how big, and on which side of the house. Does the neighbor’s lot drain onto this one. Is there a crawl space, and does it hold water. Each answer is checked against the elevation survey, and the wet edge and the dry edge of the slab almost always line up with what the water is doing.
The fixes, in order of cost
| Fix | What it does | Typical cost | Usually first when |
|---|---|---|---|
| Gutters and downspout extensions | Carries roof water well past the slab, above ground or in a buried pipe to daylight | $1,200–$4,000 | A downspout ends at a corner, or there are no gutters |
| Grading and swales | Reshapes the first ten feet to slope away and cuts a shallow channel to carry runoff around the house | $1,000–$4,000 | Water pools against the foundation or the yard slopes in |
| Root barrier | Vertical sheet in a trench between the tree and the slab, stops feeder roots drying the clay | $1,200–$4,000 | The low corner is the tree corner |
| French drain | Gravel trench with perforated pipe collecting subsurface water and carrying it to daylight | $2,500–$9,000 | Grading cannot move the water, or the wet side has no roof water reaching it |
| Crawl space moisture barrier and vents | Keeps the ground under a pier and beam house from feeding moisture into the wood | $1,500–$5,000 | The crawl space holds water or never dries |
| Soaker hose plan | Even moisture on every side through the dry months | Hose and timer | Every clay lot, after the other fixes |
Roof water is most of the water that reaches a foundation in a storm, so gutters and extensions come first and solve more than people expect. Grading is next. Root barriers go in where the tree is the cause and are planned with the city in Shavano Park, Garden Ridge and the other places with tree ordinances; a healthy live oak tolerates a barrier set at a sensible distance. A French drain is the answer when the ground cannot be made to drain on its own, which is more common in Leon Valley along Huebner Creek and on the flat former farmland of the northeast than on sloped lots.
Watering, and the rules
On clay, the dry months need water added, evenly, so the slab does not get a dry edge in August. A soaker hose 12 to 18 inches from the foundation on every side, run on a timer in the early morning or evening, is the method; the goal is consistent moisture, not wet soil, and not just the side with the crack. SAWS drought stages, set by the Edwards Aquifer level, restrict sprinkler days and hours; soaker hoses and drip irrigation are treated more leniently but still have time-of-day rules that change with the stage, so the plan is written for the current stage and says what to adjust. New Braunfels, Seguin, Schertz, Cibolo and Boerne run their own utilities with their own rules. On the limestone north side the advice flips: the soil is thin, and the job is keeping water out of the fill rather than adding it. The details are in how to water your foundation in San Antonio.
Drainage and piers
Where the elevation survey shows an inch or more of differential and brick cracks you can fit a coin in, the house needs piers on the moving section, and the drainage still comes with it, because the unpiered sections keep moving with the seasons and a pier plan on a house with a downspout at the corner is a plan that gets a second phase. The slab foundation repair proposal lists the drainage items alongside the pier count. On a pier and beam house, drying the crawl space is what keeps the new sill from starting the same clock; see pier and beam foundation repair.
What it costs
Gutters and downspout extensions $1,200 to $4,000. Grading and swales $1,000 to $4,000. French drain $2,500 to $9,000, or $30 to $100 per linear foot. Root barrier $1,200 to $4,000, or $40 to $80 per linear foot. Crawl space moisture barrier and ventilation $1,500 to $5,000. The evaluation and water survey are free for most homeowners. Every item is a line on the written proposal, and on a house that does not need piers yet, the drainage is the whole proposal, with the elevation survey kept so the next one shows whether it worked.
While you wait
Walk the house during the next rain and watch where the water goes; a photo of a downspout emptying at the corner is worth more than a description. Extend the worst downspout with a length of pipe today. Turn off any sprinkler zone that soaks one bed against the house and leaves the other side dry. If there is a crawl space, look for standing water after the storm. Then send the form with the neighborhood, what the house is doing and where the water goes, and you will get an evaluation window and a written plan that starts with the cheapest fix.
Last updated September 05, 2026
