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What Causes Basement Flooding?

Published on February 17th, 2016

The construction of a home can either help prevent basement flooding, or make it more likely that problems will occur.

To understand basement flooding, you must first understand how a home is constructed. When building a home, the contractor digs down approximately 8 to 10 feet, which creates a bowl-like formation in the lawn. The contractor then installs footings, which will bear the weight of the structure. These footing are made of concrete reinforced with steel. On top of the footing, the contractor will either build a poured concrete wall or will install block to at least grade (which is the finished lawn height) and usually a foot or two over the grade. After the walls and footing are installed, 3-4 inches of stone should be placed over the dirt and the floor is then poured on top of the stone. The floor is usually 3-4 inches of 3,000 PSI concrete. This type of basement is called a typical footing configuration.

On the exterior of the wall, the contractor will usually damp proof the concrete blocks or poured foundation. At the base of this, there should be a footing drain of 4 inch perforated pipe, routing to a dry well or “air” on a downwardly sloped hill. Theoretically, this will take any water pressure that is exerted on the exterior and drain it away from the foundation wall.

After these processes are completed, the exterior of the foundation is back-filled with the dirt that was previously removed. When the contractor backfills around the 5 to 10 feet of area surrounding the exterior foundation walls, he is using soil that is now looser and more aerated than the compact virgin soil that surrounded the building. This is due to the fact that the soil was dug up and then replaced. The virgin soil had been there (possibly for centuries) undisturbed, was very tight and not permeable. In this loose backfill area, the dirt is now porous and not as compacted as the virgin soil. Although it can be mechanically compacted, it will never be as dense as the virgin soil 5 to 10 feet out from the foundation wall. This is key to understanding why a basement leaks.

When it rains, water sinks into this backfill area and saturates it. The water will saturate the backfill area between the virgin clay soil “clay bowl” and the foundation – and will direct it towards the footing/wall intersection at the base of the new structure. This creates water pressure at this juncture.

Pressure is created based on the fact that the virgin soil is less permeable than the newly replaced soil and concrete walls cannot allow water in. Concrete walls 10 inches thick with floors 4 inches thick will not typically allow the water in. Water cannot go directly through concrete unless there is a visible joint or crack.

The poured foundation walls (if continuously poured at the same time) will not allow water in. However, if there is any type of break in that concrete structure – or if another concrete structure is poured at separate time, there will be a seam or crack on that structure. These seems can allow water to penetrate into the basement.

After the footing is poured and setup, the foundation wall is poured and the forms are taken away. The floor is then poured and finished. There is always a seam between the footing, the wall and the floor. We often refer to this as the “floor/wall joint”.

The rainwater pressure from the outside by the over-dig will exert pressure on the seam between the foundation wall and the footing. Also, it is possible for the water to go underneath the virgin soil and the footing itself. This water can seep underneath the footing and into the perimeter of the basement under the floor and come in through the cracks.

When diagnosing a basement water problem, it is important to know exactly where the water is coming from. There are some common guidelines we can use to speculate where the water is entering the basement. This is very important in diagnosing the water problem and devising how to fix it.

When it rains, the water can exert an extreme amount of pressure at the foundation wall. In a majority of basements, the water emanates from the floor/wall joint. Water leaking from the floor/wall joint emanates from the pressure outside the foundation and footing wall joint due to the over dig (which the contractor excavated out for the basement).

Clay Bowl Effect

The contractor digs holes similar to a bowl in the earth. The foundation footing and walls are installed inside this bowl. The outside dirt by the wall is called the “over dig area”. Water gathers around the exterior of the foundation and soaks through the over dig area because the back fill  soil against the wall (and approximately 5 to 10 feet away from the foundation wall) is looser than the virgin soil that the excavator dug out. The virgin soil represents possible centuries (or more) of compaction and rain water is not prone to sink into this compacted virgin soil area.

The rain water does saturate the back fill soil (loose soil) – creating a “clay bowl effect” between the foundation footing, foundation wall and virgin soil. This rain water creates pressure and will push its way through to the inside of the foundation footing and wall. This “push through” occurs over the foundation footing, underneath the foundation wall and also underneath the footing itself with the virgin soil underneath. Once again, this simple mechanic is the key to understanding why a basement gets wet.

Mythbuster: The Water Table

Basements usually leak when it is raining outside. The common belief is that water rises through a “water table”. Although the water table underneath the home does have the ability to rise, the reality is that the mechanics previously described are the real reasons why a basement typically gets wet. Building Departments and codes account for the water table presence when building permits are issued for the pouring and building of the new foundation walls and basements.

On new construction, percolation tests are performed prior to the start of construction and the possibility of the water table rising to the point of flooding a basement is very rare.

For example, if a home is on a hill and the belief is the leak is caused by the water table, then the streets leading up to the home would be a lake.

Questions? Let us know how we can help. Our team at Basement Technologies is here for you!

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