Opinion on crack in concrete subfloor?

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New Member
Aug 14, 2017
I'm looking to replace the flooring in my rec room on my first floor.
While doing that, I found a crack in my concrete subfloor, and I'm pretty concerned... Online, I've read that hairline cracks are normal, since concrete shrinks when it dries but this is no hairline crack.

The floor on either side of the crack is flat, but it does stretch all the way across the room. It looks to be 4mm large at the largest point, and it stretches in one direction. At the deepest point I was able to measure, the crack was 4cm deep. From the outside, right outside the door, there is no visible crack in the foundation. The room is about 10ft by 22ft. I don't see any other cracks. (so the crack stretches about 10ft)

The townhouse is about 20 years old, 3 stories high, and an earthquake has rocked the region maybe 5 or 6 years ago. The crack might be from that.
Is the crack a cause for concern? Does this need to be repaired? Sealed?
Thanks for any advice you can give.




Engineered hardwood will go here.

I plan on putting down underlayment, and then floating the click lock stuff.

I'm just worried that if I do have to fix the concrete, I would rather do it now than tear up any additional flooring I put on top of it.
Cracks are really not a problem if your floating a wood floor. Its the ledging from from one side to the other that's the biggest problem. It really looks like a shrinkage crack we see out here in the southwest everyday.

Anything larger than 3/8 of an inch can be called a structural defect in the slab and could possibly continue to move. On a glue down that could be a potential problem. In which case you can pin the crack with rebar and epoxy.

If it is flat across the crack what I do is simply shoot some foam sealant in there and scrape off the excess. If it's ledged the I grind then use sealant. If you use patch and that thing shrinks it can possibly push up the patch.

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