foundation wall thickness

[John Dirks Jr]

I have seen a few block foundation walls that are thicker on the bottom 4 feet or so. How common is it to build this way and why is it done?

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[David Argabright]

Hi John;

You may be looking at a crawl-space that was excavated to be used as a basement. In some places they are called Michigan basements. I find them on some older homes.

IMO; The problem I see is that the foundation for these lower walls are seldom designed with the correct design load path of 45 degrees from the bottom edge of the upper footer to the bottom edge of the lower footer. ie If the finished lower wall is 3' below the bottom of the upper wall it should also be 3' inside the upper wall. A structural engineer should be able to explain it because there may be other considerations.

One nice feature is that if the ledge is flat it can be used as a storage shelf around the exterior walls.

[Gunnar Alquist]

I have seen a few block foundation walls that are thicker on the bottom 4 feet or so. How common is it to build this way and why is it done?

Might just be a thicker block. You can use a narrow block near the top of the wall because it is not going to be doing as much work. But, the farther down you go, the more lateral stress will be placed on the wall from the retained soil. Usually, the inner wall is plumb and the blocks step out on the soil side rather than the basement side.

[David Argabright]

Hey John; Do you have a photo?

[Raymond Wand]

To buttress the wall from inward soil pressures.

[John Dirks Jr]

Hey John; Do you have a photo?

I will by this weekend and I'll post it. I was at the property for other than a standard home inspection so there wasn't really a need to take a picture. I'm returning later this week for a follow up and I snap a few then and post em.