How to Seal and Waterproof a Basement

OK, the backyard has been properly landscaped, so that water flowing towards the house is captured or diverted. The foundation drains and other below-ground drains are capturing water flowing through the soil and seeping towards the house. Most experts agree that something else is still is needed for complete protection from water penetration - the right water-proofing sealant.

The majority of residential basements are actually damp-proofed, not water proofed. Damp proofing provides partial protection that diminishes over time, and doesn’t prevent seepage if cracks develop, joints separate, or other potential openings develop. Waterproofing is a higher level of water exclusion that is called for by the 2006 International Residential Building Code (IRBC) specifically for areas with a high water table or other severe soil/water conditions. In most cases, as long as the water table is maintained 6 inches below the bottom of the foundation slab, damp proofing foundation floors and walls from the top of the footing to finished grade is all that the “Code” requires.

Water sealant products may be applied by mopping, brushing, trowel or, in some cases, spray application. Liquified asphalt is used most often to damp-proof concrete walls. The most widely used products for true waterproofing of basement walls are hot rubberized asphalt and “built-up” membrane systems, with layers of a hot or cold-applied brittle asphalt or coal tar product alternating with layers of reinforcing fabric or felt. Polyethylene sheeting, the most common method of protecting slabs in Atlanta residential construction, is actually considered a waterproofing method.

Most waterproofing systems are intended to be applied to the basement wall exterior, or the surface underlying a slab. Some cement-based waterproofing products may be applied to either interior or exterior surfaces. The waterproofing compounds prevent water migration by crystallizing within the fissures and pores of the concrete. Because cement based waterproofing products are capable of resisting intense water pressure, they can be applied to surfaces that are damp and minimally prepared in many cases.

It is always best to stop as much of the flow as possible from the outside, in order to reduce water pressure along the exterior walls and below the slab. However, it is sometimes feasible and more economical, to control dampness or minor seepage by filling cracks and sealing basement walls from inside the basement, and install drains below the slab within the basement if necessary. When drains are installed below a concrete slab, sump and pump systems are almost always needed to safely collect and discharge water away from the house.

If cracks or other evidence of foundation settlement are observed, the foundation may have to be stabilized and secured before attempting any crack repair. In Part 4, the last of this series, we will look at various types of foundation problems and the options for dealing with them.

There are advantages and disadvantages to different types of water proofing systems, and cost is certainly one factor. A geotechnical engineer can play a crucial role in many aspects of water proofing, beginning with figuring out the ground water conditions and the explanation for the cracks through which seepage is occurring. A geotechnical engineer can develop an under-slab drain system capable of handling the water pressure build-up that can lead to cracks and fissure development, can force water through existing cracks and other openings in walls and slabs, or, in extreme cases, can result in a “hydraulic blow-out” and basement wall collapse. The geotechnical engineer can answer questions such as:

  • How serious are the observed foundation cracks?
  • Does the foundation need to be stabilized before, or as part of waterproofing the basement?
  • Can the wet basement problems be tackled from the inside, avoiding the cost and disruption of large-scale excavation?
  • What is the best way to control water intrusion?
  • Is it necessary to lower the water table?
  • Which water proofing methods are unsuitable for this project?
  • And finally, what is the most cost effective way to solve the foundation and water proofing issues at hand?
    • (Not necessarily the question a contractor is going to pursue when he gives his “free” inspection!)

Copyright 2011 Daniel A. Freiberg, PE, PG

Drainage and Foundation Problems