Why a Basement Leaks and How to Fix it Permanently


Why a Basement Leaks and How to Fix it Permanently

A leaking basement is a very common occurrence in many homes.  When a basement leaks it does not imply that the home was not built well or that some extraordinary catastrophe has taken place; most likely it’s just a combination of time and the forces of nature, although there are some man-made factors that can cause or worsen a leaking basement.

Any type of foundation can spring a leak and those built of poured concrete are no less vulnerable than one made of masonry – they just leak differently.  Of course, anything that can be said about leaks in a basement can also apply to a crawl space which, after all, is nothing more than a short basement albeit one that is likely to have a dirt floor.

In examining why a basement leaks, it’s important to know that there are causes of leaks and there are sources of leaks.  The first are usually natural occurrences that take place outside the basement and the second are generally flaws or disruptions in the foundation’s structure.  The two must combine to make the basement leak and any approach to repairing the leak must take both into account.

Causes of Basement Leaks

By far the most common cause of basement leaks is pressure created by water in the soil surrounding the foundation that occurs in two forms.

Hydrostatic Pressure – Although it might not always seem like it, there is water in the ground everywhere.  Even in dry areas like the American Southwest water can still be found underground although at a much lower level than elsewhere.  The level at which this ground water exists is called the “water table” and its location varies widely.  One factor that influences the height of the water table is proximity to bodies of water so that areas around the Great Lakes, for example, will have higher water tables than Kansas or South Dakota.

When heavy or persistent rain or snowmelt occurs, water is absorbed by soil closer to the surface and, when the surface soil becomes saturated, the storm water that isn’t absorbed causes the water table to rise.  When the water table rises under a foundation it creates hydrostatic pressure against the foundation from below that can force water into the basement and create leaks opportunistically.

Lateral Pressure – The soil that surrounds a foundation between the footings and the surface can also absorb water although under normal weather conditions this soil should drain.  Certain types of soils, such as sand and loam, drain fairly quickly and don’t absorb as much water.  Clay soil, such as is common in the Chicago area, doesn’t drain as well and tends to absorb water and expand.

his situation is worsened by the fact that a 10-foot wide area around the perimeter of the house is less compacted than other soil because it was the site of the original foundation excavation that was later backfilled.  This looser soil tends to be more absorbent and expansive.

Also, this area around the home is often inundated with rain water when the home’s gutters are clogged or blocked or when downspouts are not extended and are discharging large volumes of water right next to the foundation.

When this soil expands, it creates lateral, or sideways, pressure against the foundation that can cause foundation damage and create leaks in the basement.

Window WellsWindow wells can be another cause of seepage that has nothing to do with ground water pressure.  Window wells allow light and air to enter the basement through the window but can accumulate water during a heavy rain if the drain is either clogged or missing.  When the window well fills with water, from either a clogged or missing drain or separated window well liner, the water can seep in around a poorly installed window or even create enough pressure to push the window in.

Those are the causes of basement leaks but what are the sources and how do you fix them permanently?

Sources of Basement Leaks and Fixing Them Permanently

There are several sources of basement leaks, most of which are minute openings in basement walls or floor.

Floor Cracks – Basement floors are thin layers of poured concrete, typically 2 – 4 inches in depth, that have no structural role in the home’s foundation.  They are there to provide a clean, solid base.
The same hydrostatic pressure that forces water into the basement also presses upward on the basement floor and can cause it to crack.  These cracks will allow water to seep into the basement when the water table rises.

Cove Joint -- When a foundation is built the first thing to be constructed is a system of footings, a wide slab of concrete that describes the perimeter of the foundation.  The foundation wall is either poured or built with masonry on top of the footings.  In either case, there is a minute opening where the wall meets the footing.  The same is true of where the poured concrete floor meets the wall.  This opening, called the cove joint, allows water to be forced into the basement by hydrostatic pressure.

To repair both floor cracks and cove seepage, the secret is to alleviate the hydrostatic pressure by giving ground water somewhere else to go.  This is accomplished by installing interior drain tile, a system of perforated pipe installed under the basement floor.

Interior drain tile is embedded in washed gravel to create better drainage and is wrapped in a “sock” of filtration fabric to keep it clean and flowing.  It takes in ground water through its perforations and carries it to a sump basin where the sump pump discharges it from the basement.  When installed properly, interior drain tile requires no maintenance.

Poured Concrete Wall Cracks – The most common source of a basement leak is a non-structural crack in a poured concrete basement wall.  These cracks can be caused by either the lateral pressure described earlier or by minor settling or dropping of the foundation.

The best way to repair a wall crack permanently is to inject it from the interior with expanding polyurethane.  The polyurethane fills and seals the crack out to the soil and stays flexible when cured to prevent minor foundation movement from re-opening the crack.

If the crack is inaccessible from the interior it can be repaired outside by filling a hole at the site of the crack with sodium bentonite clay.  The clay forms a pliable yet permanent barrier on the “positive side” of the foundation and keeps water out of the basement.

Mortar Joints – Masonry foundation walls are strong and capable of supporting even large structures but their weak point for seepage is the many feet of mortar joints that hold the concrete blocks, stone or brick together.  Even minor foundation wall movement can cause cracks in mortar joints and these cracks are often a source of seepage.

Porous concrete or masonry – Poured concrete is usually not porous enough to admit water but minor flaws in the pouring process, such as insufficient mixing, can create porous spots.  These spots will, over time, allow seepage through the wall.

Certain masonry materials, especially concrete block or bricks, are porous by nature and can allow seepage through the wall.  In the case of concrete block, the presence of large cavities that hold water can exacerbate the seepage.

Seepage over the top of foundation walls – There is often a small gap between the top of a foundation wall and the sill plate of the aboveground structure.   When “negative” grading or improperly sloped structures such as decks and patios cause rain water to run toward the house, it can enter the basement through this opening.

The way to repair seepage permanently from mortar joints, porous walls or over the top of a wall is to install an exterior waterproofing membrane.  The affected wall is excavated and cleaned before a technician applies a thick coating of asphalt-modified polyurethane to the wall with a trowel.  The material cures to form a waterproof barrier and can be covered with insulating material and/or heavy-duty drainage board to channel water down the wall.

Exterior waterproofing membranes are often augmented by installing exterior drain tile to carry large amounts of ground water away from the foundation to a sump pump.

Repairing window wells – If a window well does not have a drain the simple solution is to install one that leads to drain tile, a dry well or out to daylight.  If an existing drain is clogged it can be cleaned out or replaced if necessary.  In either event, a fitted window well cover should be installed, not so much to keep out rain but to exclude lawn clippings, leaves and other debris that can cause the drain to clog.

If the window well liner has separated from the foundation wall, it can be reattached in some cases but will likely need to be replaced in order to keep soil and ground water from entering the well.

There are numerous causes and sources of basement seepage and a number of ways to repair them permanently.  One thing they have in common is that it takes a trained and experienced basement waterproofing contractor to diagnose and fix them.  At U.S. Waterproofing, we have been finding and fixing basement seepage problems for more than 57 years and we have more than 300,000 satisfied customers to show for it.  Why not ask for our free advice when your basement leaks?


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