Short answer: Water that shows up after heavy rain almost always comes from outside. Clogged gutters, downspouts that dump water next to the house, and soil that slopes toward the foundation let water soak in and push against the walls. The University of Minnesota Extension recommends downspouts that discharge at least 4 feet beyond the wall and soil that slopes away at least 1 inch per foot for the first 6 feet. Fix those first, then seal cracks and consider a drainage system if water still comes in.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Water along one wall below a downspout | Downspout discharging too close | Add an extension (4 ft or more) | No |
| Water where wall meets floor all around | Hydrostatic pressure from saturated soil | Check grading and gutters | If it continues |
| Water coming through a crack | Foundation crack | Mark it; seal small cracks | If wider than 1/2 inch |
| Water below a window | Window well filling with water | Clear the well; add gravel or a cover | Rarely |
| Water from the floor drain | Sewer or drain backup | See sewer smell in basement | Yes |
| Damp walls in summer, no rain | Condensation from humid air | Run a dehumidifier | No |
Fix the outside first
- Clean gutters in spring and fall, more often with trees nearby.
- Extend downspouts so they discharge at least 4 feet away from the wall.
- Grade soil to slope away from the house at least 1 inch per foot for 6 feet.
- Clear window wells and add gravel or covers.
Then deal with the walls
Bob Vila suggests sealing small cracks with a filler compound and calling a professional for cracks wider than 1/2 inch. If water still comes in after the outside fixes, an interior or exterior drainage system with a sump pump may be needed.
Dry out fast
The EPA says wet materials should be dried within 24 to 48 hours to prevent mold in most cases. Remove wet carpet and boxes, run fans and a dehumidifier, and keep electrical devices away from water.
When to call a professional
- Water keeps coming in after gutters, downspouts and grading are fixed.
- Cracks are wider than 1/2 inch, horizontal or growing (see foundation cracks).
- Water comes up through the floor drain.
- There is mold or a strong musty smell.
What it usually costs
Bob Vila reports professional foundation crack repair at about $250 to $800, exterior French drains at $500 to $10,000, interior drainage systems at $5,000 to $18,000, and interior wall sealing around $3,000 on average.
Frequently asked questions
Why does my basement only leak when it rains hard?
Heavy rain saturates the soil and builds pressure against the walls. Water then finds cracks and the joint between wall and floor.
Will a dehumidifier fix water in the basement?
No. It helps with humidity and odor, but the University of Minnesota Extension notes it is not a complete solution for water coming in.
Related problems
- Sump pump not working
- Sump pump running constantly
- Foundation cracks
- Musty smell in basement
- Sewer smell in basement
Sources
- University of Minnesota Extension: Moisture in basements, causes and solutions
- Bob Vila: How to stop water leaking into the basement after heavy rain
- U.S. EPA: A brief guide to mold, moisture and your home
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.