How to Waterproof a Concrete Block Foundation


The second most common type of foundation in Chicagoland is the Concrete Masonry Unit (CMU) foundation, or as we like to call it, concrete block.  Concrete block foundations are commonly found in Northwest Indiana, Southern Wisconsin and small pockets throughout Northern Illinois.  This blog post will provide a brief history of concrete block construction, examine why concrete block foundations are prone to seepage, and present the different options of repair. 

History of Concrete Block

Concrete has been used in construction since ancient times.  It is made from a mixture of cement, aggregate and water.  Poured concrete (the most common type of foundation in Chicagland) is formed (casted) onsite.  Concrete block is concrete formed and hardened offsite in a manufacturing plant.  The first house in the country built using concrete block was in 1837 on Staten Island, New York.  In 1900, American Harmon S. Palmer patented the first hollow concrete block design. 

Sand and gravel aggregate is found in abundance throughout the Midwest.  They are the raw materials needed to manufacture concrete block.  Many people use the terms “concrete block” and “cinder block” interchangeably.  It should be noted that there are distinct differences.  Cinder block tends to be lighter because it uses remnants (ashes) from burned coal as the aggregate instead of heavier sand or gravel. 

In the early 1900’s, mass production methods led to a proliferation of homes being built using concrete block.  They became popular because they were easier to store and transport when compared to stone and brick.  They were also advertised to be “fireproof and weather proof”.  The great Chicago Fire is a testament to its fireproof qualities.  We’ll tackle its “weather proof” claims, however, later in this post! 

Blocks are used in both above and below grade foundation construction and come in different forms.  You might have heard of “split faced block”, which is used in above grade construction.  Concrete blocks used to build foundations usually come in 16 inch x 8 inch x 8 inch units and weigh about 40 pounds each.  They are stacked by a mason and held together using mortar.

Problems with Concrete Block Foundations

Over time, soil and water pressure exerted against the foundation causes the mortar to break down and wear away.  Consequently, water seeps through the exterior mortar joints and into the hollows of the block.  The water trapped inside the block eventually leads to foundation seepage.  It should be noted concrete block is porous by nature.  It easily absorbs moisture, which can contribute to mold problems. 

In some instances, the soil and water pressure is so great that it causes the wall to bow inward.  We'll discuss the structural concerns of this movement at another time, but we do have an answer.    

Ok, so now we have water coming into these hollow blocks and into the basement. What to do?  It's very simple.  We can either stop the water on the outside (Exterior Waterproofing) or capture it on the inside (Interior Waterproofing).

Concrete Block Waterproofing Solutions

Exterior Waterproofing:

Approaching the problem from the exterior (the “positive side”) seems to make the most sense to many people.  We are all for it when applicable and necessary.  One big draw back – COST!  It takes a considerable amount of labor (money) to dig down to the base (footing) of your foundation wall.  Once excavated, a waterproof membrane is applied to waterproof the foundation walls.  A drain tile is added to collect and dispose of the excess water.  If there are obstructions, such as a driveway, porch, or garage in the way, they would need to removed and replaced.  This also compounds the cost of the job. 

Interior Waterproofing:

There is another school of thought when it comes to waterproofing concrete block foundations.  Rather than stopping the water from the “positive side”, the water can be directed into a drainage system below the floor on the inside.  The process is known as Interior Drain Tile.  As a part of the installation process, holes are drilled into the bottom row of blocks.  These weep holes allow for water to drain out from the hollows and into the drain tile system beneath the floor.  We can install Interior Drain Tile Systems in both unfinished and finished basements, although the latter requires some prep work.

There are pros and cons to both processes.  For a more in depth comparison, I suggest reading our blog “Interior vs. Exterior Drain Tile – Which System is Better?

Getting tired of reading?  Feel free to give us a call or schedule a free consultation online.  Our Advisors are trained to diagnose your problem and present the most practical and cost effective solution.  You can also pose any questions you made have in the comment box below.  We’ll be quick to answer. 



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