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Series of holes in periphery of basement slab

19397 Views 4 Replies 2 Participants Last post by  Vincer
Hello all,

A quick summary then a couple of questions:

During the heavy March rains and melt in Toronto, my basement floor was wet. Lifting/removing the carpet and underlay revealed a layer of what I think is a thinset mortar improperly laid in an attempt to create a level floor. After a few days of running air movers the thinset has pretty much completely delaminated from the original slab, and can now be peeled away using a flatbar (heck, even my hands). I understand enough to know that without first abrading then applying a bonding agent, any type of concrete product will not adhere to an existing concrete slab.

As I was removing the thinnest, I was surprised (and confused) to find a regular series of ½" diameter holes drilled through the floor. These holes were bored after the the thinset was poured, as I can see that the underlay adhesive was acting as skim over the hole(s). The holes are exactly 8" from the perimeter of the foundation walls, and vary randomly from 12-24" on-center. It's almost like someone snapped chalk lines parallel to the foundation, then walked along with with a hammer drill and a ½" masonry bit. The holes are completely bored through the the slab (wire coat-hanger probes measure a 3-4" slab depth). I can shove the wire further in the soil below. Now that the slab is revealed, I find a decent concrete floor (sloped towards the floor drain (which was hidden under vinyl-roll-type flooring)): This slab was not floated smooth, but has generally decent slope, no cracks (happy surprise), and no signs of spalling or staining.

My Questions:
1-Why would someone drill a series holes through a slab into the ground below? My guess is that a previous owner (we've owned the place for 3 years) must have experienced a flood and drilled the holes as 'indoor weepers'. My problem with this is the regularity with which the holes were drilled - If the water table rises, you're just going to get more water back into the basement. I don't think they would have been for stud-wall anchors (8" offset for a 13-foot wide house would really reduce floor space, and you'd think they would not drill all the way through the slab).

2-Would it be appropriate to fill these holes with hydraulic cement compound? I'm thinking of either a King swift-set or plug-tite product.

Any suggestions as to why the holes are there, and how (or not) to plug them is greatly appreciated. Thank you.
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Thank you and spot on! When we got closed on the place it came with a termite-free certification from a local company (as an update from your Mississauga days - termites have made it to Toronto, and a quite a pest in the east end/beaches (where we are)). Since the house is a semi-detached (with 24" separation between the neighouring property) the exterminators treat for subterranean termites from the inside, boring holes, well, exactly like a described. Just found it on their site. Now I can contact them to see if I can't leverage a freebie or discount or the re-treatment since they didn't plug/cement the holes (again, like promised on their site). You've solved a mystery and given me chance to add extra protection against pests before I seal (or they seal) the holes, and that is a real kindness!

Your guidance is greatly appreciated! Thank you again.
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