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How to Fix a Wet Basement Floor

Jul 2, 2014 • By Matthew Stock.

How do I Determine the Source of Basement Water Problems?

One of the telltale signs of a basement water problem is stepping into a puddle on the basement floor. 

Of course, any water that comes into the basement for any reason – hydrostatic or lateral pressure, gutter spillover or oversaturated soil around the foundation – or from any source – cracks, bad mortar joints or the cove joint – ends up on the basement floor.  However, there are certain types of water problems associated with the basement floor and a homeowner that wants to keep it dry is well advised to understand them.

First of all, it helps to understand what a basement floor is and what it isn’t.  When a foundation is built, the first thing that is constructed after the excavation is done are the footings, broad slabs of poured concrete that describe the perimeter of the foundation and help spread the weight of the structure.  After the footings have cured, the walls are built, either by pouring concrete into forms or building them from concrete blocks and mortar.

After all the structural work of the foundation is done, the soil in the middle is smoothed out even with the tops of the footings, a vapor barrier is laid on top of it and the basement floor is poured.  The concrete in the basement floor is usually only two to three inches thick and the floor serves no structural purpose but is there only to provide a clean, flat surface in the basement.  The floor is essentially sitting on top of the footings and butted up against the wall.

There are several sources of water seepage in and around the floor.

Cove Seepage

When concrete is poured during the construction of a home, new concrete does not bond to old concrete.  For example, when poured concrete foundation walls are constructed on top of poured concrete footings as described above, the wall does not bond to the footings, which are already cured.  In order to prevent movement of the wall at the footing level, foundation builders create what is called a “keyway” to lock them together.

When the footings are poured and the concrete is fresh, a specially shaped form is inserted into the concrete to create a tapered channel down the middle of each footing.  When the wall is poured on top of it, the wall forms a tab in that channel that, when the wall is fully cured, locks the footing and wall together and prevents movement at the base of the wall.

Even though the footing and wall are mechanically attached the wall and footing did not bond so there is a tiny space in the keyway that can allow water to pass through it under hydrostatic pressure.  This is true also of the joint between the wall and the basement floor, a minute space called the cove joint.  Water that passes through the keyway can also pass through the cove joint and end up on the basement floor.

Floor Cracks

Because the basement floor is thin, it is not uncommon for it to develop multiple cracks that can admit water into the basement.  There are several ways in which these cracks can occur.

There are, of course, aboveground causes of floor cracks such as improper footings for support columns, heavy tools or machinery sitting on an insufficient floor or damage from falling or dropping of heavy objects.

The most common cause of basement floor cracks, however, is hydrostatic pressure under the floor.   Every piece of residential property in the United States has a varying level of ground water under it and the level of this absorbed water is called the water table.  The water table does not fluctuate quickly but is affected by climate and proximity to water, creating very low water tables in arid states such as Arizona and very high ones in the Chicago area on the shores of Lake Michigan.

The water in the ground, particularly when the soil is very expansive like clay, causes the ground to swell and this creates pressure under a foundation.  When heavy rains are added to the mix, the pressure becomes even greater and, because the pressure moves along the path of least resistance, it pushes upward and can cause the thin concrete of a basement floor to crack.

The same hydrostatic pressure that causes the floor to crack will also force water through the cracks and through the cove joint, creating puddles on the basement floor.

Perhaps not understanding hydrostatic pressure and its effects, many homeowners have tried methods of repairing cove seepage or floor cracks that just don’t work.

How Not to Fix a Wet Basement Floor

Another frequent cause of water in the basement is a non-structural crack in a poured concrete basement wall.  These cracks are best repaired by injecting them with expanding polyurethane and many homeowners have tried the “a crack’s a crack” theory and asked to have floor cracks similarly injected.  This doesn’t work.

Basement walls are up against packed soil that is essentially pushing back against the wall but they are not subject to hydrostatic pressure like the basement floor.  Injecting a floor crack with expanding polyurethane would seal that crack but would do nothing to alleviate hydrostatic pressure that would just create more floor cracks or worsen cove seepage.

Same goes for trying to patch the cove joint with hydraulic cement or some other material.  Even if the patching were successful, and it’s very difficult to force anything into the cove joint, it still wouldn’t change the hydrostatic pressure and the ground water that creates it will simply find another way into the basement.

That old standby of DIY filling and sealing, caulk, will not solve either problem and will not adhere to floor cracks or the cove joint.  Attempting to use it will just create a sticky mess and waste time that could have been used for a proper repair.

No matter what the source of seepage that creates a wet basement floor there is usually only one way to cure it permanently.

Install Interior Drain Tile

Because hydrostatic pressure is the underlying cause of a wet basement floor it stands to reason that eliminating or alleviating that pressure would stop the seepage.  The best way to do so is to install an interior drain tile system.

Interior drain tile is a system of perforated pipe buried beneath the basement floor level with the footings and connected to a sump basin.  Installing interior drain tile requires removing a 12-inch strip of concrete floor around the perimeter of the basement and digging a trench several inches deep and about a foot wide.

Once the trench is open, a layer of washed gravel is placed on the bottom.  The drain tile, perforated, corrugated plastic pipe is first wrapped in a “sock” of filtration fabric to keep out dirt and gravel dust and placed on top of the stone.  The pipe is connected around the basement with both ends terminating in the sump basin.  More washed gravel is placed on top and leveled off with the top of the footing and the strip of concrete floor is replaced.

Once in place, the interior drain tile creates a “pressure relief valve” for hydrostatic pressure under the foundation and alleviates it to stop seepage and further cracking of the floor.  The ground water that caused the pressure is collected by the drain tile and the pipe carries it off to the sump basin where the sump pump will discharge it from the basement.

Interior drain tile will stop or prevent cove seepage as well as seepage through floor cracks.  Properly installed, it will never need maintenance.

Drain tile can also be installed on the exterior of the foundation if circumstances prevent an interior installation.  Exterior drain tile is done the same way, except that there is considerably more excavation required and rigid PVC pipe will be used instead of the flexible corrugated pipe.  Exterior drain tile will relieve pressure that can cause cove seepage but it generally will have no effect on seepage through floor cracks.  It is typically used in conjunction with an exterior waterproofing membrane to solve problems with seepage through the foundation wall, such as through porous concrete or cracked mortar joints.

When a homeowner discovers a wet basement floor there’s a very good chance that either cove seepage or cracks in the floor is the cause.  Determining the source of seepage is a job for a trained and experienced professional and the homeowner will need the advice and services of a skilled basement waterproofing contractor to do the repair work effectively and permanently.  At U. S. Waterproofing, we’ve been fixing wet basement floors around Chicago, southeastern Wisconsin and northwest Indiana for more than 57 years, and have installed miles of interior drain tile.  Why not ask for our free advice when your basement floor is wet?

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