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Hello-
I was just asked to bid on a 4,000 sq ft oak floor to install over a concrete slab. Havent seen all the specs yet, but it sounds like the architect is specifying 3/4″ plywood to be screwed down over the concrete. The oak floor itself I can handle, but I’ve never installed one over concrete before. I have the option of also installing the plywood and am wondering if anyone else has done this before. What kind of screw pattern do you use? Is this something that can be done with an ordinary rotary hammer, or is there something that will work better? Any info would be appreciated.
Thanks, Mike.
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If the slab is on the ground you'll need to first lay down 6 mil plastic as a moisture barrier. Overlap the seams by at least 6".
Personally I would use 2 x 3 furring strips on 16" centers instead of plywood so I could shim any low areas. You could use 1 x 3s if the extra 3/4" of build up is an issue but they're not nearly as rigid and not realy adequet for T&G flooring.
To secure them you can shoot 3-1/4" powder loads which is quick but some of them won't hold. Alternatively you could fasten with 3" tapcons or 2-1/2" drive pins, either of which require pre-drilling.
If you're going to use plywood then use 3/4" CDX and be sure to leave about 1/8" gap between the sheets to allow for expansion. Also leave 1/2" gap at walls, protrusions, etc. Fasten it with 1-1/2" flat head tapcons or drive pins. The later would be my choice.
Also, I hope you're going to leave 1/2" gap between the new flooring and the wall, to be covered by the baseboard and baseshoe later. Don't think the floor won't buckle if you don't.
*Good suggestions Ted:There are a variety of methods and yours is what I've been accustomed to but in addition I use 15# felt on top of the visqueen overlapped 6". In the past I've used either 2" tapcons at a rate of 24 per sheet of CDX, or predill and attach with concrete cut nails.Also I've heard the use of two layers of 1/2" cdx installed perpendicluar to one another and attached together as a floating subfloor with the same mositure barriers. This way you won't have any moisture barrier punctures when attaching to the slab. Additionally you'll be able to use the 2" powercleat nail or staple which will hold much better.Personally I haven't seen the plywood on screed method(vertical height always becomes an issue) used for a few years but I know of a few builders that subscribe to that method which is one of the initial recommendations by the National Wood Flooring Assoc(woodfloors.org) or Nat'l Oak Flooring Man. Assoc(nofma.org).
*Mike,in the event that this concrete slab is either suspended and doesn't need a VB or has had a VB installed prior to pouring, I would make the following comments.You can use the same adhesive used for parquet flooring over concrete and glue the hell out of it as you lay the sheets down and this method also uses less tapcon screws and does a permanent plywood to concrete installation.Then you nail the hardwood down with proper length staples.A layer of paper between the hardwood and plywood is also recommended.Just another way......Gabe
*man, i wouldn't use plastic. mastic and felt, mastic and felt. be safe. easier to do it right the first time. see post in breaktime.
*Thanks for the info everyone. The latest word on the project is that the concrete crew will install 2 x 4 sleepers (2' o.c.)flush with the concrete that we can fasten the plywood down to. Anyone ever heard of this method before? Sounds like it would be easier for us, as far as fastening the plywood goes, but I would think that it will be a headache for the cement crew.
*Gabe,I don't want to hijack a thread here, but you bring up something I've wondered about. If I want to install hardwood over a new slab with a good VB and good drainage underneath, should I put another VB on top? If the slab is a month or two old, where will the moisture in the slab go?Jerry
*You're right, it will be a headache for them and since they will be long gone before the floor goes down, I wouldn't want to bet on the accuracy of their work. Unless they are getting paid for the extra time it will take to place those boards extremely well, you'll have a lot of time on your knees leveling it. I'd definitely be on hand to help out and be quality control.I tried it that way once on a little room as an experiment and really regretted it. I still think the idea is sound but the rest of the pour happened to be one of those less-than smooth ones (the concrete pumper's pump acted up so the pump man was watering down the mix without telling me!)and I didn't have the time to set the sleepers very accurately. Oh man I was going nuts shimmin' for hours.
*Teo:VERY good point! I can see the problems already. John: Your method sounds good too as it never hurts to overkill. Gabe: I've seen that method as well with the use of Bostiks best and a deep notch trowel such as 1/4X 1/4X 1/4"
*Jerry,This is a project I would delay for a while. Run a dehumidifier in your basement and try to dry it out as best you can, first. Normally, I wouldn't want to install any wooden floor in a basement until at least a year of drying time.If you already have a VB under the slab, a second one on the top is not recommended.Gabe