Q: We live in a 1950s Eichler built on a concrete slab. The soil immediately around the house is heavy clay and I have noticed water pooling after rain. Even after months of dry weather the soil is still wet. Could this constant moisture cause mildew to grow on and up through the concrete slab? I have noticed a musty smell and now my infant son seems to be suffering from an allergy. How do we test for this and what do we do to eliminate it?
A: We hope your infant son’s condition has improved and that it is not related to the condition of your house. But we think there is a very good chance that your wet soil conditions are creating a condition where mold could flourish.
Mold can grow almost anywhere there is water damage, high humidity or dampness. Any underlying moisture problems must be fixed to eliminate mold.
Treating mold infestations is a two-step process. First, the condition contributing to the mold must be corrected – in this case, poor drainage. Second, the affected area must be cleaned.
The clay soil prevalent in the San Francisco Bay Area absorbs and retains moisture. In wet years, as we’ve experienced this year, the soil around a foundation can remain wet for months. Also, after the rainy season ends, homeowners often overwater vegetation close to their foundation. This never allows the soil a chance to dry out.
Concrete slab foundations can wick moisture from the damp exterior soil and create perfect conditions for mold.
Our first recommendation is to determine whether the drainage around the foundation of your home is faulty. If it is substandard, as you indicate, the condition must be corrected.
Thinning out planting around the foundation might help. But if the wetness is as persistent as you describe, a French drain is the answer. This requires some work, but it is definitely doable for the homeowner.
Dig a trench around the exterior of the house as deep as the foundation footing. This could be as much as 18 inches deep. Install plastic drainage pipe, available at your local home center, and backfill the trench with gravel.
Make sure you install the pipe with a minimum of 1/16 inch per foot drop so water runs downhill through the pipe. The gravel should be at least 2 inches in diameter.
If possible, discharge the pipe into the street. If that is not possible, dig a 2-foot by 2-foot dry well for the pipe to discharge into. Make sure you locate this dry well downhill from any structures.
Our next recommendation is to check the inside of your house for any sign of mold or mildew on the slab floor. The musty odor you smell could well be mold growing on carpet padding or wallpaper, or even on the drywall close to the floor.
Pull back the carpet from the outside walls. If you find mold, clean the area with a detergent. We like to use bleach. Of course, if the mold is growing on the carpet, you may have to replace the carpet. Check with a reputable carpet cleaner to determine your best course of action.
Also check the ventilation in your home. Do you have a forced air heater with a fan that you can run continuously? If so, use it. If not, you might consider installing ceiling fans.
Also, open the windows when you can. Sunlight, air movement and dry conditions are the enemies of mold and mildew.
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