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dbhansen 04-11-2008 09:05 AM

Plastic needed under foundation slab?
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Is it necessary to put plastic sheeting under a foundation slab that sits on top of a wall going below the frost line? See attached pic. Is the point of the plastic to keep water from wicking INTO the slab, or the other way around? I expect the ground will stay dry under my foundation. Can I just pour the concrete right onto a 4-6" layer of stone, or would plastic help it cure? I would just use the plastic regardless, but a roll of 6 mil sheeting costs $24 at Lowes, and that can buy a lot of flour!

But now that I look at my own picture, I see that that half-height row of blocks (just under the top row) will let water in....D'oh!! Live and learn. Maybe I do need to put some plastic and rigid insulation under there after all (I shoulda' gone with the solid blocks!). Suggestions appreciated.

FOLLOW-UP: I confirmed with the block manufacturer that this foundation should be perfectly fine as long as I fill the holes in the half-hi block with mortar to keep the dirt and water out. Stupid rookie mistake on my part; I'll be sure to use a solid block on my next oven!


Gareth 04-11-2008 02:30 PM

Re: Plastic needed under foundation slab?
those half blocks could present a problem in the frost line, if water/mud get in there and freeze they will break your footing. Do what you can to seal those. There are many experienced on this sight that may have a solution on how. Good luck


CanuckJim 04-11-2008 06:23 PM

Re: Plastic needed under foundation slab?

Stuff the exposed half brick cavities with used mortar bags, fill with mortar. Parge all the blocks with 3:1 brick sand and Type N. Coat with foundation coating (black) below grade; you're away. Cheap solution that will work.


dbhansen 04-11-2008 06:56 PM

Re: Plastic needed under foundation slab?
Ugh, I feel like such a fool. Thanks for the help, guys. This doesn't bode well for my ability to build an oven! Unfortunately, I took that picture last fall before I filled all the dirt back in. Time to get out my shovel....

mfiore 04-14-2008 08:30 PM

Re: Plastic needed under foundation slab?
Daren, what do you plan to backfill those footings with? Does it matter?

dbhansen 04-15-2008 07:23 AM

Re: Plastic needed under foundation slab?
Mike, I put the dirt back in. Maybe gravel would have been better for drainage, but no one suggested that to me, and I really had nowhere else to put the dirt. But now I'm going to dig down a bit and seal up those horizontal holes in the bricks before I proceed with the foundation slab. If I could do it all over again, I'd probably paint the whole wall with a foundation sealer (and use a solid half-height block!). I intend to put a 4-6" layer of gravel under the slab.

mfiore 04-15-2008 08:10 AM

Re: Plastic needed under foundation slab?
I see that early on, you solicited advice about the foundation. What did you end up doing? Did you put gravel or crushed stone underneath the foundation or just put it directly in the trench?

dbhansen 04-15-2008 08:57 AM

Re: Plastic needed under foundation slab?
On the advice of a local mason, I put the cement footing right into the clay. I sprinkled some sand in the bottom so the wet clay wouldn't stick to my shoes, but that's it. The clay was very solid, moist, and easy to make level with a flat-blade shovel (not too easy to dig, though!).

Just fyi ... I dug down about 51" below grade, installed a 2x4" form, and then dug down another 8.5" from there. The footing is 16" wide by 12" deep, and measures 96x88" on the outside; 1.25 yards of cement. The 44" wall sits in the center of the footing. The top of the wall is about 3.5" below grade, so the top of my 5.5" slab will be about 2" above grade. All that (and more to go) before even starting the foundation slab! Us northerners have it rough!!

mfiore 04-15-2008 09:26 AM

Re: Plastic needed under foundation slab?
Thanks for the info! You've already done a great deal of the work for me! (except that whole shoveling 6 feet of dirt part). Did you have the cement poured or do it yourself? How did you get your 2x4 form stay in place 8.5 inches above the bottom? Did you tie in the cement footing with the block wall using rebar?

dbhansen 04-15-2008 10:47 AM

Re: Plastic needed under foundation slab?
Glad to help! I had the cement delivered to the street and I hauled it back in wheelbarrows. The neighbors helped so it only took 20 minutes or so. The added cost (compared to mixing it myself) was low. I was going to mix the foundation myself (the next step), but it looks like that too will only cost $10-20 extra for delivery vs. DIY.

After getting my trench nice and level (using a square grid of strings placed 12" above grade), I laid down the forms and pounded in 1x4" stakes behind them and attached the stakes to the forms with screws from the inside (if you go this route, be sure to pound the stakes below the top of the forms so you can run the screeding tool over the top). I then dug down in line with the inside edge of the 2x4s, so they were always resting on undisturbed dirt. Even though there was eventually only about 2-3" of dirt holding the stakes in place laterally, they stayed just fine and the forms never moved.

I did not tie the wall into the footing with rebar. The folks I talked to seemed to think it was overkill, except in places with earthquakes.


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