How Basement Waterproofing Works


How Basement Waterproofing Works

Basement waterproofing is generally not a topic of conversation around the dinner table for most people – unless they have a wet basement or they’re in the business.

Consequently, like many fields of home repair or construction, basement waterproofing remains a mystery to most people.  The popular conception is that it involves a lot of digging or smearing goop on walls and that’s not exactly wrong but basement waterproofing is much more than that and sometimes much less. 

Knowing what it is and how it works will make a homeowner an educated consumer of basement waterproofing services who will have the knowledge and confidence to choose the right contractor and best method for his or her home.

In order to understand basement waterproofing it helps to first have a picture of how and why a basement leaks in the first place.

How and Why a Basement Leaks

To reduce it to the simplest terms, for a basement to leak there has to be water in the ground around it and there has to be one or more points of entry for the water to find its way in.  Of course, it’s a lot more complicated than that.

Regardless of where a house is located there is water in the ground somewhere.  Even in an arid place like Arizona there is ground water although it is likely to be very deep below the surface.  In places close to major bodies of water, like the east and west coasts and states surrounding the Great Lakes, the water is likely to be much closer to the surface.  This point at which the highest level of ground water reaches is called the water table.

The water table is a more or less stable phenomenon but it can be affected, usually for the short term, by severe weather conditions that include a lot of rain or snow.  Soil conditions also play a role in the rise and fall of the water table because more absorbent soils like clay will retain water much more than a quicker draining soil like sand or loam.

So, when a heavy rain or major snowmelt occurs, water is absorbed into the soil around a home’s foundation and, once it penetrates low enough, can cause the water table to rise.

Of course, sustained wet conditions will also affect the soil closer to the surface, specifically the soil that surrounds the foundation.  Heavy rains or thaws will cause this soil to become saturated and retain water around the perimeter of the foundation.

Along with holding water in the proximity of the foundation an elevated water table or saturated soil near the surface will also create pressure on the foundation.  Below the foundation, the rising water table creates hydrostatic pressure that pushes upward and can cause cracking in poured concrete basement floors.  At a level closer to the surface, saturated soil also creates lateral pressure against the foundation walls that can create nonstructural cracks in poured concrete and masonry walls and can actually damage the foundation by causing walls to move inward.

Besides this physical damage, both hydrostatic and lateral pressure can force water into the basement by means of a number of different types of openings, some accidental, some opportunistic.

Here’s how water can enter the basement:

Wall cracks – The most common source of basement seepage is a non-structural crack in a poured concrete basement wall.  Also, when mortar joints in a masonry wall begin to crack or deteriorate they can also allow seepage.

Floor cracks – When hydrostatic pressure pushes upward long enough on a basement floor, the floor, which is only a thin slab between 2 and 4 inches thick, will crack and allow water to enter the basement.

Cove joint – Because a foundation wall has no bond between it and the footing on which it sits, there is a small opening between the two.  Add to this the cove joint that exists between the basement floor and the wall, and it becomes an avenue for water under hydrostatic pressure.

Porous concrete or masonry – In general, poured concrete is not very porous but if mistakes were made during the mixing and pouring, porous spots may exist and they can admit water to the basement.  Depending on the material used, masonry walls can be considerably more porous.  Concrete (or “cinder”) block, for example, is a fairly porous construction and can admit water through the concrete itself.  Brick, too, is a porous foundation material.

Top of foundation wall – Poor grading, incorrect landscaping or improperly sloped exterior structures like decks and patios can cause rain water to run towards the house and seep through the tiny gap where the aboveground structure of the house sits on top of the foundation.

So, how does basement waterproofing stop water from entering the basement through all of these possible openings?

How Basement Waterproofing Works

There are essentially two main approaches to basement waterproofing: sealing techniques and methods that relieve pressure and drain water.  Both are equally effective when applied under the right circumstances.

Wall Crack Repair – Repairing wall cracks is a classic example of sealing an opening to prevent further seepage.

In a poured concrete wall, a seeping non-structural wall crack is best repaired by injecting it from the interior with expanding polyurethane.  The crack is cleaned out, plastic injection ports are inserted and a coating of quick-curing epoxy is spread over the crack to close it off.  When the epoxy sets, the crack is injected through each port and the urethane material expands to fill and seal the crack all the way to the outside soil.  It remains flexible when cured to prevent re-cracking from minor foundation movement.

If the crack is inaccessible from the inside it can be sealed from the outside with sodium bentonite clay.  A small hole is excavated down to the footings at the site of the crack and filled with the granular clay, which absorbs water from the soil and forms a permanent water barrier on the “positive side.”

Exterior Waterproofing Membrane – Another common sealing technique is to apply an exterior waterproofing membrane to prevent seepage through or over a foundation wall. 

To install an exterior waterproofing membrane, the affected foundation wall (or walls) must be excavated down to the footings.  The wall is cleaned of dirt and debris and the membrane, asphalt-modified polyurethane, is applied with a trowel in a thick coating.  The membrane may be covered with insulating material and/or heavy-duty plastic drainage board that will protect the membrane and channel water downward.

An exterior waterproofing membrane is completely different from “damp-proofing,” which is a thin coating that is sprayed onto a foundation wall during construction to help prevent condensation and dampness.

The other approach to basement waterproofing, relieving pressure and draining away water, is accomplished by installing drain tile on either the interior or exterior of the foundation.

Interior Drain Tile – Interior drain tile is the most versatile method of basement waterproofing as it eliminates seepage from several sources, including floor cracks and the cove joint.

Installing interior drain tile begins with removing an approximately foot-wide section of basement floor along the affected wail or walls.  Once the concrete has been removed, a trench is dug in the underlying soil down to the bottom of the footings and a layer of washed gravel is poured in.

Lengths of perforated corrugated rain pipe, warped in a “sock” of filtration fabric, are laid on top of the gravel and connected at one or both ends to a sump basin.  The trench is then filled with more gravel to promote water flow and the strip of concrete floor is replaced.

When hydrostatic pressure begins to build the water is forced into the drain tile instead of the basement and the pipe carries it to the sump pump for discharge from the home.  When properly installed, interior drain tile never needs maintenance.

Exterior Drain Tile – Exterior drain tile performs similarly to the interior version but uses rigid PVC pipe with perforations instead of corrugated to better handle pressure and movement of soil.

To install exterior drain tile, the ground around the foundation must be excavated down to the footings where the same process is followed to embed the filter-wrapped pipe in washed gravel and connect it to a sump pump.  Exterior drain tile is often installed in company with an exterior waterproofing membrane, especially in instances where the ground water is very high.

Regardless of the source of basement seepage or the method recommended to stop or prevent it, a homeowner with a wet basement will require the advice and services of a professional basement waterproofing contractor.  At U.S. Waterproofing, we have been diagnosing and repairing basement water problems in the Chicago area, including southeastern Wisconsin and northwest Indiana, for more than 57 years and we have served more than 300,000 satisfied customers who now enjoy dry basements.  Why not ask for our free advice when you see signs of basement water problems?


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