Basement Waterproofing: 101

Published on August 5th, 2019

There are 6 locations where water can enter the basement. The floor/wall joint, floor cracks, wall cracks, the bulkhead, windows or over the sill plate and down the wall. In this post we’ll be discussing each.

1. The Floor / Wall Joint

Also known as the cold joint, the floor/wall joint is the most common entry source of water infiltrating the basement. This is where the foundation wall meets the basement floor. The water comes in from the outside over the footing under the foundation wall and up through the the intersection where the foundation wall and basement floor converge. This type of wall seepage is seen most commonly when it rains. The reason for this is due to the over-dig mechanic previously described. If you have 3 to 4” or more of water in your basement, the source of the water is probably from the floor/wall joint. Water may seep in from other locations, but the most heavy water intrusion will be from the floor/wall joint. Due to the nature of concrete, when one concrete structure is poured and another concrete structure is poured next to it (or on it), a seam is created between the two concrete structures. For example, the footing is poured – the foundation wall is poured – and finally, the floor is poured. Each concrete structure (the footing, the wall and the floor) is poured separately. There is now a seam between each of these three structures. This allows the potential for water (under pressure) to come into the basement area. The points of entry are over the foundation footing, between the foundation wall, underneath the foundation and up through the seams between the foundation footing and the floor. This floor/wall joint seepage is very common and is a predominant reason for basement flooding.


2. Basement Floor Cracks

Water cannot enter into the basement without having a visible crack, break or a seam. Concrete cannot “leak”. If there is a visible floor crack in the poured floor, the water can enter from underneath the slab floor – through the visible cracks. Lally columns are a common source. If a footing was poured and the column was sunk under the floor, the intersection of the steel and the floor can create a floor crack. After the water has penetrated over and under the footing, water is present underneath the floor. It is common for floors to produce cracks after they are poured and water can and will infiltrate through these floor cracks. Floor cracks can occur due to settling and temperature differences between the interior and exterior.


3. Basement Wall Cracks

A visible wall crack in a poured foundation wall is another common source of entry by rain water. Wall cracks are easier to see than the floor/wall joint or a floor crack. Wall cracks would also include any pipe penetrations or tie rods that were used to construct and hold the forms in place when the wall was poured. Tie rods are metal pins that go all the way through the foundation wall and tend to rust over time. This rusted pin allows the water to enter between the rusted pin and the concrete. Wherever a pipe penetrates through a wall – between the exterior of the pipe and the concrete foundation wall – there is a seam. Water can enter through this seam.


Other areas to inspect for water infiltration are windows, bulkhead entries and over the top of the foundation wall. Quite simply, a wet basement is caused when rainwater gathers around the foundation/wall footing due to the over-dig from the original construction. This rain water, under pressure, pushes through to the inside of your basement through a variety of different ways.

4. Basement Windows

It is not uncommon for basement windows to be below the grade of the lawn. Rain runoff can penetrate windows and leak down the wall onto the floor.


5. Bulkheads

It is common for some homes to have an entrance into the basement from the outside yard. It this case, it is typical to build three walls protruding from the primary foundation wall and installing stairs to and from the basement as well as a cover for access. This area can be a significant source of water infiltration. There are many varieties of bulkheads and bulkhead styles and the best solution for water infiltration is to stop the water from getting onto the floor of the basement.


6. Sill Plate

This is the intersection where the wood framing sits on top of the foundation wall. This area can allow water infiltration depending on the grade of the lawn. If a house is at the base of a hill, water will flow down and pool at the foundation. When this happens, the water can come in over the foundation wall and underneath the sill plate.


The cause of a wet basement is usually the rainwater that has saturated the exterior of the home in the over-dig area. Water creates pressure at the base of the virgin soil and the foundation. The water can then infiltrate over or under the footing and enter the basement through any one of the above mentioned avenues.


Although it is possible that water tables can rise and flood a basement, the reality is that 9 out of 10 homes that suffer from wet basements do so because of saturation of the over-dig area. Heavy rainfall can enter the basement through any one of these (6) described methods. Flooding water cannot penetrate through concrete – unless there is less than 1” of concrete in the basement structure. Foundation walls are usually 10” thick and floors are normally 4” thick.

Water must enter through a visible crack, a floor/wall joint, a bulkhead, a window or sill plate. Homeowners must keep this in mind when diagnosing the source of the water in their wet basement.

Questions? Contact us today!

Back to News

"Basement Technologies has restored my faith in customer service."

Betsy M. Southborough, MA