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Old 04-01-2012, 05:08 PM   #1
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Concrete Porch Slab-Support plywood


We have poured cement walls under our porch with one inch of plywood going across the top of the walls, forming the ceiling, that supports the concrete slab of the porch. The house was built in 2004 old and the plywood under the slab is flaking off and has a coat of mold on it. What do we need to do: can we rip out that plywood, do we need to leave it up? What about the wood between the walls & the slab? Looking for suggestions

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Old 04-01-2012, 07:51 PM   #2
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It should have been ripped out after the pour. BTW, if that space is a cold room, it should have a exterior door to close it off from the rest of the basement space. As for what is between the slab and walls, spray down in a pump garden sprayer a treatment of Water treatment, along with termite treatment, or brush on Polyurethane to seal, if you are not able to chisel out, if it goes too far across.

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Old 04-01-2012, 08:19 PM   #3
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Since you can remove the plywood I would guess there is a door or some access. Where is the access from, outside, or cellar? Is it a crawl space or full cellar? There must be moisture or humidity causing the mold. is the floor dirt? You should be able to remove plywood. I can't imagine it is structural. How thick is the slab and is it reinforced? By your description there is plywood between walls and the slab you cannot remove. If so you would have to seal that inside and out to prevent it from rotting out, letting the slab settle. If it is a dirt floor a heavy layer of polyethylene for a vapor barrier would help.
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Old 04-01-2012, 10:47 PM   #4
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Concrete Porch Slab-Support plywood


Thanks for the input! It is greatly appreciated. The door to the area under the porch leads into the basement. Right now it is just a four foot opening. It is a nice area with the concrete walls and floor, just have this plywood below the porch slab as the ceiling. We think the slab could be eight inches thick and have no idea about the reinforcement. We are thinking of using a circular saw to cut between the metal joints to be able to pry the plywood off of the ceiling. Below are a few images. Do you have any other suggestions?




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Old 04-01-2012, 10:51 PM   #5
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Can you resize the photos and repost please? Problem that I see, unless you use Steel shims as you remove the plywood, the slab is going to start cracking as you remove it, then you have bigger problems on your hand, and possible death to the person standing under it.
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Old 04-01-2012, 11:01 PM   #6
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Got them resized. What do you mean by steal shims. New supports under the concrete to keep it up? or place metal between the plywood and concrete to press against?

As you can see in the first image the plywood doesn't extend very far over the wall. (At the plywood joint the corner gone and you see the edge.) The Second one you can see some of the mold on the wall in the corner. The third picture has form joints and a little of the mold with the black plastic in the background of the joint, I think. We were thinking of running the circular saw along side those and pulling the plywood down, but you are suggesting we don't do that? The fourth picture shows the plywood pealing apart.

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Old 04-01-2012, 11:14 PM   #7
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That's a new one on me. I can not imagine someone even having done it this way.
Rule #1 in building is wood should never be in direct contact with concrete unless it's pressure treated.
Anytime I've seen a porch built like this it's pored over steel panels. Even then at some point it rust and starts leaking.
Your between a rock and a hard place. The plywood needs to come out but once it's out there's nothing left to support the concrete above.
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Old 04-02-2012, 02:58 PM   #8
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Is the plywood exposed around the outside perimeter of the slab? If not, I think the first thing I would do is to cut out a small piece of the plywood and see if it runs the full depth of the wall, or if it stops just past the inside lip of the wall. How thick are the walls supporting the slab?
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Old 04-02-2012, 08:52 PM   #9
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That's not just plywood, it's actually 1 1/8" plywood forms that are meant to be re-used. They're likely either "Duraforms" or "Advance" wall forms that have either seen better days, or the contractor simply didn't mind sacrificing a few of them...........

The steel "shims" that are being referred to are the bands of the form which add rigidity to the form, as well as contain the hardware that holds the forms together. The bands are about 2.5" wide and about 1/4" thick, and are centered on the tie openings you see below. If you cut anywhere between the bands, you'll only need ot cut through the wood. The issue I see though is that the hex-bolts might be impregnated (ie:stuck!) in the concrete, making it extremely difficult to remove the bands. You may luck out though, as it appears there may be tarpaper, or some other black membrane, layed directly over the forms.

I can't tell you how many times I've used these style forms to form a ceiling pour (I'm doing it again later this week) for a porch/pool equip. room/holding tank/etc., but I've never laid them with the outside upward....
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Old 04-02-2012, 10:41 PM   #10
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Originally Posted by jomama45 View Post
That's not just plywood, it's actually 1 1/8" plywood forms that are meant to be re-used. They're likely either "Duraforms" or "Advance" wall forms that have either seen better days, or the contractor simply didn't mind sacrificing a few of them...........
I thought they looked familiar! I've certainly never seen them used that way and really wonder about the long-term prognosis for this slab. Obviously the metal parts of the forms are going to rust over time in that's going to create some awful good places for the slab to start cracking. I really wonder what the best course of action is for this.
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Old 04-03-2012, 11:56 AM   #11
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Is the plywood exposed around the outside perimeter of the slab? If not, I think the first thing I would do is to cut out a small piece of the plywood and see if it runs the full depth of the wall, or if it stops just past the inside lip of the wall. How thick are the walls supporting the slab?
No the plywood is not exposed to the outside perimeter of the slab. The slab it self I think goes over the edge of the wall and hangs down over the edge of the wall a little bit. The plywood does just hang on the edge of the wall. I can see the end of the plywood in a couple places. I think the guys commented after you are correct. I think the plywood are cement forms that are approximately one and an eighth inch thick. The walls going into the room are ten inch walls, I would not think the other walls would be any different.
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Old 04-03-2012, 12:02 PM   #12
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I really wonder what the best course of action is for this.
That is our question!
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Old 04-03-2012, 12:04 PM   #13
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That is our question!
The correct course of action for this is obvious, take it out and start over again to prevent long-term problems. Barring that the question is what's the most economical and effective method of correction?
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Old 04-03-2012, 02:44 PM   #14
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I'll bet if you remove the temporary studs & top plate shown in the second picture you'll be able to wiggle the forms out w/o even cutting them. Logic, and being in this trade for years, tells me the concrete contractor wouldn't form the underside without a good, easy way to strip the forms out, seeing as those forms are likely their lively-hood.

The pealing faces of the forms you see is extremely common as well, they always did that after a ton of pours, and especially when they don't get enough form release on them.

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