Subfloor repair question

I've ripped out old tile and underlayment in preparation to lay down a solid wood floor, 2 1/4" x 3/4". (I've already gotten some help with this project on two other threads). My subfloor is made up of diagonally running solid 7 3/8" planks, 3/4" thick.

Having gotten to this point, I now realize that I have never had any insulation in this floor which isn't good because under it is an unheated garage. Also, this project is actually part of a larger project that involves removing a load bearing wall and as a result of that, I'll need to relocate some ductwork which will also involve tearing up the subfloor. And beyond that, some of the pieces don't look so good. For all I know this is the original floor from the 1950s when this house was built

Long story short, I'm going to need to remove and replace a good portion of this subfloor.

Questions:
1) When I replace the pieces, can I just use 3/4" plywood? I've checked a modern piece of ply against the thickness and it matches up very well. Is there any problem with a mix of solid and ply subfloor when placing solid 3/4" red oak flooring on top? I can't tell what the original plank is made of.

2) When I do cut up parts of the subfloor, cutting over the joists, is it okay to reuse the same wood and put it back? Is there any rule about how long of a run I should have?

3) The subfloor planks stop at the wall that runs the length of the room. When this wall is removed, I'll need to put down new subfloor again with the solid 3/4" wood going over the top. I can just cut back to the nearest joist and fill in more plank, but is there any problem with having all the planks cut at the same point? In other words, how important is it that the subfloor joints be staggered as you might be concerned when placing underpayment for tile. Not sure if I've explained this well or if it's even an issue. Being I'm repairing and replacing some parts in the kitchen area already, I could wait until the wall is gone and put down longer runs.

4) Any general comments on condition of floor (see image)? Also, when everything is where I want it and it's time to lay down the solid wood floor, is it a good idea to go over the whole thing and add fresh screws just to firm it up?

Image Kitchen Area:


On the right hand side from this angle is the wall that will be removed. Looking closely, you can see the vertical ductwork that will need to be moved. For those following my other thread about ripping out or toothing in, the doorway in the back right (and short wall to the right of it) represent the area I will have to tooth in.

Image of dining room, looking in the opposite direction of the photo above.:


From this view the wall to be removed is on the left. For those following my other thread, this is the section I was asking about toothing in or ripping out.

Closer image of subfloor condition and thickness:

 

No problem with using a mixture of the old boards and plywood providing the thickness is the same. It gets done that way all the time.

 

Thanks Mark. Is my new hardwood floor gonna care if I reuse the old planks or have runs as short as 16" in places?

 

Not really, the only downside to the planks is if you try to nail the hardwood in the gaps between planks. That's why many will overlay it all with plywood first.

 

Is the existing hardwood set directly on the diagonal planks? or is there a layer of ply between the floor and the subfloor.

Rule of thumb is that you want a patch to span to 3 joists (not just 16") as there is strength from this set up rather than spanning just one joist. Strengthens the deflection between the joists. A layer of 5/8" ply on top of the diagonal planks would make for a very sturdy floor.

 

Yes, the existing floor is laid directly on the planks. I understand and agree with your reasoning for using 5/8 ply on top of that, but it's out of the question because the entire first floor was laid this way and I want a level floor.

What you say about trying to span at least 3 joists makes sense and I'll try to follow it while I do repairs. My plan is to use 3/4" ply cut into the size of the planks or in some cases where I'm replacing side-by-side planks I might go wider than the plank width. In fact, in some areas I might deliberately cut sections of plank out and use plywood pieces to avoid having to use short planks. The thickness is a good match.

Whether between planks or between planks and newly installed plywood sections, what is the preferred gap to allow for expansion? 1/4"?

Also, there is no underlayment, not even thin wax paper. I could add this to the area that I'm re-doing, but I'm concerned it will raise the floor level so I'll probably just skip it. What would be the purpose of using such a thin underlayment anyway? Dust or sound barrier?

 

The use of 15# roofing felt is to cut down on squeaks, crackles and such to make the floor quieter. In some instances, it also acts as a vapor barrier.

Cut your plywood patches in in large Rectangle pieces only, set perpendicular to the joists. Do not cut into plank size pieces and try to run it diagonally to replace a single plank. Plywood/OSB is not that strong cut into thins strips. It is designed to be laid perpendicular to joists and have enough material to prevent deflection. So, cut out a 24" by 32" section of plank and inset plywood perpendicular to the joists.

 

czizzi,
Okay, I'll do as you say. Thanks for all your help with this and other questions. After ripping out the floor on the other side of the room today I did find some sort of thin barrier. Maybe wax paper, but I don't think it's roofing felt. I'll see what they offer when I buy my new floor and just use it through out.

thanks again!

 

czizzi,


Bumping this old thread rather than start another...

For plywood subfloor, or any subfloor really, is it necessary to leave any room for expansion between the pieces? 1/8"?

 

You are dealing with a combination of plank and plywood. Each reacts differently to changes in ambient humidity and temperature. The plywood will be more stable but will still move, just not as much. I don't think you will hurt yourself by leaving a little gap between boards.

 

So... only gap between plywood and planks? Plywood to plywood can touch?

 

If the ply is tongue and groove yes.

 

No, not tongue and groove, just regular 3/4" plywood. So I'll only leave a small gap between the plywood and the plank but the plywood can be snug against the other pieces of plywood.

Thanks for the clarification!