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Old 04-24-2010, 10:39 AM   #1
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Gap between slab and wall

I am in the process of preparing to finish a space in the basment... along with sealing off the rim joists from the outside, I was looking at the floor... I noticed the poured slab as about a 1/4" gap along the wall the slab never seems to touch the cinder block wall... Is this from shrinkage? i dont think its french drains because wouldn't that be larger then a fourth an inch?... can this be sealed up? I think bugs are crawling though it... or at least down into it to hide... if it can be sealed... can you just use silicon caulk? or what would be best? thanks


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Old 04-24-2010, 12:04 PM   #2
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The gap is almost certainly unrelated to a French drain, as you noted it is too narrow. It is almost certainly not from shrinkage, shrinkage typically produces hairline cracks, not a straight line, and certainly not immediately adjacent to the block. The gap is most probably there as a crack control joint, and may not extend all the way through the concrete. Check it with a probe, a coat hanger would work well.

If the joint does not extend full depth, bugs are probably not crawling through it, and you can leave it alone. If it does extend full depth, and you wish to caulk it, make sure you use a flexible caulking like silicone, since the width of the joint probably varies seasonally, and you don't want to damage either the caulk or the concrete as the joint moves.


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Old 04-24-2010, 03:38 PM   #3
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I tried to push a 12 gauge ground wire down into the crack and I can push it almost 7 inches deep before it stops and pulling it up it has some dirt on the end... must go all the way down
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Old 04-24-2010, 07:37 PM   #4
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If your poking down to dirt then yes the bugs can get in through. Don’t try to fill the entire 7" gap with caulk. Stuff a backing down there first. You can get ’ to ” round foam backer at any good hardware store, comes in a package 20-25’. Stuff that down there so all your caulking is a ” x ” gap. I’d use a polyurethane caulk like Vulkem or Quad, they will stick to concrete way better than silicone.
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Old 04-25-2010, 06:31 AM   #5
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its a version of a ' slot drain ' & was formed when someone plac'd the floor then ' pull'd ' the ( form/foam/board/isolation jnt) which had been placed against the wall,,, in theory, any infiltrating water from the wall's supposed to enter that slot & drain out below floor,,, never saw it work like that, tho.

backer rod & sealant ( silicone-polysulfide-polyurethane ) [ NO CAULK! ] as suggested.

in theory, there's no difference 'tween theory & practice,,, in practice, there is !
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