Fondation Concerns – Check for Drainage and Check the Soil
San Antonio is known for expansive clay soils. Fortunately, there are resources available that can help you make an informed decision when purchasing a home. Slab foundations are built with either post-tension cables or rebar so that they move as an entire unit instead of breaking apart. Pier and Beam foundations are not tied together and are more susceptible to sinking. Since a majority of San Antonio homes are built on expansive clay soils, a sufficiently compacted lot prep and a well engineered foundation based on soil samples from the area is a homeowners first and best line of defense. Unfortunately, homes built the same way in the same neighborhood can react differently. This is where additional information can help.
Small cracks can be found in virtually all slab foundations, and no foundation is perfectly level. Here are a couple rules of thumb that can help. Is the crack is large enough to put a coin in? Can you feel the level change (either by walking around the home, observing a marble rolling across a hard floor, or by observing doors swaying in one direction). These can be significant indicators of additional problems. If you like the home, and want to know more about the potential for future movement, check out these three things.
(1) Standing water. A flat lot with improper grading for drainage can eventually result in insufficient foundation support. Fortunately, proper grading and gutters can help mitigate this problem. If you love the home and want to make an offer, including these problem mitigation terms in your offer or in your budget. Sellers know it will be difficult to move a home that has further potential to shift and they may feel more inclined to fix the problem since it would help move the house if you were to back out.
(2) Check the USDA Web Soil Survey site for soil compositions. If the home is built on clay, how far down does it go? A home with rock at 18 inches is less likely to move than one built on nothing but clay. If it is on clay, make sure the drainage is sufficient and check for a sprinkler system. You may have heard that it’s recommended to “water your foundation regularly”. This recommendation is actually encouraging you to keep your clay soil at a more consistent moisture content. Watering during the dry season will help keep the soil from shrinking and cracking, while limiting water in the wet season helps to keep the clay from expanding.
(3) Check the San Antonio River Authority flood Risk MAP. This map is useful for general flood plane awareness, but it can also help identify areas where erosion or flat lots may be problematic. Land development engineers have become very adept at building water management into our newer developments in the form of water retention areas, drainage easements, etc., but older developments can have little or no water management built in. This map can help you understand what elevation your lot is with respect to the lots around you and give you a sense of where the water will or will not flow.
There is no crystal ball that we can ask how happy we will be with our home purchase in 10 years, but information can make our own predictions more accurate. If you have questions or additional resources regarding foundation concerns, please leave them in the comments below.