I live near Toronto, Ontario, Canada. I have a victorian home - 1876, recently renovated by previous owners.
Hydronic radiant heat was installed - I believe gycrete was used, no sleepers.

I want to install an orginal style hardwood (thin strip oak floors, quartersawn mostly).

I will be using plywood as the underlayment, and these are my questions:

1. Should I use CDX or OSB?

2. Should I glue the plywood to the gypcrete (I understand gypcrete doesn't take glue very well), or "float" it?

3. CAN I float the underlayment? I read that two layers of 1/2" OSB alternated direction and screwed together can be layed loose, with 'H' clips at the seams, then the hardwood can be installed with serated nails like they used to use.

4. Will the plywood move up and down if installed like this and assuming the gypcrete is flat? Will I get squeek?

I believe I'd essentially be creating my own engineered floor using this manner. I'm just not sure about floating the underlayment. I want the R-value of my floor to be as low as possible, so I won't use roofing paper, but will use plastic for the vapour barrier.

What do you think?

Please check out these comments by another guest here who used a floating plywood subfloor over concrete then attached 3/4" solid oak over that.

VP



Joined: 29 Oct 2004
Posts: 17


PostPosted: Wed Dec 29, 2004 11:03 am Post subject: Reply with quote
Just wanted to let everyone know how it is going on my naildown over plywood and slab project… Slicing old carpet and pad into 3' wide strips and then rolling was a great idea (from "How To" article on this site). Flatening the slab took longer than anticipated. There were several cracks where there was slight heaving. Ground down these and then used thin set to flatten slab in places where it was low. A couple places took two passes with the thinset. The self leveling stuff would be nice but wow would that be expensive on 2800 feet of floor! Am now putting in the plywood underlayment over the 10 mil poly. Found 2x8 sheets easy to work with. Fitting bathroom corners was a pain. Put bottom layer at 45 degree and then top layer at 90 to anticipated floor alignment. Both layers at 45 was going to be ALOT of work. This way I get the benefits of not having overlapping joints but it is a little quicker. Glue and screw layers together. 3 ply ½" plywood works ok but you have to be careful to weight down stack as you use it. That way it stays as flat as possible before use. I imagine 5 ply would be better but it is an extra $4 per sheet here in Phoenix. Home Depot cut 160 sheets into 2x8 panels for $20. This is a great labor saver. Am picking it up in 20 sheet batches as needed. So far, have half the house done with the poly and plywood. My prefinished flooring supplier slipped their forcast delivery date so am taking my time with the plywood install. Should have it all done by the time the flooring arrives. I like the slightly soft feel of the two layers of 1/2" plywood. With the 3/4" nail down on top it should be very nice. I do check the top layer of plywood as I go to make sure it is securely glued and screwed to bottom layer. Add screws where it feels mushy. For those concerned with moisture, using 10 mil poly under the plywood up the wall slightly and forming a "pan" over your concrete together with "floating" the plywood and keeping everything spaced from the walls – should all but elimate moisture concerns even in a wet climate (which mine is not). Will be removing about a 150 sq ft of glue down next week and that should be interesting! Hope my pogo stick power scraper works as advertised. Well, thanks again everyone for the help and advice. Back to work!



Joined: 29 Oct 2004
Posts: 17


PostPosted: Mon Nov 08, 2004 11:05 pm Post subject: Reply with quote
Anyone have experience with eflooringwholesale.com?

My plan (but I am open to suggestions):
1) Remove existing carpeting and baseboards
2) Level and clean concrete slab as much as possible
3) Spread 10 mil poly sheeting over entire floor, extending slightly up baseboard and gluing any seams with construction adhesive
4) Lay 2’ x 8’s of ½” CDX in same direction as final nail-down flooring boards, leaving recommended spacing between panels and walls
5) Lay second layer of 2’ x 8’ ½” CDX 90 degrees out from first layer with correct spacing
6) Fasten second layer to first with ¾” finish nails (have a 16ga air finish nailer on-hand)
7) Cover entire plywood sub-floor with 15# roofing felt
icon_cool.gif Install nail-down 3-1/4” prefinished T&G
9) Install 6” MDF baseboards and quarter round molding

You mention the “Pergo feel”. Yes, I would like to avoid this. With two opposing CDX layers, will that “feel” be minimized and/or be eliminated? Will this hybrid floated floor generally feel like the layers were glued?

Will there be a movement problem given that we are talking about a near solid 1-3/4” of wood over a smooth and level concrete slab?

With a floated floor, how does one ensure a tight fit around “no molding” areas like door jambs – without using caulk or fitting tight (and risking buckeling)?

Gary: would you still go with your glued-down vinyl and single layer of glued/nailed ¾” CDX?

While flooring is enroute, I have 3-4 weeks to decide. All comments appreciated!

Thanks!


Have you considered NOT fastening your floor to the concrete and instead floating the whole thing? I am in middle of whole house floating install of nail-down over two ½” layers of plywood sub-floor that was laid over flattened concrete. Process I'm using is described elsewhere on this board. Feel when walking on just the plywood sub-floor is slightly soft but it is very nice underfoot. Spent a lot of time flattening slab with thinset and a grinder. Nailing down the actual flooring is simple when compared with the all the non-seen stuff under it! But as with any project, preparation is the key to a successful install. With additional ¾” of nail-down over the ply, floor softness is much reduced but not totally eliminated. That's a good thing! No problems with moisture either as the poly forms a pan in each room that is not broken between the wood and slab. Have some old oak engineered that was glued to slab. The softness difference between glue down engineered and nailed down T&G over plywood that has been floated is noticable. But I like the floated better. Plus it is warmer in the winter. Doubt that with weight of a total of 1” of plywood under ¾” of nail-down plus the 10 mil poly and 15# felt that I will have problems with it moving. Elected to not use T moldings but am instead doing it without joints because it looks better. Left 1" space around all walls and jambs just in case something expands or moves slightly but I doubt that will happen.

Floating is an alternative to the glue mess and smell but not even close to being less work. Doubt that any professional installer would even consider this method due to extra labor and costs involved. But for a serious DIYer willing to take the time to do it right, I think it is a good alternative. Something to think about...