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Basement Waterproofing Membranes: Exterior Wall Treatment & Sealers


Materials needed: asphalt base foundation sealer, elastic roof sealer, hydraulic cement, trowel, stiff bristle brush, waterproofing membrane

Optional: chalk line, reinforced polyethylene sheet, muriatic acid or etching product

All exterior wall treatments, regardless of other local conditions, are based on at least one application of an asphaltic base, a tar-like sealer designed for exterior foundation walls. For most homes, a properly applied coat of foundation sealer should prevent any water from entering the basement.

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If your home is in a low spot or in a chronically high water-table area, the enormous pressures exerted by the water against the walls will require more than a coat of sealer to prevent leakage. For these conditions, the only solution is to install a membrane, a continuous layer of material impervious to water, against the exterior wall.

In the past, the installation of a membrane was referred to at the “sandwich method” because a layer of heavy 30-lb. felt paper was sandwiched between two coatings of tar. This method is still perfectly good, its disadvantages being that it is time-consuming to install, and that felt paper comes in relatively narrow rolls requiring overlapping to form a good seal.

Today, new membrane systems and materials offer almost foolproof water protection. Applied to the exterior of the foundation, these impenetrable membranes can withstand changes in temperature, continuous exposure to water, and are relatively easy to apply.

Equally effective and easier to install is a fiber glass reinforced polyethylene sheeting that comes in rolls up to 8 feet wide. Do not use a standard unreinforced polyethylene sheet. It tears too easily, negating its purpose. A single roll of this sheeting will cover the entire wall of most homes without any horizontal overlapping. It is available at well-stocked masonry supply outlets under a variety of brand names.

If a contractor is doing the job, ask if he is going to install a membrane barrier. If he says no, find out why. It doesn’t take that much time to install and for the extra material cost, it may be well worth it, especially if you have any doubts about the condition of the walls and the local water-table height.

basement waterproofing sealer

REMEMBER: If your house is in a high water-table area, water may still come up through the floor, and for obvious reasons it is impossible to place a membrane barrier on the exterior side of the floor. In an extreme case it might be possible to tear up the floor, put down a membrane and pour a new floor, but that seems an excessive solution. Good perimeter drain tile should keep the water table below the level of the floor slab.

After excavating a comfortable work space, clean the wall with a stiff brush and a garden hose. If there is a lot of sediment or crystallized deposits on the wall, wash it thoroughly with a weak muriatic solution or use a commercially prepared etching product (UGL Drylock Etch, for example). Always wear eye and hand protection when working with these products.

Patch all visible cracks with hydraulic cement. If the wall is bare concrete or cement block, it should be given a coat of mortar. This treatment on block walls is referred to as “parging” and results in a smooth surface upon which to apply the sealer. Being introduced on the market is a fiber glass reinforced bonding cement that could be used instead of parging and forms a dampness barrier of its own.

The wall should be absolutely clean when using it, however. Few homes in northern latitudes have block foundations because they are more susceptible to shifting and settling due to ground movement caused by deep penetration of frost. Block foundation homes in northern climates must install a membrane barrier to insure dryness.

Once the wall is thoroughly cleaned and prepared, elastic roof sealer should be applied around any pipes, wires, or conduits that pass through the wall. Elastic roof sealer is different from roof cement in that the roof sealer can be applied to a wet surface and remains more pliable. Use roof sealer to coat the wall-footing joint, to fill any depression or holes in the surface, and to seal any other joints or corners that look like potential weak spots.

Now apply the foundation sealer. Measure the square feet covered after using the first can. The amount of coverage should correspond with the manufacturer’s recommendation. If more space has been covered, then it is being spread too thin. Roof sealer and foundation sealer are available at any building-supply store, and can be purchased in one- or five-gallon cans.

To apply the reinforced polyethylene sheet, start in a few feet from a corner and strike a vertical mark with a chalk line. Press the edge of the sheet up to the line and secure it with a coating of roof sealer. The top can be temporarily tacked to the bottom of the house frame and the bottom held down with a shovel full of gravel. Roll it out tightly around the house as if you were wrapping a Christmas present. Any cuts to fit the sheet around pipes or wires should be completely sealed with roof sealer. When you get back around to the starting point, overlap at least 12 inches and completely seal the joint with the roof sealer.

The same procedure is followed on any vertical joint if a second roll is necessary. After the trench is backfilled, the excess at the top can be cut off down to the level of the grade.

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