Foundation Drainage

Most homes around Pflugerville, Hutto and Manor sit on a concrete slab poured directly on the ground. Many newer slabs are post-tensioned, which means steel cables inside the concrete are tightened to help the slab resist movement. That design works best when the soil around and under it changes moisture slowly and evenly. Rain that runs off the roof and soaks in right at the slab edge does the opposite. It wets one side of the house while the other side stays dry, and the soil swells in one place and shrinks in another. Foundation drainage is the set of fixes that interrupt that cycle. The goal is simple. Catch roof water, move it well away from the house, and keep the ground beside the slab sloped so surface water leaves instead of ponding. We do not lift, pier or repair the structure itself. We work on the water.

Here is the soil problem in plain terms. East of I-35, much of the ground belongs to the Blackland Prairie, a belt of dark clay that includes the Houston Black series. The Texas Water Development Board describes Houston Black as one of the classic expansive soils in the world. Expansive means the clay swells when it soaks up water and shrinks and cracks when it dries. Over a wet spring followed by a dry August, the soil under a typical lot can rise and fall by an inch or more, and a slab riding on top of it moves along with it. You cannot stop clay from reacting to weather. You can reduce the difference between the wettest and driest spots around your house. That is what good drainage does. It keeps extreme moisture swings away from the footing line, which gives the slab and the soil under it a calmer year.

The most common cause we see is a gutter or downspout that dumps water beside the house. A single downspout on a 2,000 square foot roof can shed thousands of gallons in a wet season. If the pipe ends at the foundation or a short splash block, that water sinks straight down along the slab edge. Extending the discharge, or piping it underground to a pop-up emitter or the street side of the lot, often does more for a foundation than any other single change. The second cause is grade. Soil settles after construction, mulch beds pile up, and flower beds get edged with stone that traps water against the house. Building codes commonly call for ground that falls away from the foundation, and the usual rule of thumb is about six inches of drop over the first ten feet. Check your city and builder documents for the exact standard that applies to your lot.

When grade and gutters are not enough, a perimeter drain may help. This is a trench along the part of the house where water collects, lined with fabric, filled with washed gravel, and fitted with a perforated pipe that carries water to a safe outlet. It is closely related to a French drain, but placed with the foundation in mind. The trench stays shallow enough that it does not undermine the footing, and it is kept a sensible distance from the slab edge. Where water cannot drain by gravity, a sump basin with a pump can lift it to a discharge point. Outlet location matters as much as the pipe. The best drain in the world fails if it dumps onto a neighbor's lot or into a low corner with nowhere to go. We plan the outlet first, then work backward to the trench.

Be careful with any claim that drainage alone will fix a damaged foundation. It will not. Drainage reduces one of the forces acting on a slab. It cannot undo cracks, uneven floors or stuck doors that have already developed. If you see stair-step cracks in brick, doors that rub their frames, gaps between walls and ceilings, or floors that slope, call a licensed structural engineer or a foundation repair company first. Many homeowners get an independent engineer's report so they have an unbiased view of what the slab needs. Once the structure has been evaluated, drainage becomes the maintenance side of the plan. A repaired foundation that keeps getting soaked on one side can move again. A stable foundation with good drainage is more likely to stay that way.

A word about watering foundations. In dry summers, some Central Texas owners run soaker hoses around the slab to keep the clay from shrinking. Opinions on this differ, and the advice we found is mostly from contractors rather than neutral research. Watering unevenly can cause the same differential movement you are trying to prevent. If you want to try it, ask your engineer or foundation repair company what pattern and schedule fit your slab and soil, and keep hoses away from areas where water also ponds. Finally, before any trench is dug, Texas law requires a call to 811 at least two business days ahead so buried lines are marked. Your HOA or city may also have rules about changing grade or draining toward easements, so ask before work starts.