Q&A / 

Poured Concrete vs. Block Walls

Poured Concrete vs. Block Walls TIPS

DEAR TIM: What's the difference between concrete block and poured concrete foundation walls? Which one is better? The lot I'm 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.

Concrete Block Is Concrete!

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'd find in a poured, or cast, concrete foundation.

The only difference between the two finished products s the size of the gravel. Typically you'll see gravel stones as large as big juicy grapes in a poured concrete foundation. A concrete block may have gravel no larger than the size of a standard green pea or a dried raisin.

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Poured or Cast Concrete

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. Poor workmanship can yield hollow spots in poured foundations. The contractors use special vibrating tools to prevent this.

The typical concrete block foundation is not solid. The concrete blocks that are used to build block foundations, by their very nature, are hollow.

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Make Block Walls Solid

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.

The concrete used to fill the block voids must have small gravel so it can flow readily into all the narrow voids. To maximize strength, the voids need to have 1/2-inch steel rods centered in the voids from the bottom of the block walls to the top. Put this steel every 2-feet on center.

If the builder does this, then the filled concrete block walls are nearly identical to poured concrete walls.

Both Weak If Bent

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.

This means when you squeeze concrete it's very strong. 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 to it. Tension is a bending or stretching force.

You can easily picture this. Imagine pouring a 4-inch thick slab that's 10 feet wide and 40 feet long that's up three feet in the air and just supported on each end.

It would bend in the center under it's own weight and crack long before it would if you tried to drive across it!

Massive Soil Pressure

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.

Poor drainage around a house creates pressure as tons of water collects in the soil.

This is why roof water should NEVER be allowed to run on the ground near a foundation.

The use of splash blocks at the base of a full basement is comical. Roof water needs to be piped far away from the house to the lowest spot on the lot or into an approved storm sewer.

Steel = Strength

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 is usually found about one foot up from the bottom of the foundation wall and a foot down from the top.

There are usually two rows of the steel and it's continuous around the entire foundation. When the steel bars overlap, it should be no less than 16 inches.

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 Steel

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.

Not Waterproof

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.

There are many different systems, but my favorite when I was building was a rubberized asphalt called Tuff-n-Dry. Once the material was sprayed on the walls it was about 1/8-inch thick. If the foundation did crack, this material could stretch and bridge a crack up to 1/4-inch I believe. The specifications may be different today.

Liquid asphalt is often sprayed on a new foundation wall. It's like a coat of paint and very thin. This is NOT waterproofing. It's just a vapor barrier.

Drain Tile

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.

Column 439

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45 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?
    Thanks
    Jeremy

  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.

  14. Hi, I'm renting a house that I also have the option to purchase in 5 months in Winnipeg Manitoba Canada. It has a block foundation and a fully finished basement so it's hard to tell the shape of the foundation. The reason why I suspect there to be a foundation problem is throughout the house (only on main floor not second story) there are cracks in the walls almost around every door frame . I was told that's signs of stress and I could be looking at foundation problems . The furnice room is the only room in the basement that isnt finished and there is a small horizontal crack right across the middle of the walls , the crack isn't big, can't fit a dime in it. Also underneath the stair case is another little closet with bare foundation showing and I looked and there is a large crack that seems to have some sort of filler in it but the filler is loose I can wiggle it with my finger.

    Please tell me your thoughts on this and if you think I'll be looking at some hefty repairs on the foundation in the near future if I choose to purchase this property.

    Thanks !

    • It's really hard for me to speculate without being there, but based on your description, I'd NEVER buy this house. READ all my columns here in my Structural category. Just type that word into my search engine to get to it. I have a column about the EXACT cracks you see at each doorway!

  15. Tim
    I recently had a soil engineer out on a piece of vacant land in which will require a mound type septic system. Everything is a go for that. I do however have one concern we talked about. The land has some very shallow areas toward the back in which he called shale rock. My concern is building a forever home with a basement as well as having a backyard for the kids to play in and not a swamp. Is there things that can be done to drain/remove some of this excess water that sits back there. Any expert advise would be appreciated. Thank you so much. Also want to keep a dry basement. Have a great day.

  16. Tim, no question, just wanted to thank you for all your info. I have been debatting on which way to build (block vs poured) I live in a house with a block basement now that I did not build and it was not build corretly and it has been nothing but trouble. (Water, cracks, mold, etc. I'm selling and starting over. I am looking to start building a house with a full basement and the info you have supplied is a great help.

  17. Tim,
    I have a question I live in the Philippines, i would like to build a basement, I was told every 2-3 years the area I live in if the drainage is clogged then I might get about 2ft of water outside. Now I could raise my house but I was wondering if I went along with building the basement, had an outer and inside wall 1ft apart and then poured concrete in the middle and then treated the outer wall with a concrete sealer, would that be ok? Maybe I'm over thinking it I'm not sure. I would just hate to build a basement and then have to worry about it flooding. I did intend to put a sump in the room as well. Your advice would be appreciated.

  18. Hi, I just constructed a small "wall" 18" high x 32" long x 8" thick to support 4 x 4 8ft posts for a lean-to roof over an existing basement door. It extends down 12" below adjacent grade. It is formed with wood boards. How soon can the form be removed? I know that the concrete needs light wetting while curing. The roof is eager to be built.

  19. Gary Jenkins
    [email protected]

    We are having a concrete retaining wall for a fence footing built by a sub-contractor of the Fencing company. The contractor is preparing to pour concrete. My concern is that per my request the metal fence posts were seated in PVC and then concreted. Because there has been delay due to weather (rain) there is now dirt that has built up over the concrete seating for the posts. I'm questioning if there is a concern over pouring the concrete over the dirt (which will cause a layering effect of concrete/dirt/concrete). It's my thinking that if the dirt then washes out (from natural drainage) between the layers that there will then be space and will the weight of the fence then lead to cracking in the will. The subcontractor says no, but since he has been less than knowledgeable up to this point, I'm reluctant to trust him. Your thoughts would certainly be appreciated!

  20. We have a walk out basement. The earth around it is clay. The basement is 12 inch block. My husband is good about keeping gutters and downspouts clear and directing water away from the house. The basement wall is 9 or 10 feet tall. We recently noticed, after living there eight years (house is 12 years old) that the wall into the hill leans forward one inch at the top. We removed the Sheetrock to see if the wall had damage. It is perfect, no cracks, very dry. One structural engineer told us the wall had moved and was compromised. Should we get a second opinion? The whole basement is tiled. Not one cracked groutlines, no cracks, not even hairline, in Sheetrock, glass shower doors close perfectly, doors and windows close perfectly. The interior walls lean by same amount, but doors are square. We think the wall always leaned, was constructed that way. Decks are flush, concrete patio is not cracked and is flush, stacked stone on exterior is perfect. We also believe the previous home owner, who finished the basement himself, came off the back wall. In our minds, if the walls were pushed, things would not just be slightly tilted in but also off square. Look forward to your opinion on our second opinion. Also, if we do brace the wall, what would you recommend? He recommended steel h beams from footing to floor joists. Upper level is log.

  21. We are planning to raise the Ground Floor by 10Ft height and construct G+2 Floors. i.e ground floor + 2 additional floors.
    Can we have the Large rock based foundatation for this 10Ft wall on all 4 side or just concrete with Steel road + bricks for 10Ft.
    Pl. suggest for cheap and best solution

  22. I have a question that I've been unable to find an answer for; but then, it may just be a stupid question.

    I'm thinking of building a shed, with a "basement"
    I've researched all what I think I need to understand how to do the concrete floor and I want to use cement blocks for the walls, with rebar reinforcement.
    I would then build supports to pour the "basement ceiling/shed floor" on top of.

    My question is this:
    After having the basement hole excavated, I'm going to end up with a large amount of dirt.
    What would happen if I used this dirt, to fill in the gaps in the concrete block?
    I could do it two ways.
    One is to fill it in as each layer is cemented in place and tamp it down solid.
    The other way is to wait until the walls are finished and then fill them in, tamping them down by building 2 different kind of tampers.
    One made with a solid bottom for the holes with no rebar and one build with a hole in the middle for the ones with rebar.

    Please don't laugh to loudly.

  23. We're a fourth generation family owned and operated design-build firm with nearly 70 years experience in building homes. Our background is in masonry and we've been doing block foundations going back to my great grand father. It wasn't until recently, some prospective clients brought to our attention concerns of block foundations that we discovered articles saying block foundations are hollow and not reinforced with steel. Since the age of 4 years old, walking around the job sites with my father and uncle, it was common knowledge to me that block foundations were reinforced with vertical and horizontal steel and filled with concrete. Reading up online and finding where builders are not doing this just blew all of our minds. Prior to reading online, we were really struggling to understand these clients admit concerns of block foundations. Now I see where they're coming from. Thank you for the article!

  24. I'm in the desert with 6' tall block fences around my whole property. The fences have stucco applied on the outside, looking away from my yard. My property is 38 inches above the grade of the surrounding area and four rows of the fence block retain dirt. The ground pitches rapidly down just outside my fence. When it rains, water soaks the soil and eventually loosens the stucco on the outside. I'm tired of paying to have it replaced every 5 years. What can I do to eliminate this problem.

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