Double plywood base/Wood floor install
Hello All,
Will be laying wood floors (5″x11/16″) in a 12 x12 room over concrete slab.
It was reccommended by the supplier to prepare as follows:
Plastic vapor barrier- double 5/8″ plywood, with top layer set on a diagonal and screwed together.
Is anyone familar with this technique, that may be able to give me some insight on how to layout the diagonal plywood over the first layer?
I am assuming the first layer of plywood is cut to fit the entire floor and just laid flat, correct? Should this be T&G or just butt up together? Gap?
I’m not sure exactly how to do the diagonal part. Also what would be the preferred screw/size?
I was also wondering how to deal with the sheets of ply that may not be real flat. Will screwing the second layer to it true it up if I run into this situation, I’m kinda concerned about that.
Appreciate any insight.
Mick
Replies
Hey Mick:
I just posted regarding the same thing, sort of.
There was an article in FHB in 1994 by Don Bollinger
Search hardood over radiant slab and it will come up.
He details it pretty well but I my post to everyone was asking if they had done it and what success they had and what to be wary of!
Have you recently done this Jeffin?
Re. "have I done this"
No Mick, I have not.
I have a laminated hardwood floor glued down on gypcrete and have had no issues for the 8 years i have walked on it, but never sleepers before.
I am curious about it as well.
Jeff
Mick, did a basement floor similar to this 2 years ago now. Is still looking good. What we did was start with a dimpled plastic product made by Schluter ( I think, I can check my file and see.) All the seams get taped. First layer of plywood was 5/8, second layer was 1/2. Seams were staggered half way in both directions, screwed together about 8 " on center. We then installed concrete screws about 16" on center. 3/4x3 1/4 quarter sawn white oak went down over felt. This floor had no water issues. Foundation was dug out and waterproofed, so I was comfortable putting hardwood down this way.
Rich
Rich,
I had planned on using 1 large sheet of plastic to cover the entire floor so I won't have any seams to tape, but I was curious if I should tape the perimeter?
How exactly did you lay the sheets of ply? is the top layer laid in the opposite direction of the bottom? I'm a little confused with how to lay the top layer on a diagonal as was mentioned to me.
Thanks for input
You sure the guy giving you instructions did not mean the second layer of ply is perpendicular to the first layer? As someone else pointed out, just make sure the joints are staggered and not lining up with the layer below.
I had used the heavy dimpled plastic product by schluter which raises the plywood above the slab by 1/4 to 3/8 inch. This would keep the ply out of any minor water intrusion or leaks. I didn't tape the edges. Seems fine this way. The ply was not run diagonal, just offset 4 feet in the 8 foot direction and 2 feet in the 4 foot direction.
I think diagonal would be fine. If this is what the architect specified then do it this way. Lay down your first layer. Snap 45 degree layout lines and start with the second layer. Figure your layout to minimize waste, and screw together well. IIRC we had to countersink for the screw heads so they wouldn't spin before pulling in flush.
I had thought at first the guy from Lumber Liq. had meant to lay the ply perpindicular also, but he specified diagonal. Do you think one way is better than another as far as perp, diagonal or just staggered seams?
As I mentioned in the original post, does anyone think that any "bowed" sheets of ply will be a problem, or will the double layers "true" them up? Supplier mentioned "relief" cuts in the back of the ply, any thoughts on that?
Diagonal is probably more stable. But remember I attached with concrete screws.
You just fastened the first layer of ply with the comcrete screws and then screwed the top layer to the underneath layer correct? How do you install concrete screws? Do you drill a hole and they self tap? What size did you use?
Concrete screws went through both layers ply and the plastic, they were probably 2 1/2 inch. They get hammer drilled with the correct size masonry bit and ply countersunk. Run them in but not so tight as to crush the plastic.
I checked my file the membrane was Cosella Dorken Delta FL
Rich
Rich,
Those cement screws, are they the blue coated ones that you would find on the shelf at the Depot?
Yes Mick, those are the ones, good luck.
Gotcha'
On a side note to this job, I need to build a kneewall on top of the slab, would these concrete screws be a good choice for attaching the bottom 2x4 to the concrete?
The easiest way to install plates is with a PAF, (powder actuated fastener) . If you don't own one, it could be rented, but for one wall, you won't save anything. Screws will work well.
Rich,
I was wondering what size screws I should use to secure the top layer of 5/8" ply to the bottom layer. Being that I will have 1 1/4" between the two layers, if I countersink the screws the 1 1/4" screws will be to long and I wouldn't imagine a 1" screw would be enough, would it? What would be the recommended screw type? Would drywall screws do it?
I think you'll have to try each. If the 1 inch screws hold tight without spinning, they're ok. If they spin easily then go 1 1/4 inch. Try to get galvanized, or ceramic coated. Drywall is last choice. EDIT; on second thought, use the longer screw, set flush. 1 inch are going to be too short.
Rich
Edited 2/19/2008 5:36 pm ET by rlrefalo
Thanks
I picked up 1 1/4" coated ext. screws for the plywood. When I hold up the screw to the double sheets of ply, the point extends just a bit below. I don't see how I can use these as they will not sit flush. I've looked up and down for a smaller acceptable screw and all I see is 1" and only for drywall. I thought about countersinking and going with a 1" screw (would this be a good method?) but can't find a 1" other than drwall screw. Any suggestions?
Try the 1 inch screws with countersink. Don't predrill, just a countersink. As long as they dont spin they should be ok.
That's what I figured I would do. I can't locate any 1" screws in HD or Lowes other than drywall, any idea? Thanks for your input.
Go to swansecure.com , they have SS screws, but big $$$$$.
Ufortunately I need them now. Thanks for link, will save for future use.
You should read the article you don't need to screw to the conc. just glue and screw to each other. Humity does have it's place here too, speaking from experience. Read the article and learn more. Good luck!
unless you are mechanically fastening the new floor to the plywood, why even use p.w. substrate? I would just install a layer of rigid insulation over the slab. This will give you a warmer floor that wo`nt be affected by moisture as much as plywood.Also, in some areas , only treated wood is allowed to cotact concrete by code. Good Luck!