The #1 cause: negative grading
Soil sloping toward the house. The fix: regrade so the first 10 feet around the foundation slopes away at 6 inches of drop.
The #2 cause: downspouts
Downspouts discharging at the foundation. The fix: extend discharge at least 6 feet from the foundation. Splash blocks alone are insufficient.
The #3 cause: foundation cracks
Even hairline cracks pass water under pressure. Polyurethane injection seals individual cracks for $300–$800 each.
Cove joint seepage
Where the basement wall meets the floor slab. Indicates hydrostatic pressure outside. Interior drain tile and sump systems address symptoms; exterior waterproofing addresses cause.
When you need a contractor
For persistent moisture not solved by grading and downspout fixes, an exterior waterproofing contractor evaluates options. Plymouth-area exterior waterproofing runs $8,000–$25,000+ depending on linear feet.