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I've got a section of my basement that I want to turn into a workshop, but one of the many things to improve is the floor. I want a smooth level floor for this section of the basement (I might eventually take lessons learned and do the rest of the basement, but I don't want to make this project bigger than it already is).

the current floor is very rough, and seems to have been made a few different concrete slabs that are now uneven, damaged, cracked.

Is there any hope of repairing this? breaking or grinding down high spots, filling in the damage with a repair filler, and then pouring a thinner layer on top to level it out?

Or is this a case where I just need to sledgehammer the whole area and pour something from scratch? If so, what are some concerns and gotchas that I might need to worry about with a basement floor like this?

Any advice would be much appreciated for what approach is going to be the most DIY friendly. My concrete working skills are pre-amateur, so even tips on tools and supplies would be valuable.

so rough that it's hard to even sweep

Seems like a relatively thin slab. damaged down to the dirt underneath

one slab is significantly higher than the other

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  • Hae you got some wooden posts going into that slab as well as the steel ones? Those will rot (or already have started to.)
    – Ecnerwal
    Commented Nov 3, 2023 at 1:49
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    How much headroom do you have now? How much headroom can you stand to lose if you pour an overlay?
    – Ecnerwal
    Commented Nov 3, 2023 at 1:59
  • The wooden posts are already rotted, but aren't supporting anything important, I'll be taking them out before any floor work. Commented Nov 3, 2023 at 4:56
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    I'm fine with losing an inch, preferably not much more than that. If for no reason other than it will put that part of the basement at a different level than the rest. Commented Nov 3, 2023 at 4:57
  • If the surface doesn't have to be concrete, you could consider building something out of plywood. Use shims to provide flat enough attachment points. The plywood won't crack as easily if there is some movement.
    – jpa
    Commented Nov 3, 2023 at 14:46

3 Answers 3

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The best solution would be to break it all up and re-pour a nice monolithic reinforced slab. That's also expensive and a lot of work, so you might want to consider attempting "not the best" to see if you can manage "good enough for what you're actually doing" at a lower price point with cost being an issue.

Quick, dirty, far from the best but "something" and no loss of headroom would be to remove the loose broken bits, clean out the crud in the gaps or former walls, and mortar the holes & gaps flat with the remaining floor. If it breaks up again in a year or two, you haven't spent much on trying it, and you'll know you need to do a better (and more expensive) job. If it lasts 5 years it might be "perfectly adequate" depending on your expectations of an old basement shop floor.

I would NOT suggest trying to follow that up with self-leveling compound - I'd expect it to move at least a bit, and that will break the (expensive, delicate) self-leveling layer. SLC is only suitable on a stable, but unlevel or rough base. This isn't that.

As one example that won't be getting featured in "Fine Homebuilding" or "This Old House" - where you have workbenches what you need is a set of level spots for the legs, and tolerably level floor around them to walk on. Apart from where the legs are, the floor under the workbench can be all kinds of wonky without affecting anything but your sense of the perfect basement workshop - which is either something you have to decide that you want to pay for, or something you have to face down where it doesn't really affect your use of the space, in the interest of practical budget considerations. It's your call, at that point.

Likewise, once you get the canyons and crevasses filled in, minor cracks vanish under a rubber mat that makes it more comfortable to stand on the floor.

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  • Agree with this approach. That slab doesn't have proper drainage, insulation, thickness, vapor/radon barrier. Unless you have 20-30k sitting around I'd just patch it. I would buy a Airthings Corentium Home radon detection kit to determine what levels you currently have so you can plan on active or passive mitigation when/if you start using your basement for living space. Curious why the recommendation for re-inforcement in the slab, is the soil type in MA typically of low compressive strength? Commented Nov 3, 2023 at 5:17
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    Because reinforcement is dirt cheap compared to the overall cost of pouring concrete, if you bother to put it in when pouring, and makes the result considerably stronger so that it's less prone to crack for whatever reason in the future. Of course, you'll commonly see the result of not reenforcing, and not see the result of reenforcing, because the reenforcing is doing its job...
    – Ecnerwal
    Commented Nov 4, 2023 at 0:28
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Any half hearted attempt to "repair" the floor will eventually end up cracked again, in all the same spots as they move independently from one another. The only real chance would be to cut the cracks out cleanly with a concrete saw and try to dowel in rebar horizontally before filling back in. The problem I see is that it doesn't look thick enough to even attempt. Hard to say from just a picture.

Depending on the overall size of the room, it's probably best just to rent a small jackhammer and start fresh. If it's as thin as it looks, it should come apart fairly easily.

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    No argument that cracks will come back. But eventually might be 6 months, 6 years, or 60 years. It's quite clear that a lot of the damage shown has been left for decades with no attempt to fix it. And there generally should be little to no frost movement in a typical basement floor. Of course, if it's expansive clay, yup, might be significant movement in 6 months - and you'd find that out in 6 months...
    – Ecnerwal
    Commented Nov 3, 2023 at 3:11
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The good news is that you do not necessarily have to worry too much about future cracks in the concrete because I strongly recommend that you cover your floor with a tiny bit of wood. Nothing fancy, the cheapest 12/13mmm or 18mm plywood you can get is fine (as long as it is not warped in any way).

That way you will not ruin your chisel if you drop it on the floor, and whatever is underneath is no longer that important as long as it is flat.

In my workshop the floor was originally nowhere as bad as your case, just a bit uneven. Some of the leveling compound I used in one corner turned out to not bind/solidify properly and could be scratched into with hard objects/disintegrate into sand. But since was planning on laying a sheet of plywood on top it did not matter, as long as it is flat underneath.

So depending on how uneven your floor is you might even get away with filling in a bit of sand in the cracks before laying plywood on top.

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    Those look like some pretty significant changes in elevation. I doubt that plywood covering it would be able to level it (without significant support underneath the plywood to hold it level). If the OP just dropped some ply on top, it would end up being just as uneven as the concrete is now. Additionally, while protecting the chisel from the floor is a great idea, there's no guarantee that "workshop" == "wood shop". And, here in the US, 1/2" or 3/4" plywood isn't exactly cheap (though better than a couple of years ago). For that money, just fix the concrete, then protect tools.
    – FreeMan
    Commented Nov 3, 2023 at 15:48
  • After reading q, before reading a, this was my answer. BUT only if the room was not damp in any way, and with a membrane underneath for good measure. Part of my basement workshop also has an old carpet on top of that, making it very comfortable to work in. It does mean any welding or spark-making gets done elsewhere, but a small price to pay. A large angle grinder will make short work of any really high high spots first. And use 3/4" marine ply. Do it right, do it once.
    – Tim
    Commented Nov 4, 2023 at 11:14
  • It's very uneven, so that's the heart of what I'm looking for some guidance on. and as the other comment suggests, this basement is damp enough that I'd be worried about plywood directly on it. Once I solve the uneven-ness and holes, I will definitely consider some kind of layer over it though so I'm not continuing towork directly on the concrete. Commented Nov 5, 2023 at 23:21

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