Water in the Basement: Find the Source Before You Pay

A modest storm produces far more runoff than people expect.

DOE’s building science guidance notes that just a few inches of rain on a roof can produce several thousand gallons of runoff, which has to be channeled away from the foundation to keep a basement or crawlspace dry. The arithmetic is simple: one inch of rain on 1,000 square feet of roof is about 623 gallons. All of it leaves through the gutters — and then goes wherever the downspouts point.

The cheapest fixes are usually the ones that work.

• Gutters: EPA advises cleaning and repairing roof gutters regularly. Overflowing gutters dump water straight down beside the wall.
• Downspouts: DOE’s guidance is to terminate downspouts at least 5 feet from the foundation, or at least 10 feet if they discharge to an underground catchment system. EPA’s house tour gives the same 5-foot distance.
• Grading: EPA says the ground near the house should slope downward and away from it, so water does not collect around the foundation.

Walk around the house during a heavy rain with a flashlight. Where the water goes in the first ten minutes tells you more than any quote.

Three sources look alike on a basement floor.

Water that appears after rain and near exterior walls points outside. Water that appears regardless of weather may be a plumbing leak; the water meter test tells you. Damp walls or sweating pipes in humid weather may be condensation, a humidity problem rather than a leak — EPA notes that condensation on cold surfaces signals high indoor humidity that should be investigated.

Speed is what prevents mold.

EPA says that if wet or damp materials are dried within 24 to 48 hours after a leak, in most cases mold will not grow. Move stored items off the floor, run fans, and use a dehumidifier. EPA also advises keeping indoor humidity below 60%, ideally between 30% and 50%, and notes that an inexpensive humidity meter costs about $10 to $50. If a basement runs humid, EPA recommends a dehumidifier, cleaned regularly to the maker’s instructions.

Some water is not a weekend project.

Get help for water that keeps coming after the outside has been corrected, for cracks that are growing, for any water near the electrical panel or outlets, and for contamination from sewage or floodwater. If mold has spread over more than about 10 square feet, see the guide to mold testing and cleanup.