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Water Down Below

Steps for Preventing Basement Leaks

Article written by Jack Carr, P.E., R.S., LEED AP, Criterium Engineers
Published in Condo Media, October 2013

Wet basements are an annoyance not just to unit owners but also building committees and property managers. Just when you think it is only an intermittent problem, along comes a Patriots Day Storm and everyone wants to build an ark.
Fixing a wet basement can be complicated because not only is the source of the problem not well known but selecting the method of repair can be more complicated than buying life insurance. An additional worry may be that there is not only a water infiltration problem but also a structural problem.
To simplify our discussions, let us only consider relatively modern basement foundations of monolithic poured concrete, as repairing stone or masonry foundations can be more complicated. Typically, a basement foundation is made up of a concrete spread footing pad many feet underground supporting a concrete basement wall. Most of these types of foundations have foundation drain pipes around the exterior perimeter at the footing and some buildings have foundation drains under the basement slab. These pipes drain to the outside by gravity or to a sump pit and pump.

Addressing the Problem

When basement water infiltration is first observed, the initial reaction should be to control the easiest source of water, i.e. the ground surface. If the roof has gutters and downspouts, make sure the downspouts release the water flow 10 feet away from the foundation. On the ground at the drip edge, ensure the ground slopes away from the foundation. Place a sheet of polythylene on the ground 18 inches wide along the foundation and cover with stone or other material to hold the membrane in place.
The easiest foundation leaks to fix are the wall cracks that show signs of dampness or mineral efflorescence. These vertical cracks are rarely of structural concern and can be easily scaled by professional companies who drill holes in the walls and inject a poly or epoxy fluid filling the cracks. This technique is fairly inexpensive and produces good results.
More challenging is when the water infiltration comes through the joint at the base of the foundation wall. This would indicate the positive pressure on the ground side of the wall is higher than the negative pressure on the basement side, overwhelming the foundation drains designed to reduce hydrostatic pressure by diverting water away from the wall. The ground water is finding a path either under the footing and through the slab or through the joint between the footing and the wall. In concrete block walls, the water can even be driven vertically up the wall, causing the wall to bleed or sweat.
  • The first thing to do is have a plumber flush out the foundation drain pipes, as sediment can bypass the filter cloth wrapping the perforated drain pipe and clog it up. If flushing does not work, then three repair methods are often offered. The simplest is coating the wall with a thick water-sealing product. These products often fail, especially the one-part coatings, as they do not address the real problem - namely, the hydrostatic pressure trying to lift the coating off the wall. Some of the modern two-part crystalline coatings not only fill the small cracks with expanding crystals but also penetrate the concrete to hold better to the surface.
  • A second method to address the problem from the basement (negative pressure) side by installing a "baseboard" drain system at the base of the interior basement wall. A trench is cut in the concrete floor slab a few inches wide along the entire perimeter of the interior wall. Holes are drilled through the base of the wall. The trench is filled with stone, and one of several manufactuers' drain pipe systems is placed in the trench and connected to a sump pit and pump. The trench is then covered by concrete level to the existing slab.
  • The third repair method is to excavate the backfill to repair the positive pressue side of the foundation wall. Once the soil is removed to the footing, the foundation drain is repaired or replaced with a modern waterproof membrane applied to the wall with a drainage plane fabric that directs the water to the new foundation drain. This method can be combined with the interior "baseboard" method. This is expensive and disruptive but is a very effective technique.

The key to a successful dry basement project is selecting the right method for your condo's conditions and the right contractor to execute the plan. With some understanding of the causes and remedies available, you will be able to file away your plans for an ark.