Stains on Concrete or Stone & Mortar Basement Walls

Stone or Block Foundations

Home owners with stone and mortar foundations or block and mortar foundations must deal with the fact that 90% of these basements are damp. Over the life of the foundation, the walls shift very slightly. This slight movement opens microscopic cracks along the joint where the mortar is bonded to the stone or block. Moisture moves through the foundation and evaporates on the surface in the basement. Water migration through soil will dissolve minerals from the soil and then as the moisture evaporates on the inside, the minerals are deposited on the walls. The minerals stains are dark in color and mimic the appearance of mold.

The same staining condition can occur in poured concrete foundations when there are slight imperfections in the original pour. Over many years, moisture migrates through the imperfection and deposits minerals on the inside surface of the foundation.

To determine whether the stain is substantially mold or mineral, try the following:

Mix Bleach & Water (50-50) and splash some of this mixture on the stain.



  1. If the stain substantially remains, the material is a mineral deposit.
  2. If the stain substantially disappears, the material is mold. Retreat the area with bleach and water mixture; but this time, add a little Dawn Dish Soap and brush the debris away.
  3. If you have mold on the foundation you may have mold elsewhere and a mold inspection is appropriate.



These foundations are notoriously damp because of moisture infiltration.

Installing drywall on the exterior walls usually causes problems because the drywall becomes damp and this is an ideal condition for mold development.