Lowering Small Area of Concrete Basement Floor

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Old 07-30-19, 02:27 PM
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Lowering Small Area of Concrete Basement Floor

Hi and thanks in advance for responses/advice.

From my limited knowledge, it's not a good idea to simply lower a concrete basement floor by cutting the concrete and digging, since it introduces soil into the foundation and messes with structural integrity. Is this the case even if the area and depth are small enough (e.g. lowering the floor on a 5' by 5' area half a foot in a basement that's 30' by 30')? Our basement has ceilings just barely my height and I'd like to have a section with a couple more inches to put gym equipment. Area wouldn't need to be big at all.

Figured to ask since when you put a sump pump you cut down the concrete and dig out some soil, and I haven't heard people mention of structural integrity when doing this.

Thank you!
 
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Old 07-30-19, 02:43 PM
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You can remove the concrete and dig down so you'll have more headroom but you can't just dig down with no consideration of the footers. My grandfather had a cellar with a little over 4' headroom. He dug down 2.5' BUT not all the way to the foundation walls. He built a new wall [with footer] a foot or so away from the original foundation wall and blocked that wall up past the bottom of the house footer. Then poured a concrete ledge between the 2 walls.
 
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Old 07-30-19, 04:58 PM
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My inspectors generally consider the soil under a 45 degree angle out from the bottom of a footer to be the no-go zone without Engineering approval. So, in general, if you excavate and stay out of that 45 degree area below the bottom of the footer you are OK to dig.
 
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Old 08-03-19, 03:38 AM
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Thanks for the advice mark and pilot,

My buddy does concrete work and would give me any tool Iíd need and some advice on moving forward. Based on above (around a 5í by 5í or so area half a foot or a foot deep), do you guys have any idea on the difficulty or time this would take? Thanks again!
 
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Old 08-03-19, 05:02 AM
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Hard to estimate the time without seeing it in person. It will be hard work as you'd need to break up and remove the concrete before you can excavate [which also needs to be removed] How far is the 5x5 section from the exterior walls or support posts? Seems like a lot of work for such a small area. You'd also want room for steps.
 
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Old 08-04-19, 02:24 AM
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Thanks mark,

Basically the basement has ceilings just high enough to stand in, but nothing more. The small area would be a place to put something such as a treadmill, etc that would require a half foot or foot more height to be functional.

That being said, the location isnít critical. The basement is 30í by 22í with support beams running along as to divide the 22í into two 11í sections. The area I was thinking would be a couple feet away from both an exterior wall and support, but based on your expertise could be flexible.

Bottom line is thereís no problem putting in the work - it would be very beneficial even though small and I enjoy this type of project - but I donít want to create a situation Iíll regret or would be detrimental to our home.

Thanks again!
 
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Old 08-04-19, 06:24 AM
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Given those dimensions, it would make more sense to just lower the entire basement floor to create a usable basement.
 
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Old 08-04-19, 08:51 AM
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Roe: Thought I would tell you about a "similar" project I undertook earlier this year. I needed to gain access to a septic tank that had a massive (to me) lid and had sprinkler system lines laid directly over the lid. I settled on the idea of cutting a 4" x4" window in the tank top so I could measure the contents. Renting a concrete cutting saw was possible but I had an old Skilsaw and concrete cutting blades, bought some new concrete drill bits and went at it. The concrete probably was used in the same pour as some nuclear plant and ate up the blades very quickly. I was able to make a deep scratch around the edges of the intended window and then took the bits to try to cut through the corners. The bits went through the concrete satisfactorily but when I hit the granite aggregate, the bits did no good. So, I took my three pound hammer and decided to just knock a hold in the lid. That finally worked but with extreme effort. I finally got a 2" x 2" hole opened where I could insert a stick to measure.

I know you will have much better equipment for your project than I did but I am thinking this is going to be a major job unless you use a jackhammer. Best of luck.
 
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