For more information, The hardwood is 3/4” x 5” solid white oak. The joists are standard 16” spacing and they are 2” wide. Advice I’ve been given from contractors is to screw down the plywood subfloor every 6”, then use PL 400 to glue down the hardwood and then nail. They seem to think this will keep the hardwood down and prevent cupping. Does anyone agree or have further suggestions? Thanks so much!

I think that is correct .

You also should have a full HVAC system and keep the humidity in the proper range (35-55%) year round for wide plank flooring.

My floor joist change directions in the middle of the house and I laid the floors on a diagional. I didn't want to loose 3/4" from my 8 feet ceiling height to install an additional layer of plywood. Also I picked 4" white oak so I wouldnt need to glue down.

The previous home owner installed part of the floors parallel with the joist and it had started to gap and buckle in areas where the subfloor was sagging.

I can't imagine how joist direction has anything to with a plywood substrate floor cupping.

There is no point in adding screws every 6" on a panel, unless you're adding another layer. You would be screwing down to an empty bay and possibly into a pipe or wire that does not have efficient clearance. Also, why is the contractor recommending PL? It is too brittle. It will crack and come loose. There are plenty of flooring adhesives available, designed for wood flooring. And many manufacturer's make it in sausage tubes for applying strips of beads. For 5", I would use a roll-on vapor retarder and do a glue assist, applying a bead of flooring adhesive to the back of the floor board and nail/staples, no changing direction. You won't get any cupping. The roll-on will prevent most of it. If, you are really concerned of cupping. Do a full trowel with nail assist.

Maybe I didn’t explain it correctly, our contractor is worried about sagging between the joists and cracking boards.

Since you have an unfinished basement, 2x joists, and it is a small area, just add blocking to the joists in that area. It will eliminate the need for more subfloor while fixing the problem.

Thank you everyone for your input. It seems like adding blocking to the joists is the solution. We have diagonal braces between the floor joists. Is this common? Can we still add blocking? The hardwood installation begins today. Could we add blocking after the hardwood has been installed?

Diagonal braces photo

Yes. diagonal braces is common, and yes you can still add blocking. If you are nailing down the hardwood then I would not add blocking afterwards. The nails should penetrate the subfloor into the braces.

The only way to install parallel to your joists is to add another layer of five eights plywood. Glued and screwed. This is the only way.

We changed direction with our floor joists and I like it! Keeps the place from looking like a bowling alley :)

The only way to install parallel to your joists is to add another layer of five eights plywood. Glued and screwed. This is the only way.

You have one room out of an entire house that goes in a different direction, with exposed joists below. Why in the world would anyone want to add 5/8" plywood to the whole house when you have access to the joists? If you add blocking on 16" centers to essentially turn the direction of the joists, you don't have to deal with all of the problems that come with floors and stairs in an entire house that are 5/8" higher.

Is there some reason you can think of that it would not accomplish the exact same thing?

ETA... It really is a sincere question...

The purpose of the pictured blocking is to prevent joist rollover, not to stiffen the floor.

I can tell by his answer that G & S knows what the hell he's talking about. I'd do what he says and forget about it.

Joseph, to elaborate... if your joists are exposed, add full blocking, the same dimension as the joists. That’ll stiffen everything up.
The only other concern is the thickness of the plywood?

Oh yes. One other. As G&S from the Commonwealth of Massachusetts said, please no PL. even the 400. It isn’t flexible enough for 5”. Procure Wakol MS262 adhesive sausages and a sausage gun from a wood floor distributor.
If I can help you on that regard, Reach out

The purpose of the pictured blocking is to prevent joist rollover, not to stiffen the floor.

Where do you see a picture of blocking on this thread? The only thing I see is bridging. Bridging and blocking are different things and although they both offer rollover protection, blocking serves additional purposes.

It still prevents roll-over but also has seismic protections, stiffens the floor, and forms a mechanical point for fastening. Flooring with blocking every 16" will be as stiff parallel to the joists as it is perpendicular to the joists. When done properly for wood flooring you will stagger the blocking 8" (between joists to make installing blocking easier) and chalk line the floor above so that every other nail in an 8" nail schedule goes into the blocking or joist. This will create a floor assembly much stiffer than adding 5/8" plywood.

Bry911:

Additional blocking is last on the list of methods to stiffen a floor.

" When done properly for wood flooring you will stagger the blocking 8" (between joists to make installing blocking easier)..."

Not a recommended procedure.

"This will create a floor assembly much stiffer than adding 5/8" plywood."

4]Weakened planks and second-floor
bounces are best fixed with a new layer of plywood

"Not a recommended procedure."

By whom? The floor has bracing, the purpose of the blocking is not roll-over protection or hurricane protection or even seismic protection... it is to limit subfloor sagging between joists... PERIOD!

"Weakened planks and second-floor
bounces are best fixed with a new layer of plywood"

Either the problem is solved or it isn't, either the solution is cost effective or it isn't. The best solution in one situation may not be the best solution in another. When the typical solutions don't work, you look for other ways to accomplish the same thing.

The solution to add plywood is the best solution in most situations, but is suboptimal when the problem exists in only a small area. Suppose the living room is 300 square feet of a 3,000 square foot home. The best solution in that case may not be add 3,000 square feet of plywood nor would it be transition down...

The best solution would be find a way to achieve the same results without adding height to part or all of the floor, e.g. from below. At times, that is not possible, but sometimes it is. This is something that my floor guy has done several times. It does work just fine.

bry911:

We agree that there are several cost effective options to solve this issue.