I understand that my subfloor (24" o.c.) does not currently meet specs for a hardwood install. A keyword search of this forum yielded a post from 2005 by someone with a similar issue in which Gary suggested the addition of layer of 1/2" CDX in order to further stiffen the subfloor. I hope that this solution can apply to my particular situation, so now I have some specific questions about how to proceed. Please note that the last question may open the proverbial "can of worms".

1. Gary seemed to imply that the procedure required screwing and gluing. If I understood this correctly, then what sort of glue should I use? Also feel free to tell me what type of screws to use.

2. Should I flatten the existing subfloor before I add the new layer of CDX or wait and see what it looks like after?

3. How much should I worry about the additional load (1/2 " CDX + 3/8" hardwood) on the subfloor support frame? This room is a 1980's addition to a 1962 house and appears to be built over a former concrete patio (above grade). I removed a small piece of the 3/4" CDX along one edge in order to verify the thickness and found that it was mounted to 2x4 framing which was itself in contact with a concrete slab. It seems quite solid - there is only one squeak in the whole room - but I felt the need to ask because it is obviously a non-standard configuration.

I am grateful for any and all input.
I sincerely do not want to go back to carpet.
Thank you.

Link to 2005 post referenced above: http://www.hardwoodflooringtalk.com/forum/post8652.html#p8652

1: While the addition of glue is recommended by most contractors, it is not absolutely necessary. Having said that, I would not do the job without it. Use a "urethane" adhesive in tubes laying it out in zigzag pattern on the botton of the new plywood.

2: Simply adding 1/2 inch plywood on top of an irregular subfloor will only raise the irregularities by 1/2 an inch. Do all flattening to the existing subfloor prior to adding the new plywood.

3:The additional 1/2 inch plywood will not affect the "load" of the subfloor, since your description indicates that the entire system is mounted on a concrete base.

An additional tip. If you are planning on installing a solid wood floor product, you must moisture test your subfloor. Especially since it is built over an old concrete patio. Odds are, no attempt at controlling moisture migration through the concrete was taken. It is quite possible the slab underneath that wood platform is emitting moisture vapor, which in turn, could be migrating into and through the plywood. Extra care in your situation needs to be taken to ensure a problem free and lasting install.

Dennis, thanks for the advice about gluing and flattening. I do not want to cut even a single corner during this project.

Gary, I'm glad that you mentioned moisture testing. I've been checking around and the only moisture meter that I yet can find that is available for rent is the Testo 606-1. Perhaps you have an opinion about it. It is a pin type meter and will probably be fine for testing the CDX subfloor. However, the manufacturer of the strand woven bamboo that I plan on using recommends a "non-intrusive surface meter" for the acclimation testing because of the resin content and the high density of the product. I have yet been unable to find one for rent.

Here are some other questions regarding moisture testing:

1. If I choose to use an engineered floating floor instead of a solid floor, is moisture testing less critical?

2. Should I wait until there is wet weather before I rent the meter in order to get a better picture of the actual situation?

3. If I do find that the moisture content of the CDX is > %10, is the only solution to remove the subfloor and put plastic film down on the underlying old concrete patio?

Thank you for your time.

Guys,

First let me say this forum is great. Fantastic source of info and advice. Much appreciated.

I have a question also about 24"OC, though my situation is 24"OC with 23/32 OSB, and is on a second floor. Is the recommendation to go down with the 1/2" CDX over the OSB? Hardwood is 3/4 x 3 1/4 Pat. Rosewood. My condo is 2004 construction and appears to be engineered wood I-joists. What other options would I have other than the additional 1/2" CDX?

Since it is a condo I am using the insulayment product from MPGlobalProducts. Assuming this needs to go between the rosewood and the CDX (if needed), correct?

Final question is how to match against carpet through a doorway. Carpet appears to have a standard 1/2" pad and is a berber style. If I have to use the 1/2" CDX, this is looking at nearly 1 3/8" total thickness for the wood and like 3/4" for the carpet. Big change.

I have never understood why the manufacturers of OSB and trusses and the local building codes allow 3/4" OSB subfloor panels to span 24". The few jobs I encountered with this "system" had significant sag between the trusses. And talk about flex and bounce. There outa be a law. I think the ONLY flooring that can be installed over that is carpet, to hide the flaws in the subfloor.

But to your situation. After removing ALL the old floor covering, you will need to flatten that OSB subfloor by sanding the high seams/joints and filling in the low places between the trusses. After getting that done, I'd glue and screw/staple some 1/2" CDX plywood very well so it's solid and tight. Then double check for flattness again and rectify any places out of spec. Since you do not wish to have such a large discrepancy between the hardwood and the remaining carpet, I'd suggest one of two things.
1) use thinner flooring. A 5/16" or 3/8". Both thicknesses can be had in a solid pre-finished.
OR
2) Use the flooring you want ( 3/4" ) and after the wood installation, the carpet and pad will need to be pulled back where it butts to the new wood flooring. Then carpet shims will need to be installed over the subfloor to raise the carpet up in those areas. Then re-install the pad and carpet.

Gary, thanks for the answers. Since I already have the 3/4 rosewood I will deal with the carpet transistion at a later time as I intend to replace the carpet in the future. Looking a bit closer the difference doesn't appear to be too bad. I think with a threshold between the wood and carpet it won't look to noticable. Sorry in advance for the long post.

I did some further investigation of the subfloor and found that the trusses are not as equally spaced as I thought. Most of them are on 20-21" spacing from what I can tell using the stud finder and looking at the nails in the OSB. The outer edge of the room has one span that is 24". Still sounds like they are too far and best to use the additional CDX, would you agree? Could I get away with 3/8 over 1/2 since most joists are on 20ish spacing?

One problem area I would like your advice on dealing with. I have a heater vent near the outer edge of the room. It is actully placed very close to, but not in contact with, the outer joist that is on the 24" spacing. When the builder or whoever put this in, they didn't build any bracing around it, so the OSB has a big 4x14 hole for the vent right near the edge of the sheet. If I walk on the floor in that area, it sags terribly between joists and causes the vent to pop and distort shape since they only supported it by nailing to the OSB. My thought is I will need to cut out a section of the OSB, build in bracing using 2x4s around the vent to provide support, tieing between joists, and also build in a frame where I cut out the OSB, then reinstall the OSB in that area. Am I on the right track (diagram can be provided if needed)

Any special tips with installing the Patagonian Rosewood 3.25" boards? I have seen doing reading here that tounge splitting can be a problem. Reading some of the other posts, it looks like best thing is to staple with an air powered floor stapler. Also, any special tips with using the Insulayment under the wood (condo HOA required insulated underlayment). any other words of wisdom?

Personally, I would use cleats to install that floor since they will have less tendency to split the wood than the staples.