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cinder block on slab?


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I have a coworker who owns a home in the midwest. It is a tudor built in 1974. He is moving some walls on the first and second floor. He is trying to understand his foundation before he starts upstairs.

It is obvious that footers are poured on the exterior walls. However, in the basement he has cinder block walls that partition the space. Would footers be poured under these cinder blocks or would it have practical to lay them on top of the slab?

I know he would have to ultimately dig to know for sure. Just wondering what some experts would believe is under the slab.

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Those block walls that "partition space", if they do not do anything else, may or may not be supported by footing thickness, which should at least be six inches for residential for a wall that supports structure above. I have seen a lot of big cracks in basements where control joints were not cut, but have never seen boring/testing for thickness on remodels. The question is logical, but then you never know how home handymen might change things in a basement.

Some of the worst I have seen is where home handymen have created "man caves" in basement/crawls out of osb that butts right into soil and nailed onto spruce framing that is equally risky re termite invasion.

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4 inches of slab on undisturbed soil works fine as footer.

Marc

Mark,

I disagree. It depends on the bearing capacity of the soil and the width/height/weight of the wall.

The footing design (depth, width, type of concrete, and possible reinforcing) is based on that information.

I have seen many failed footings due to improper design and installation. Keeps me busy with legal work too!

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Construction varies quite a bit in different areas and at different times of construction. I assume the concrete block (not cinder block) wall is a bearing wall to support floor joists and loads from above. There should be a footing under the slab (or the slab should have been thickened at this area). If the slab is sitting on bedrock it would not matter. If sitting on poor soil it could matter.

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4 inches of slab on undisturbed soil works fine as footer.

Marc

Mark,

I disagree. It depends on the bearing capacity of the soil and the width/height/weight of the wall.

The footing design (depth, width, type of concrete, and possible reinforcing) is based on that information.

I have seen many failed footings due to improper design and installation. Keeps me busy with legal work too!

Yeah, I did forget geographic differences in soil content. The figures I gave pretty much covers anything in south central Louisiana.

Marc

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I have a coworker who owns a home in the midwest. It is a tudor built in 1974. He is moving some walls on the first and second floor. He is trying to understand his foundation before he starts upstairs.

It is obvious that footers are poured on the exterior walls. However, in the basement he has cinder block walls that partition the space. Would footers be poured under these cinder blocks or would it have practical to lay them on top of the slab?

I know he would have to ultimately dig to know for sure. Just wondering what some experts would believe is under the slab.

Hire an engineer to determine where the load bearing walls are and if any foundation modifications are needed.

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Are the floor joists above supported by the basement block partition?

One way to tell is to look for splices in the joists where they rest on the wall. If so, the block wall is supporting the structure, it is likely original and probably has a good footing. Moving the walls above is not an option for a novice.

A simple basement partition that runs parallel to the joists or has joists continuing full length to a beam beyond, is likely just sitting on a thin slab. That wall would not qualify as a support for walls above, although it could be upgraded with posts and a beam. It needs engineering.

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  • 3 months later...

I have never seen a 4 inch slab as an acceptable footing for a cement block wall. You can tell if the purpose of the wall was to bear weight by looking at the structural members resting on the wall. The age of the home can tell you what the codes were at the time of construction, but you would have to assume that they were followed. There were and still are some areas that do not require permits or inspections. The materials bearing on te top of the wall are your best clue. On way to check for a footing, would be to use a hammer drill with a bit that's at least 12" in length. Drill a few holes along the wall and check the depth. The same would go for interior partition walls. If you have access to the attic, check to see if there are joists that split on the wall. Are the joists ceiling joists or floor joists? You would have to determine the proper size and span for the current members. You can sometimes remove bearing walls if you install properly engineered girders headers etc.

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