How to waterproof cinder block walls

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Damp breeze block walls are cause by hydrostatic pressure: Water from the outside is pushing its way into the dry interior. Applying a sealant to the inside of the wall is only a temporary fix since the water pressure will eventually force its way through even the tiniest crack.

You need to seal the outside of the breeze block wall. The best time to waterproof the walls is when the building is under construction and there is no excavation required.

Excavate around the walls to a depth of 18 inches below the base of the wall. Lay a drainage pipe in the trench and fill the trench to the level of the wall base with gravel. Cover with tar paper.

Sandblast the breeze block exterior to remove any dirt.

Coat the exterior with a commercial water resistant sealant. After it dries, re-coat with a thicker layer.

Apply a sheet membrane to the exterior wall below the soil level if the area is known for being excessively damp.

Backfill the trench with the soil.