Re-doing Basement slab

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Old 01-13-21, 09:03 AM
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Re-doing Basement slab

House: 1950's Ranch. Mud/rat slab currently in the basement.

Due to both radon issues & desire to finish our basement, we are going to start a project to remove our existing basement slab & re-pour the concrete. We've tried a radon mitigation company but despite their efforts they are unable to resolve the radon problem.

Asks:
- What would be the negative consequences of pouring the slab in sections, if I were to do this myself? (unlikely that I do this since it is 1200 sq feet) - this would allow me to do it over the course of a few weeks, section by section

- when is underpinning required? there is currently no gravel under the existing slab, so we will need to dig out an additional 4" minimum. The goal of this new slab is to add gravel & potentially insulation under the slab. What is the criteria for when I need to underpin the exterior footing?

Thanks for all of your help.
Jon

 
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Old 01-13-21, 09:45 AM
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You should not have to underpin the foundation as long as you don't break it when you remove the floor. Your footing is thick (likely 6-8" or so) and it should extend out from the foundation at least 4" or more, and it usually supports the outside edge of the floor. As long as you don't undermine or jackhammer on top of the footing, it should be fine. You DO NOT want to disturb the soil under the footing. So when you break out the floor, stay about 6" away from the edges at first. Then break out the remaining portion carefully, so that you don't break the footing or wall.

Pouring the slab in sections is likely going to result in a very uneven finish floor, no matter how careful you are to keep it level. This would mean you would need to cover it with a self leveling compound after you are done, and even then, the self leveling will likely crack along all your seams.

I assume part of your goal in tearing out the floor is to address the radon, so if you are doing mitigation along with the floor (sub-slab mitigation along with drain tile) all those cracks would first need to be sealed with polyurethane sealant before the self leveling compound goes on.

Concrete is also not really one of the things one person can do and have it turn out well. That's why professional concrete crews who work on large projects have 4-6 guys on them.
 
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Old 01-13-21, 11:09 AM
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IMO, large cement projects are one of those things that are better to hire out. Bringing in a cement truck and a crew to get it finished in a few hours is well worth the cost.

I feel the same way about large drywall/mudding projects. I'm more than happy to do a room on my own, but when planning a few rooms or a house, sure I can do it, but for the reasonably low cost of labor to do it, it's amazing to watch a good crew crank it out.
 
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Old 01-13-21, 01:29 PM
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Thanks for the quick replies. I agree entirely on having a crew do it and I've already gotten a handful of quotes.
I would be surprised is there was any drain tile under the existing slab & I hadn't considered installing one to be honest.

So right now it looks like I'll be digging down 10" total, being careful around the perimeter not to compromise the footing or wall. 4" gravel, 2" insulation (in Massachusetts), 4" of concrete.

As far as radon - the mitigation company that we worked with advised that we just do crushed gravel under the slab & have a thick vapor barrier between it and the concrete. Issue is right now that the slab is right on the dirt, and therefore there is very little airflow. Obviously I'll need to seal the vabor barrier with concrete anchors and caulk, & type but otherwise, fingers crossed, should do the trick. I'll reach out to a few other companies to see if they suggest the same, since I know there a a few approached out there for new construction.
 
 

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