What Do You Use to Seal Below Ground Cinder Blocks?

Liquid Membranes

• A liquid membrane can be sprayed or painted on the below-grade cinder blocks. Make sure to use a waterproofing rather than a damp-proofing material to create an effective seal. Material options range from modified asphalt or asphalt emulsions to synthetic polymer or rubber-based membranes. A waterproof membrane must be flexible and durable. Liquid membranes must be applied to a clean surface and are relatively fragile so they are often covered with rigid insulation or similarly protective board to preserve the material during back-filling.

Solid Membranes

• A solid membrane waterproofing system provides an alternative to a liquid sealant. Peel-and-stick type membranes are generally thicker and more durable than spray-on membranes and consist of a sheet or roll of rubberized asphalt and polyethylene with a backing that is peeled off to reveal a sticky side that adheres best to a pre-primed foundation wall. A waterproof barrier can also be constructed from two-ply hot-mopped feet, adequately thick polyvinyl chloride or polyethylene sheeting, 55-pound roofing paper or polymer-modified asphalt paper. Overlap the joints and seal them with an adhesive suitable for use with the specific membrane material.

Drainage Considerations

• Waterproofing a cinder block or other foundation type will generally only prove effective if water does not remain standing against the exterior for any extended amount of time. When the foundation is being sealed, install or repair foundation drains so that water will move away from the cinder block quickly. A simple drain can be constructed by digging a trench to the depth of the footing that extends to an outlet or sump and filling at least the bottom few feet of it with coarse gravel. Lining the sides of the trench with filter fabric or landscaping cloth and installing a perforated drain pipe at the trench bottom will increase the drain's effectiveness. Make sure that water from the roof or downspouts is directed away from the cinder block and the landscape slopes away from the structure.

Exterior Waterproofing Alternatives

• In some situations, it may not be feasible to excavate the ground next to the block in order to implement exterior waterproofing. In this case, the interior surface of the below-grade cinder block can be treated and an interior perimeter drain installed. Break up concrete to create a trench around the perimeter, install a drain pipe that leads to a sump pump and cover it with gravel. Drill a series of holes along the bottom of the cinder blocks to facilitate controlled drainage, install a flange that extends between the holes and the drain pipe to channel the water, cover the drain pipe with concrete patch and waterproof the wall above the drain holes using paint-on waterproofing.