Hello,
We want to rip out our carpet and replace with beautiful hardwood in two rooms. We live in a condo and and are not allowed to screw into the concrete (pycrete) floors. Most of the condo floor is already Maple Hardwood 3/4" and we want to follow the same pattern as it will be the same height where the two floors meet.

Three options are presented to us and we are at loss because of conflicting opinions.

1\Our favorite option) Purchase 3/4 plywood and cut it in 16" strips. Stagger the strips while leaving 3/4" all around the rooms. Float all the plywood (not gluing or screwing) over a moisture barrier (plastic), nail/tack the hardwood to the plywood.

See our plans: https://www.dropbox.com/s/6y8pjsve0191cz9/subfloors.pdf

page 1 - floor plan
page 2 - plywood cuts

Pros: We prefer this one because we feel it's simpler for us, we don't have power tools and would have the plywood cut in strips at the store making it easier to haul in the stairs to the 3rd floor.

Cons: Everyone says: Well this sounds like an interesting idea! But I definitely never heard of it, I would go with option 2 (see below) instead. All the while offering no reason why it should not work.

2\The prevalent opinion) Purchase twice as much 3/8" plywood and lay down in opposite ways, the first layer at the bottom in the same direction as the Maple planks, the second in opposite direction while taking care to avoid overlapping edges. Some people call for gluing, others for screws, others completely floating. We are told that "this is the way it is done" with no real pros or cons and we feel that moving around twice as many sheets, laying down and assembly, is over-complicated. All the while, if "this is the way it is done"... well maybe we should follow along?

3 The simple/simple option) Purchase 3/4" plywood, float it over a moisture barrier in opposite direction to the hardwood and tack/nail the hardwood to it, end of story.

Pros for us: Definitely simple

Cons: We have to get 4'x8'x3/4" sheets of plywood up to the 3rd floor in the stairs. We also fear that the floor may buckle because the whole sheet is simply floating, but this may be exaggerated...

What do you guys think? What should we do?

Last question, most installers told us the layering should be as follows: plastic, plywood, foam (sound insulation), hardwood.

Others said: 2-in-1 moisture and sound, plywood, construction paper, hardwood.

Would you please spare some of your valuable insight on this, we are going crazy!!

Thanks a lot in advance.

Option 2 is the best of the listed options. Glue the two layers together. I have seen option 3 before (although it was only on repair jobs.) Maybe just use a good adhesive with moisture barrier built in and adhere the plywood with that. That is if that is allowed there.
We always put 15# tar paper (roofing felt) between wood and plywood.

Make sure whatever you do to keep the plywood spaced 1/8-1/4" apart. It should have those spaces to help breath, also that is a big place for squeaks is sub floor rubbing on itself.

Ive seen it done all three ways successfully. Id have to ask why gluing two sheets of 3/8 is any different than one 3/4". You are essentially building a 3/4" ply by gluing two layers of 3/8 together. I know in Florida where my little brother did a ton of high end high rise work they moped on black mastic/ 6mil plastic/ 3/4 ply / felt then the hardwood. They would cut the plywood in 2 x 8 lengths stagger the joints and lay it perpendicular to desired floor direction. Being on the third floor would normally cut down the potential for moisture problems too. Make sure to check the moisture on the plywood with a meter, sometimes they store it outside and it can be pretty wet. Bottom line I would go with the system that the flooring company I chose will stand behind, a warranty is only as good as the company that stands behind it.

Gluing the 2- 3/8 layers would be better because they would be laid in opposite directions of each other then essentially creating a whole sheet of contiguous plywood throughout the whole area. Which if you cannot adhere the plywood to the concrete would give the best results.

Yes here in florida we put roofing tar, plastic, plywood, felt paper, then wood. The purpose of the tar is to seal around the nails that hold the plywood to concrete.

staggered 16" plywood strips are a newer method and highly successful.

Having done all, floating subfloors are the best.

you van even thorw some sound absorbtion for cushion and added insulation.

the other methods and tried and ture old school practices we've done dozenbs of times.

ive even mopped on cold tar, 2 layers opposite felt, then strip glue with more cold tar, and nail for wide planks..floors will never move.

the floating subfloor is outstanding and leaves no room for failure.

it is basically the old screed over concrete system advanced.

Thank you for your replies, it's very much appreciated.

We not yet 100% decided but I believe we can forget about option 3 at this time and focus either on 16" strips or 2 layers of 3/8".

5thgenfloorguy mentioned keeping a gap between each plywood sheets, does this apply to both options? I am asking because I thought that once sheets are glued together, then there should be no rubbing?

Thanks again for your help

always allow space between any method of sub floor..a nail space -1/4" is sufficient.

it allows for expansion and prevents any rubbing that may cause squeaks

All right!

Your input has been invaluable, thank you for your help so far.

We decided on laying down two layers of 3/8" plywood that will be staggered and laid in opposite directions on top of a 2/1 soundproof/moistureproof barrier.

We are starting the work over the weekend and I have one last question, I searched the board extensively but I have not found the answer to this question:

Should I glue and/or screw and/or staple the 2 plywood layers together?

And if glue is required, what kind of glue should I use?

Thanks again!

From what I have seen from nofma you glue and staple the layers together. I would use the cheapest flooring glue, but I have also heard you can just use wood glue and squirt it liberally. Use 5/8" staples every 6-8".