Q&A / 

Poured Concrete vs. Block Walls

DEAR TIM: What is the difference between concrete block and poured concrete foundation walls? Which one is better? The lot I am going to build upon has a seasonably high water table. Which of the wall systems would insure a dry basement? Diane J., Redwood Falls, MN

DEAR DIANE: I think it might have been easier to answer a question such as: Is a glorious sunrise more beautiful than a magnificent sunset? The truth of the matter is that both building materials - concrete block and poured concrete - can yield superior foundation walls so long as they are installed correctly. All too often builders and sub-contractors fail to realize the limitations of certain masonry materials. When this happens, foundation failures are a common occurrence.

The first thing to realize is that concrete block is made from poured concrete. The primary ingredients in concrete block happen to be: Portland cement, gravel aggregate and sand. These are the same ingredients you would find in a poured concrete foundation. The only difference being the size of the gravel used in each application. Typically you will see gravel stones as large as three-quarter inch diameter in a poured concrete foundation. A concrete block may have gravel no larger than the size of a pea.

Poured concrete foundations are solid through and through. This happens by default as the plastic concrete flows from the ready mix trucks into the foundation forms. The typical concrete block foundation is not solid. The concrete blocks that are used to build block foundations, by their very nature, are hollow.

When concrete blocks are stacked on top of one another, you can look down through the center of the foundation. After the concrete blocks are laid, the voids can be filled with a cement-based mortar or poured concrete that contains small pea gravel. If the builder does this, then the filled concrete block walls are nearly identical to poured concrete walls.

But simply doing this is not enough to satisfy myself, code officials and structural engineers. Concrete and things made from concrete or cement tend to have fantastic compressive strength. Often it is measured in the thousands of pounds per square inch range. But the same material typically has only one-tenth the strength when you apply a tension or stretching force to it. Bending a poured concrete or concrete block wall creates tension. Backfilled soil against a wall produces tension as the soil pushes against the wall. Poor quality soil creates tension on a foundation as more and more weight is added to the foundation as the house is built.

You can strengthen poured concrete and concrete block walls by including reinforcing steel. Often you will see horizontal steel bars placed in the lower and upper sections of poured foundation walls. This steel often has a tensile strength of 40,000 pounds per square inch. This horizontal steel helps prevent vertical cracks should the foundation drop or heave. Horizontal wire fabric can also be placed in the mortar joints between layers of concrete block to achieve the same result in a concrete block wall.

Vertical reinforcing steel is also very important. This steel can be put in both a poured concrete foundation and a concrete block wall that will have the hollow voids filled with cement grout or pea gravel concrete. The reinforcing steel should be one-half inch in diameter and these rods should be connected to the poured concrete footer that the foundation rests upon. This steel should be placed every two feet on center.

Neither wall system will be waterproof. If you want a dry basement or crawlspace you must apply a foundation waterproofing system to the exterior of the foundation walls after they are built. In addition, a drain piping system needs to be at the base of the wall adjacent to the footer. This pipe is covered with two or three feet of washed gravel. The gravel is then covered with six inches of straw or a sheet of tar paper before dirt is placed over it. The straw and tar paper prevent silt from the fluffed up backfill dirt from clogging the gravel. The water that flows through the soil and makes it to the pipe is then drained to daylight if the house is built on a sloping lot. If the house is built on a level lot, the drain pipe often empties into a sump pit.

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33 Responses to Poured Concrete vs. Block Walls

  1. If your foundation has the drain piping system on the perimeter, does it still need the outside of the wall coated with a tar mixture?

  2. You always want exterior water proofing, even if interior drainage is good, because any water will bring mold, algea and in cold climates, frost and cracking.

  3. Is it common for the sand to migrate out of a pored foundation wall? In one area, I have a light tan "dust" on the wall (top to bottom). I vacuum / brush it off and it re-appears.

  4. I was recently looking to buy a house with a block built foundation/ basement. It appears that someone has installed some sort of piller supports on all walls except one. The wall that has none is a little bowed, and has a nice sized angled crack that leaks water. I just witnessed the sump pump running you could hear the water going out of the house overhead and 3 minutes later the water would literally pour back in the crack! Besides getting the water away from the house how could you fix this?

  5. On a poured concrete or block wall, when furring strips are used to attach drywall, How do you obtain the depth to install electrical boxes? We. Are building a home in southwest florida.

  6. Hello, I want to buy a house that has block foundation. The house has a plastic that covers the blocks and you can see white pots on the block. What is your advise?

  7. I have a seasonal home built with concrete block foundation on sandy soil. Last time I was there I noticed on one corner of the wall the blocks had filled with water, about 25 of them. I did not have the time to empty them out.
    Do you think the water entered from the outside water line or from under the cove joint and worked its way up.
    I guess they had about 9 inches of rain the week before.
    Furthermore I noted a horizontal crack in the front facing wall but I did not detect any bowing in that wall

  8. I'm going to replace one of the block walls in my basement thanksgiving weekend. All four walls are bowed in and they are bad at the cornets too. How do you tie in a new straight wall to a bowed at the corner? Lots of mortor?

  9. I want to build a tornado shelter because I live in tornado alley. There are lot size construction so it has to be aboveground essentially. I'm concerned that CMU blocks or just strong as a poured wall, even if I backfill it and reinforce it with rebar. I am using FEMA's drawings.there will be a solid poured top and a solid poured foundation with the rebar tight end at least every 16 inches on center. Am I mistaken that it won't be as strong as a solid wall?

    • The block MUST be filled with pea-gravel concrete. Hollow block can be EASILY penetrated with a flying 2x4. Then, imagine a car or pickup truck flying through the air that RAMS your shelter!

  10. I want to build a 30x60 single story building with a 12 ft ceiling and a pitched roof. I live southern Indiana. And the building site will most likely be in a cleared out space in a wooded lot. Not terribly sloped but not exactly level either. What sort of foundation would you recommend?

  11. I'm looking at purchasing a house with a stone foundation/basement in the Midwest (Kansas City). I would like to add a two car garage with a suspended floor with space underneath that could be finished and connected to the existing basement. Can this be done?

  12. I live in the Kansas City area. I have a basement that is referred to as a Michigan basement. The basement is rectangular with a 7' ceiling. Half of the basement wall is a full 7' tall. The other half has been dug out, leaving a 2' knee wall on a 5' tall dirt ledge. A poorly formed sister wall was put in some time in the past to hold the dirt ledge in place. that sister wall was not reinforced and is starting to fail.

    My question is:

    Is there anyway to fix this wall? Replacing the wall exceeds the value of the house. Many engineers have told me that the sister wall is not load bearing and therefore does not need a footing under it. I have been monitoring the sister wall and it has not sunk any in 10 years, but it is bulging in to the basement. the result is the knee wall is starting to sink.

    Is it possible to hand-stack concrete and fix this wall?

  13. My builder is installing foundation blocks 6x8x16. He places the steel from the base of the trench within the core of the blocks. But as he lay more blocks and the foundation gets higher the steel is no longer visible. Was this done right?
    It's like 4 rows height of foundation blocks. He argues that bringing up the steel is not necessary because of the height. Please help.

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