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Waterproofing Basement Walls

Basement, Insulation and Energy Conservation, Painting, Stone and Concrete, walls, Waterproofing November 30, 2009 Matt Weber


Nobody likes unsightly stains from mold, mildew or water damage, but that’s what occurs in countless basements across the nation. Basements have a tendency to be damp by simple virtue of being the lowest level of the house, often below grade. The basement walls are the barrier between your living space and the gallons and gallons of accumulated water that your home sheds during rainfall.

However, roof runoff is usually not the only contributing factor to the massive amount of water collecting at a home’s foundation walls. Basement waterproofing should begin outside your home. Whenever you discover wetness on interior masonry walls, observe what is causing the seepage problem. Check for leaky gutters and downspouts or blocked drainage pipes. Ruts in the landscape or improper grading can also direct water towards your foundation. As the soil around your foundation becomes saturated, hydrostatic pressure forces water through the porous masonry, creating dark stains on the interior surface. For the best waterproofing results, correct all of these exterior problems before applying a masonry waterproofer to the interior block.

By installing a gutter and downspouts, water on the outside is diverted away from the basement walls.

For example, the house shown in this article had no gutter along its rear roof line. As a result, during a rainstorm the water was cascading off the roof and pounding a trench along the foundation wall, where it was seeping through the ground and migrating through the cinderblock, as evidenced by stains on the interior. By installing a gutter and a couple of downspouts, the water is channeled away from that troublesome trench and diverted across the yard and away from the house walls. The gutter system will address much of the water intrusion on the outside, and the application of a masonry waterproofer to the interior walls will prevent future stains from occurring. Together, both the exterior and interior approaches will ensure a clean, bone-dry basement for years to come.

Get rid of ugly stains.

Prep the Surface

Applying a masonry waterproofing product is basically a painting project, so begin with prepping the surface of the wall for the best adhesion. Clean the block thoroughly with a wire brush to remove all loose and broken mortar, dirt and dust This particular house had many dirt spots caused by dirt daubers—a type of wasp that builds dirt nests on virtually any flat surface it can grab. Another obstacle to any masonry waterproofer is efflorescence (salt deposits). This white, powdery substance is formed when water-soluble salt compounds in the masonry are drawn to the surface by water seepage. These deposits prevent the waterproofing product from adhering to the surface of the wall. To control this problem, wash with a solution of muriatic acid, according to the manufacturer’s directions. Remove old paint by wire-brushing, sand-blasting or scraping (beware of lead paint). Most masonry waterproofers are warranteed when applied over bare masonry only.

Scrub away any loose debris.

As mentioned earlier, the foundation walls of this house also required the removal of mold and mildew stains. Bleach is a well-known killer of mold and bacteria, so I used a solution of bleach and water to scrub away the stains. When all of the mold and other debris is removed, dry the area completely.

Next, use a hammer and cold chisel to chip off any unsightly excess mortar protruding from the joints. If necessary, use a trowel to fill any shallow spots or holes with mortar. Small cracks can be patched with a concrete repair product and troweled smooth.

Also, inspect the floor-to-wall joint for evidence of seepage. When concrete floors cure, they normally shrink and pull away from the wall, creating a gap where water can enter. If this is the case, fill all gaps between the floor and walls with a concrete patching product.

Fill cracks and holes with concrete patch.

Allow all repairs to dry completely before painting. Then, clear the room of all obstacles and mask off everything you don’t want painted. Sheets of cardboard work well to cover the floor during painting. Also, wear clothing that you’re willing to sacrifice after painting. The solid material in the waterproofer has a tendency to fly off a paint roller and splatter all over the user.