How badly cupped are some of the 1x8's ?
Depending on how badly cupped they are, you may want to do some sanding, before sheeting out. It's hard to tell without seeing, but it sounds like sheeting out may be something to think about, to span those 1x8's, and firm things up.

Those old subfloor systems of 1 by 8's running diagonally across joists 16" OC are actually pretty good for hardwood floors. What I find is after 50+ yrs, they need re-nailing. I use screw shank 2&1/4" nails instead of screws because screws always seem to split the old wood and I'm not going to pre-drill everything. Using a pneumatic framing nailer is fast and easy. You can always sand those subfloors a bit to even them out as well. I've laid some 3/4" strip floors across those old 1 x 8 diagonally laid subfloors and it worked out very well. NOFMA at one time said that subfloor system held fasteners the very best.

I was a little concerned about the firmness, (shoring up the subfloor was mentioned). But the hardwood should firm it up pretty good I would say.

The 1x8's aren't cupped badly, although that is relative. I would say slight cupping at the edges. What is okay to me might be bad to you. Can you give me some indication of what bad would be?

The floor is firm now especially since they've added several beams and multiple piers. Although my original reason for wanting to sheet out the floor is to add some firmness back to it. And avoid having to sand the 1x8's. I pulled up 15/16" particle board and I know how much different the floor felt before and after I pulled that up. I just wasn't sure if the 3/4" oak (5" width) would add that stiffness back to it by itself. I have been on the fence whether to sheet it out at all and if so, whether to use 1/2" or 3/4". By the way, what grade plywood should I use?

One of my concerns is with height of the other flooring. I am putting carpet in the bedrooms so I currently have the 3/4" particle board in those rooms. That will put my carpet above the 3/4" of the wood if I put the wood directly on the subfloor. The wet areas are all getting tile which will have some sort of underlayment which I can adjust to match the wood.

Any and all opinions are welcome.

Thanks,

Jeff

If you still have the 3/4" p. board in the carpet areas, then it could very well make sense to sheet the hardwood areas so they aren't way lower than the carpet. But the tiled areas are more important and less "flexible". So, figure out EXACTLY how high the finished tile floors will be off the wood subfloor. Do not even think about installing tile ( ceramic ) over p. board. So let's assume for a minute that your tile will be 1" above the subfloor. If you added 1/4" plywood, then you hardwood could be at the same height. If you could get the tile height to 1&1/4", then a single layer of 3/8" CDX grade plywood would be ideal, putting both the tile and the hardwood at the same height. The carpet would be very close as well, slightly higher but that's good. Better than lower, IMO.

Thanks Gary. I'm an amateur tile setter (considered that for a livelihood) so I'm aware that the p.board is out for a substrate with the tile. It's funny you bring that up though because apparently the contractor who did the remodel of this house in 2000 knew nothing at all about tile since they did lay the ceramic directly to the p.board in both bathrooms.

My thoughts were to pull up the p. board in the tiled areas and then I can adjust the tile height using a combination of plywood/backerboard to get close to the wood floor height. My actual concern is that I will get the wood floor to low and my tile will be too high after I put down my backer board etc. I mean, 3/4" p.board, 1/4" backerboard, 1/4" min thinset, 3/8" tile thickness puts my tile at well over 1-1/2". If I pull the p.board, I can lay 1/2" ply, 1/4" backer, 1/4" thinset, 3/8" tile and that will put me at 1-3/8". Or just go with 1/2" backer and I can get it down to 1-1/8" which would get me really close to the wood at 1-1/4" (1/2" ply + 3/4" finished floor).

When I sheet out the floor do I need felt between both layers or just between the finished floor and the plywood? CDX is the plywood grade I want to use? The X is only to indicate the different type of glue for exterior use correct? It isn't treated, that would be bad since it is wet.

Do I need to check the moisture of the 1x8's before sheeting out with 1/2"? Or do I just need to sheet out the floor then monitor the moisture levels in the plywood?

Thanks for all of the great information. I can't wait to get this going.

Jeff

No need to install anything between the subfloor and the plywood underlayment. CDX works fine for hardwood flooring underlayment. C and D are the grades of each side and X does signify that it's made with exterior glue and can be used in an exterior application. Some prefer higher grades. You'll pay dearly for higher grades of plywood and my opinion is, if it's being covered, who cares what it looks like. You can check the existing subfloor for moisture before sheeting with plywood. It's probably a good idea. If you have a crawl space, you MUST install 6 or 8 mil polyethylene plastic on the entire ground as a moisture vapor barrier. Most big box stores ( who have the best plywood prices ) store the plywood indoors ( so should you ), so the plywood ( unless improperly stored ) most likely will be in the normal moisture content range, which is around 10%.