Pouring concrete deck against sheathing?
A stampcrete deck is being poured around my new house tomorrow. The inner part of the deck is being supported by the top outside edge of the foundation and is going to be poured against the OSB sheathing which is now covered only by a layer of Tyvek.
My concern is that the moisture (from snow, rain, etc.) in the porous, sponge-like concrete slab (long-term) will not be kept away from the OSB sheathing sufficiently by the Tyvek to prevent rotting of the OSB. I would like to add a piece of 6 mil poly (say, 12″ wide) held against the Tyvek by Tyvek tape at a height a few inches above the finished slab level. This would separate the concrete and the Tyvek, and thus isolate the wood from the slab moisture. Any moisture coming down the outside of the Tyvek would be directed away from the OSB. The poly brick base flashing would then be taped above this and the brickwork done on top of the slab on up the wall. My builder says that Tyvek is waterproof and will keep the sheathing dry. I disagree. However, my builder is receptive to my wants, and will probably allow me to put the poly up if I push him and if I can do it before the pour. He feels that in his experience that they never use poly to separate wood and concrete.
My question – I there likely to be harm done by putting up the poly?
I know from 2 years’ research in the field of ICF construction that putting concrete directly up against untreated wood is an absolute no – no as far as long-term durability goes. This must also apply in conventional construction?
As someone repeatedly states in these forums “The Devil is in the details”. But if other builders out there routinely pour against Tyvek and sheathing like this then maybe I’m splitting hairs unnecessarily.
Anyone out there tonight have any comments for me? See anything I might not?
Thanks in advance – Brian.
Replies
Mark,
Around here NJ, all the masons that I've seen pour concrete up against wood puts a roll of aluminum flashing up first and then pours the concrete up against that , never seen anyone pour concrete in new construction up against wood befor. Old houses , yes. I've also never seen anyone pour concrete up against tyvek befor either.
Come to think of it. One job that I was on, the mason put snow and ice shield over the sheathing and then poured the concrete up against that. I'll have to find out what happened with that one.
Joe Carola
Edited 6/2/2003 10:44:29 PM ET by Framer
Hiya Joe, I'd never want to see it either.
Ice and snow shield (vycor) would be my choice for a situation like this.
Plastic wouldn't hurt but not as effective either since it's not slef healing around the nail/staple holes..
Excellence is its own reward!
Piffin,
How are you? Staying out of trouble? ;-)
What do you normally do or see it that situation? Like I said above, the masons around here always put flashing in first.
Joe Carola
Depends. There's trouble and then there's trouble...
The flashing works, for h emost part, but like said, the asphaltic based stuff works better. The impregnated 1/2" homasote is great because it gives the separation forom water/condensation and provides a better slip joint too. I thought that ELCIDs idea had merit. The moisture you are separating from with any of these materials is both ground water migrating through the concrete and condensation it attracts out of the air.
now, if it were my place, and I had to do something quick, and had no time to run to the lumberyard before they poured, I would tack up some plastic. That would be a wise choice anyway because I could run it three feet up the wall too, as masking, and protect it from spashes.
Not that concrete guys ever splash any material around...
;).
Excellence is its own reward!
It's funny about this. ICF manufacturers recommend 6 mil poly or galvanized steel. If that is good enough to protect entire wall/floor systems, shoot. Maybe in this case there is more exposure to weather? Anyway, it probably works as a minimum of protection.
Alternatively, we put treated pine on top of concrete all the time. Just the treatment is supposed to be enough protection. I kind of feel like the staple holes are moot to a good degree because they won't get enough oxygen to cause rot so poly should be okay.
But why take a chance, right?
It's pretty easy for us as builders to come up with small amounts of ice and water, left over from other jobs, but I can't see the homeowner buying a 65' roll to cut into strips though it is no doubt a good barrier.
Edited 6/3/2003 11:50:55 AM ET by Mad Dog
A strip of poly like you said, and then a strip of 1/4" foam.
Do it right, or do it twice.
Elcid's idea addresses two problems, moisture and motion. The foam will compress and let things move a little without building up big stresses, thereby reducing the probability of cracks.
-- J.S.
What about a srip of rubber roofing ,if you have any laying around . then the extruded foam.
I would put the extruded foan in in two pieces, with the top 1" just stuck on with some spray adhesive. After the pour has cured a couple of days go back and peel out the top layer of foam. Replace it with backer rod and seal the joint with PL 7 self leveling sealant. We use to do that with the old black asphalt impregnated expansion joint, but could never get it to stay put without nailing it, which made it a pain to dig out. Tried the low tack foam sray with foam board and it worked great. The important thing is to create not just an expansion/slip joint, but also one that can be properly sealed.
Dave
Epilogue - Based on your suggestions here's what I did.
Got up two hours earlier the next morning, got permission by phone from the builder for me to put up the poly. (The "extra" charges have been piling up). I just happen to have a roll of 4 mil X 12" wide poly and some 1/8" thick foam sheets on hand (not ideal, but instantly available). Started at a point where I expected the pour to begin in case they arrived to pour before I was done. Stapled the foam over the Tyvek just up to but not covering the chalk line they had placed to indicate the top of the slab. Then stapled the poly up over top of the foam, then sealed the top edge of the poly (over the staples) to the Tyvek with red sheathing tape. The bottom of the poly rests on the inside corner of the angle iron which supports the pan.
There is 96 linear feet of house wall in contact with the deck. Part of it is over a cold room, and the rest will be lying on a corrugated steel pan with 6x6 mesh sitting in it, so there really isn't a lot of space now to place anything thicker between the ends of the mesh (or the raised corrugations) and the wall.
Other than snow melt there should not be significant amounts of water on the deck as there is a roof over 3/4 of it. Also, the brick/stone will be placed on top of the deck over a poly base flashing which will be taped to the Tyvek and the brick/stone will continue on up.
Here's the beauty of the whole thing... As of Wednesday (the pour was supposed to be Monday a.m. - weather was perfect for it) they still haven't poured and the weather has now deteriorated so rain will render the area inaccessible to cement trucks. And so we wait.
Thankyou for all of the the valuable responses. Without them I would not have been able to get this detail right.
- Brian.
ps. to DAVERICHESON (RE: post 14) - I've also seen that technique done with asphalt-impregnated type expansion joints, where the top 3/4" or so is cut with a saw kerf about 3/4 of the way through. After the concrete is set, the top piece is torn up at the kerf line, and the space is properly sealed up. It's a nice detail.
Stick with the old method, using an asphalt based separator, in this case the best available is the Ice Guard!
I will not add to the plastic/tyvek/icedam thoughts, but one thing that comes to mind is green plywood here instead of OSB, along with whatever barrier you choose. But it's too late for that much change, I guess.
At a minimum, if they are looking for a way to try to avoid the effects of stupid planning, the isolation membrane should have roughly the same life expectency as the components on both side (concrete/osb)
Whoever planned this should be held back.
_______________________
10 .... I have laid the foundation like an expert builder. Now others are building on it. But whoever is building on this foundation must be very careful.
11 For no one can lay any other foundation than the one we already have--Jesus Christ.
1 Corinthians 3:10-11