Plastic needed under foundation slab?

[dbhansen]

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!


Daren

[Gareth]

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

Gareth

[CanuckJim]

Re: Plastic needed under foundation slab?

DB,

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.

Jim

[dbhansen]

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]

Re: Plastic needed under foundation slab?

Daren, what do you plan to backfill those footings with? Does it matter?

[dbhansen]

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]

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]

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]

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]

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.

Daren

[mfiore]

Re: Plastic needed under foundation slab?

Can I assume this is for a 42 inch oven? Did you use 8x8x16 inch blocks throughout?

[dbhansen]

Re: Plastic needed under foundation slab?

Yes, 42" oven. I used one row of 4x8x16" (half-height) blocks in the below-grade foundation wall, but otherwise they were all 8x8x16".

[mfiore]

Re: Plastic needed under foundation slab?

OK, Daren. Now that I've started digging, I have even more questions. Your frost-line is 42-48 inches. If I read the numbers above correctly, you excavated down to 59 inches. Is that needed for some reason, or just extra precaution?

If your footing is 96x88 inches, the foundation ends up 88x80 inches (is that correct? Is that the size you plan to pour for the cement slab on grade?

[dbhansen]

Re: Plastic needed under foundation slab?

Mike, I just saw your picture - you're going fast! By the time I fix my mistakes, you'll be way ahead of me and I'll be asking YOU the questions!

I didn't base the 59" on anything scientific, and I probably should have eliminated at least the half-hi block, if not more. I just sort of made it up as I went along, making sure I would get to or below 48" down and reach the proper height without having to cut any blocks. You'd probably be good with only 4 rows of 8x8x16 blocks (maybe 5) + the footer. I just know that it DOES sometimes freeze to 48" here and I read that the footer should be below the frost line, so I went with that. Or maybe I'm just crazy about digging in clay!

Yes, with a 96x88" footer, my wall is 88x80" and my foundation slab will also be 88x80", which is close to the 86x77" recommendation in the FB plans.

Happy digging!