Basement flooding

How the water gets in

FEMA's guidance for homeowners describes sewer backup during heavy rain, when there isn't enough capacity in the sewer system and sewage flows back up through floor drains, toilets, sinks and other fixtures (FEMA, Urban Flooding). Stormwater also comes in through windows, air vents and other openings near the ground, and over the thresholds of outside basement stairs (FEMA).

Speed differs. Sewer backup is relatively slow because the drain pipes are small. Surface water through a low driveway, a stairwell door or a window can equalize with the water outside in minutes, and a basement wall that collapses under outside water pressure can fill the basement in less than a minute (FEMA).

The volumes are bigger than they look. An inch of rain puts about 1,250 gallons on the roof of a 2,000-square-foot house, and without grading, gutters and downspouts some of it ends up in the basement. The water table under the house can also rise with flooding or with the seasons (University of Minnesota Extension). Window wells that are built badly act as drains aimed at the basement wall (UMN Extension).

What FEMA advises

• A backwater valve lets wastewater leave the house but stops sewage from flowing back in when the city's sewer mains are full (FEMA).
• A sump pump isn't designed for large amounts of water and stops when the power goes out (FEMA).
• Don't pump a flooded basement dry too fast. If the soil outside is still saturated, the outside pressure can collapse the walls or push up and crack the floor as the water inside drops (FEMA P-312).

Where basements flood most

Counties with the most flood losses in buildings with basements since 2016, from FEMA's NFIP records (refreshed September 9, 2026). The basement share is of losses whose foundation type is recorded.

Where the water table is shallow

Among counties with 50,000 or more homes, these have the largest share of land where USDA rates a shallow water table a limit for basements. More in the wet basement guide.