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Basement Floor Cracks: How They Occur and Why They Seep Water

May 17, 2015 • By Matthew Stock.

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Basement floor cracks are so common that many homeowners don’t even notice them – until they start to seep water into the basement.

The seepage, regrettably, is pretty much inevitable because the same forces that created the crack in the first place are also responsible for the water.  Fortunately, a typical floor crack doesn’t result in a huge amount of seepage but is typically an indicator that a major seepage problem elsewhere in the basement is imminent.

Let’s take a closer look at basement floor cracks and what they can mean to the health and safety of a home’s basement.

How Do Basement Floor Cracks Happen?

Most homeowners have at least a fair idea of how their foundation is built.

First, there are footings, wide slabs of poured concrete that describe the perimeter of the foundation and are built deep, as much as ten to twelve feet below the surface.   These footings may be anywhere from 16 to 22 inches wide, the width determined by the load-bearing capacity of the soil and the size of the house to be built.  Footings may be between 12 and 18 inches thick.

Once the footings are in place, the foundation walls are built.  Depending on the height of the wall, a poured concrete foundation wall will be eight to ten inches thick.  A masonry wall built of concrete block is typically eight inches thick.

Walls and footings may be reinforced with metal rebar and some or all of a concrete block wall may be filled with concrete.

These structural elements of a home’s foundation sound pretty burly and they must be to support the home and keep it stable.

Then, there’s the basement floor with the current standard of three to four inches of poured concrete.  Done right, that concrete was poured over a vapor barrier and reinforcing metal grid.  Even then, the floor is pretty thin, especially when compared to the other elements of the foundation.

The lower standard for floors is acceptable because they are not a structural element of the foundation, serving only as a solid, smooth surface for the basement.  However, even a reinforced basement floor is prone to cracking due to forces in the soil below that come from water under pressure.

Why Do Basement Floor Cracks Seep Water?

The source of seepage through basement floor cracks lies in the water table.  “Water table” is the term used to refer to the normal level of ground water in the soil in a particular area.  Topography, soil composition and proximity to large bodies of water all play a role in determining that level.

When heavy rains or snowmelt are absorbed into the soil, the water table will rise, albeit temporarily.  This increased level creates hydrostatic pressure in the soil that pushes up against anything resting in or on the soil – like the basement floor.

This pressure causes the relatively thin concrete layer to crack and, once the cracks are open, forces water through them onto the basement floor.  This usually results in minor seepage from floor cracks but, as noted above, indicates that a bigger problem may follow.

The place where the foundation footings, walls and floor come together Is called the cove joint and there are infinitesimal openings where floor meets wall and wall meets footing.  Hydrostatic pressure can also force water in through this cove joint, which can result in significant amounts of water entering the basement.

The solution for seepage through basement floor cracks and the cove joint lies in eliminating hydrostatic pressure and this is done most effectively by installing interior drain tile.  Generally, floor cracks cannot be repaired through any type of injection process because sealing floor cracks will allow hydrostatic pressure to build and force water to enter the basement elsewhere.

At U.S. Waterproofing, we understand how foundations are built and the problems that can occur with even the best of them.  We have diagnosed and solved seepage problems caused by hydrostatic pressure for many thousands of the more than 300,000 customers we have served since our founding in 1957, so why not ask for our free advice?

If you’d like to know more about basement floor cracks and how to stop them from seeping, please post your questions in the Comments box below.

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