[Greenlight1]

Hi guys. We currently have 7/8” tongue a groove subfloor running throughout home. I was originally going to refinish it but several sections were rotted away and needed to be replaced. So with that being the subfloor, I am going to cover it with plywood before we install the new flooring. Should the new plywood be tongue and groove or not? And should we go with 1/4” 1/2” or 3/4”??? The floor is very strong but I feel laying a new level surface would make whatever we put down a lot easier.

 

[chandler48]

How sure are you that it is 7/8" thick? It would be an oddity. Normally subfloors that are tongue and groove are 3/4" thick. If that is the case, then replacing sections that span more than two joists would be acceptable with Advantech. It is tongue and groove, but it may not match the T&G you have down, and if you are replacing only sections, you will most likely need to cut the tongue off the Advantech for it to fit properly. Be sure to glue and screw down the new subflooring.

 

[Marson]

Hi guys. We currently have 7/8” tongue a groove subfloor running throughout home. I was originally going to refinish it but several sections were rotted away and needed to be replaced. So with that being the subfloor, I am going to cover it with plywood before we install the new flooring. Should the new plywood be tongue and groove or not? And should we go with 1/4” 1/2” or 3/4”??? The floor is very strong but I feel laying a new level surface would make whatever we put down a lot easier.

No need for tongue and groove for the plywood going over the existing subfloor. What thickness plywood depends. If you are going to tile, I believe you can get away with minimum 1/2" going over a board subfloor in good condition (don't quote me on that.) And for other flooring, I think unless it's exceptionally smooth and continuous, I would also use minimum 1/2".

 

[Greenlight1]

Yes the existing sub floor is pretty much flat and continuous. I screwed any/all popping/buckled boards down to the joists and all the patching I did is pretty tight with sub floor (1/8”/3/16” gap)
I just want a nice smooth floor to install new flooring on. So, I will plan to use standard 1/2” ply and leave 1/8” gaps between pieces and walls to allow for movement. Should the plywood get glued and screwed through the flooring INTO the joists? Or just glued and attached to the sub floor, intentionally missing the joists? I have read mixed reviews about attaching to the joists and just want to make sure I don’t have movement with one of the floors that pops something.

 

[chandler48]

The second layer of subflooring gets screwed to the first, and NOT into the joists and not glued. It allows a slip factor between the two layers, although minimal.

 

[Greenlight1]

The second layer of subflooring gets screwed to the first, and NOT into the joists and not glued. It allows a slip factor between the two layers, although minimal.

Thank you for the info. So I’ll just plan on using 1 1/2” round shank nails and avoid the joists.

 

[carpdad]

Thanks for not-gluing reason. Forgot or first time reading why. If it's about the movement between 2 layers, I still don't get it. Example is schuluter system which is, technically, anchored to the subfloor. Gluing is similar in that the glue anchors the two layers. So why is one type of anchoring is ok and not the other? Isn't this usually the recommendation when the floor is tiled? Is it the same when the floor is wood or engineered?


OP: what kind of flooring are you using? I'm wondering why you'd add extra plywood if existing subfloor is smooth enough? Instead of adding, why not replace any boards that are too warped or such? Look at the amount of fixes you may have to do vs covering the whole thing? Even half inch ply may have humps if the rough floor you have is not relatively flat. Even some engineered flooring may bridge not absolutely flat subfloor.
If extra plywood makes you happier, ok. Make sure the edges at least meet even and may have to patch the joints. 6d ringshank nails from a nailer gun may splinter the ply face, esp along the edges. Or drive the nails too deep.
Also look at what the changed floor height may do - including the finish floor? Entrances probably will need threshold trims which is raised above the finish floor. Room to room transitions, baseboard heating height, door gaps, entrance door gap, door to bathroom? Staircase riser height? I was forced to use cheap 3/8 engineered flooring because a staircase was part of the floor I needed to fix and I didn't want to risk getting flagged on the riser change. If this area was carpeted especially, change in height may matter a lot.