Moisture Control for Tapcon Screw Penetrations in a Thin Slab
Hello everyone,
I have a home built in 1930 and I am finishing the basement. The existing slab is not insulated and there is no vapor barrier below the slab. The slab is also very thin – in some places 3/4″ which I learned when having an interior drain and sump pump installed. The basement is dry – I just wanted to install the perimeter drain and sump pump before finishing the basement to cover my bases. I am planning on adding either 1) Dricore or 2) 1″ of EPS + plywood or OSB + thinset and tile for the floors. I understand it’s recommended to fasten the Dricore or OSB through the insulation with Tapcon screw sto the slab to make for a secure flooring substrate. However, I am concerned that the Tapcons will fully penetrate the slab and create moisture issues. Is this a valid concern? Would adding 2 layers of 1/2″ plywood/osb on top of the EPS suffice in providing a solid substrate without anchoring to the slab? And would this require a perfectly level slab (which mine is not)? Any recommendation or advice is much appreciated and thank you.
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Replies
You could squirt a little silicone or urethane caulk into the hole prior to driving the tapcon. The caulk will seal the hole. I’ve often done this outdoors to prevent the hole from accumulating any water where freezing might be a concern.
Bill
Thank you Bill. I'd be drilling the screw from above the osb/plywood so I won't be able to access to fill any holes in the thin slab. Any recommendations?
Smear the sealant on the tapcon before putting it in the hole. Put a relatively heavy layer. Some will ooze out onto the finished surface though, so be aware of that. Since you're not trying for a "pretty" job here, you just want the hole sealed up, you don't need to try for precision.
Note that I doubt you'll get much leakage around the fastener in your application anyway, so this is a belt and suspenders sort of thing. When I seal these holes outdoors, it's usually with things mounted onto slabs, usually using some type of sleeve anchor that has extra room in the hole. Water can get in, freeze, and fracture the concrete over time so that's why I like to seal those holes. The sealant in this case isn't as much to keep out water as it is to make it so that water has no place to go -- the sealant fills up the empty space in the hole around the anchor.
Bill
Thank you Bill. I just heard back from Tapcon and they require 4" min. thickness of concrete to drill into so I'm shying away from screwing into my slab, especially since Dricore's recommendation is 5 screws per panel. But this is a good suggestion if I ever need to.
That's only necassary to meet their pullout specs (and I can't see the short screws needing 4" of concrete to bite into). If you only need the screws to keep some flooring from sliding around, you don't really need to worry about it -- you're not really putting much load on the tapcons in this application since they're just pinning the floor together so that it won't slide around.
I'd be more concerned with a hammer drill cracking the thin slab than I would any issue with the tapcons themselves. I can't see how a 3/4" slab will hold up very well, even if they used the reinforced fiber cement. Usually 4" minimum thickness is standard for a slab. A lot of outdoor stuff is probably more around 3.5" if done with 2x4 forms though.
Bill
I think it's usually fine to do two layers of plywood or OSB on top of EPS, with the layers screwed together and not screws into the concrete. Extras that could improve chances of success include a vapor barrier on top of the slab, under the foam, heavier tongue-and-groove OSB such as 3/4 advantec, and perhaps foam glue between layers.
Why only 1" of EPS? Would you loose too much headroom with two?
Correct, I don't have much headroom. If I float the floor with two layers of plywood or osb, does the concrete floor need to be 100% level using self leveling? I really only want to level locally where needed, and am okay if it's not completely level. I was thinking of using Schluter's Dietra uncoupling membrane on top of the plywood or osb. Suggestions welcome, thank you!
Why did the slab end up being so thin? I can't imagine 3/4" concrete not cracking when a Tapcon screw is driven through it.
Great question - I have no idea why they made such a thin slab! It's an old home built in 1930. I leaning towards doing a floating floor without tapping into the slab.
Have you considered replacing the slab? I’m in a similar boat. What is your foundation construction?
I thought about it for two seconds before deciding it's not worth it in my situation. Replacing the slab would be too much money and trouble. We also living in a rocky area and already have some rock outcroppings in our basement, I feel I may run into trouble there too.
Hi all, just wrapping up this conversation as I found a solution. Wedi board can be adhered directly to the slab using modified thinset - this provides 4.3 R value per inch and is also rigid enough to hold tile. No need to screw down to the slab. I just spoke with a Wedi board rep and they confirmed this application. Also no OSB or plywood (organic material) needed in the basement! Win-win. Hope this helps anyone else with this issue.
Interesting solution. Thanks for sharing.
I just wanted to chime in on the slab itself...what I think you might have there is more of a "rat slab" than a proper basement floor. A rat slab is often just an inch thick or whatever they have left in the mixer since it's only there to block bugs and rats from burrowing up into your basement. They're also not known for being perfectly flat or well finished.
Sounds about right, thanks for sharing Andy.