Hi, me again. My solid walnut now has a net thickness of 5/8" from the remilling process, and will be a little less stable than 3/4" product.

I'd like to kerf and glue 5/8" plywod over concrete sealed with MVP, no nails.

Can I nail or screw 3/8" plywood over this, then glue and nail the solid planks to the second layer? In case of another failure, I'm thinking this would protect me from tearing out the "whole enchilada", and that I could remove the flooring with the top layer of plywood, thus salvaging the glued plywood. Is this possible, or recommended over one layer of
3/4"? What do you think, Gary, Perry, kls, et al?

Thanks for all your feedback, I will really ponder it before starting my reinstallation.

Why do you think 5/8" thick wood is less stable than 3/4" wood? Stability refers to a wood's natural inclination to expand and contract. This varies depending on the species and not so much the thickness of a wood. How the flooring is constructed also plays a factor in this as engineered flooring is much more stable than solid wood, for the most part.
On your idea to glue down a 5/8" subfloor, that's fine; it's done all the time. But 5/8" plywood is not thick enough to nail to. You need a minimum of 3/4". Your idea of screwing down 3/8" to the 5/8" then gluing and nailing your flooring to that should work. But the idea that you could remove the 3/8" and wood flooring from the 5/8" plywood is faulty thinking. For one, how are you going to get to the screws to remove them to remove the 3/8" plywood? Plus, the flooring fasteners (cleats or staples) will protrude through the 3/8" and go into the 5/8" plywood, nailing them together even more. I doubt very much that you would be able to successfully remove the flooring without damaging it. But you will have a nice 1" substrate to nail to. Tell me again, why did you decide against the floating subfloor?

Hi, Gary. To answer your question, the person who milled my wood said there will be less stability in 5/8" thickness than 3/4" in terms of expansion and contraction. The wood is walnut, BTW.

I recently walked on a solid oak strip over floating subfloor and it sounded "hollow", reminded me of laminate, and I don't want to cheapen this project. So I thought I should go back to the idea of a solid feel, and the only no-nail option for a subfloor left is the glued plywood.

I see your point about the screws, I wasn't thinking! Gary, I remember in one post you recommended 1/4"(or 3/8"?) layer of plywood over a 3/4" plywood substrate (2nd story) in case a floor ever had to be repaired or replaced, and not to use glue between these layers. I thought that was a great suggestion. I got the impression that the top plywood layer could be pried off of the substrate without damaging it, if needed. Now that I think about it, you probably recommended stapling the second layer. Would a stapled top plywood layer be easily pried away from the subfloor? If so, would 5/8" be enough thickness for the staples to grab into ? If you think this would work, would it be just as stable as the 3/4" layer alone? I have allowance for about 1 1/2" of floor height.

Thanks so much. I really appreciate the time you have taken to help me and others on this site to get through this journey.

This suggestion is refering to glueing down an engineered wood floor to a wood subfloor. The idea is that without underlayment (3/8" plywood), if the engineered floor needed to be removed for some reason, it would trash the subfloor in the removal. In your situation, it is different in that you are nailing as well. Since the fasteners will go through the underlayment (the 3/8" plywood) into the subfloor, it will make removal much more difficult in terms of salvaging the flooring.
I would suggest you lay a 3/4" nailing base (3/4" plywood ripped into 16" or 2' widths and kerfed on the backside) glued down with Bostick's Best over their MVP. You may need to anchor the plywood here and there with weights if you do not want to drive nails into the concrete. Then, nail and glue your walnut flooring to the plywood nailing base with 1&1/2" fasteners (staples or cleats). You will need to perform moisture tests along the way to see if your plan is working. Do not rush the install. After laying the plywood, check the MC. Wait a day or so and check it again to see if there is any MC gain. Personally, I'd hold off on installing the flooring till the wood subfloor equilized with the environment. About a couple weeks or so. Try to keep interior conditions even at about 65 to 75 degrees F in temp and around 45 to 50% RH. Acclimate the flooring as well. If the plywood goes much above 11 to 12 % MC, you know that your slab is emmitting too much moisture and you'll need to rethink your plan. At that point, you'll need to think about sealing the plywood and back priming the flooring as well. THat's about the best you can do under the circumstances, IMO.

Thank you, Gary, I will follow your plan. I hope you won't mind clarifying a few of your explanations.

1.) I presume your suggestion of 2' widths would mean 2' X 8' plywood strips and staggered every 12", kerfed every 8-10"? Does 2' X 8' follow the contour of the concrete better than 4' X 4'?

2.) Does the temp. have to be maintained at 65-75 degrees forever, or just during acclimation? I know RH should always be consistent, but can the temp. fluctuate to lower temperatures later on, after installation is complete?

3.) I want to be able to see the progression of the MC for myself without having to invest a ton of money for a one shot deal. Can I use one of those $20 moisture pin meters to test the plywood? Someone on this site mentioned one from Harbor Freight.

1) Yes, 2x8 will conform better than 4x4. Stagger every 4', not 1'. Kerf every 12" 3/8" deep for 3/4" plywood.
https://www.nofma.org/Portals/0/Publica ... 010_04.pdf
2) It always is best not to subject any wood to extreme temps, whether it is a dining room table, an acoustic guitar or a wood floor. Temporary drops and jumps in temperature will not harm the floors but may increase the likelyhood of gaps or a minor cupped appearance. Maintaining temps and humidity go hand in hand. It is prefered but we all know many do not abide by the recommendation. All we pros can do is educate and advise. The rest is left up to the consumer.
3)I haven't used that one so I cannot recommend it. I have no idea if it is accurite or not.