University of Minnesota Extension

FO-07051     Revised 2006  

Moisture in Basements: Causes and Solutions


Typical Causes of
Basement Moisture Problems

Inadequate Grading

PROBLEM: If the ground around a foundation is level or slopes toward the house, water is directed into the basement. The soil next to the house is often backfilled without proper compaction and later settles. This is especially true under stoops where water can collect next to the basement wall.

SOLUTION: Place earth around the house so that it slopes away from the foundation wall a minimum of one inch per foot for at least six feet.

 Inadequate grading diagram

Defective or Missing Gutters and Downspouts

PROBLEM: Missing gutters and downspouts cause rainwater to be directed toward the foundation perimeter. A downspout without an extender or splashblock is worse than no downspout at all. It is depositing the huge volume of rainwater from the roof in a single concentrated location near the basement.

SOLUTION: Place a minimum of one downspout per 50 linear feet of roof eave. Extensions should discharge water at least four feet beyond the wall. Sloped concrete sidewalks around basements are very effective in directing rain runoff.

 Defective/Missing gutters diagram

Improperly Designed Window Wells

PROBLEM: Window wells are like a drain right next to the basement wall. Often they are improperly built so that any water is directed toward, rather than away from the foundation.

SOLUTION: Window wells should be filled from the footing to the window sill with 3/8- to 3/4-inch coarse aggregate. A supplemental drain tile extension should extend from the footing to the base of the window well.

 Window well diagram

Ineffective Drain Tile and Sump Pit

PROBLEM: Many existing houses simply have no subsurface drainage system. This comes from a time when basements were not used as habitable space. In other cases, the systems do not work for a variety of reasons, such as collapse of the pipe, clogging of the pipe with silt and/or tree roots, or a broken connection to the sump. The sump pit usually contains a pump designed to lift the water to the ground surface outside the foundation wall. This pump can fail.

SOLUTION: See Approaches 2 through 5 that follow.

 Drain tile/ Sump pit diagram

Improper Drainage with Underslab Ducts

PROBLEM: If heating ducts are installed beneath a basement floor slab, the drainage system may be inappropriately left at a level higher than the duct. In effect, the duct becomes the drainage system, and with standing water within the heating duct, there are potentially serious health consequences from mold contamination.

SOLUTION: Heating ducts placed beneath the basement floor must be insulated, watertight, and sloped to collection points for drainage and cleaning. A drain tile and coarse aggregate can be placed under the ductwork.

Drainage diagram

Structural Cracks

PROBLEM: Concrete and concrete block foundations usually develop some cracks. They can be severe if floor joists are not properly connected to the foundation wall, thus permitting the wall to move. Also, soil settling causes cracking. Places where walls meet rigid structures like the fireplace often crack as well. Usually, drainage removes the water from cracks, but repair may be necessary.

SOLUTION: Proper footing design and proper connection between the foundation wall and the structure above are required (e.g. anchor bolts or straps at the sill plate and floor joists nailed to the sill plate).

 Structural cracks diagram
HOME NEXT: An Overview of Solutions to Basement Moisture Problems
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