Diagonal plank subfloor preparation for Hardwood Flooring?

Fingers

Hello,

I'm renoing a kitchen in a 1950s bungalow. All the plumbing and electrical is finished, the uppers are hung. But we stood back and looked at the existing floor (which we were hoping to salvage) and realized there are too many fit and finish issues to address before refinishing ($400 to refinish 100sq ft of prefinished oak is a bit much IMO too). Instead, we're opting to replace it all.

We got an amazing deal on some Canadian made prefinished hardwood, 3.25" for $2.50/sq foot (clearance, they only had a small quantity, which ended up being the exact amount i needed, plus 20% for waste).

Presently, the layers are as follows:
-3/4" Prefinished oak (does not continue under cabinets)
-1/8" Vinyl
-1/8" rigid linoleum tiles
-3/4" particle board (that has swollen from a few faucet leaks)
-3/4" diagonal plank subfloor.

I will be ripping everything down to the 3/4" diagonal plank subfloor. My question is: do I need to add another layer of ply on top of the plank flooring? Or is it sufficient to just screw the old boards down, lay down roofing felt, then the hardwood?

Follow-up question -- if I must use ply/osb, do I NEED to use 3/4"? I don't want a drastic transition. If the goal is to level everything prior to HW install, then I'd like to see if I can get away with 1/8" ply.

I have a healthy quantity of leftover 15# felt from a roofing job, and I have the hardwood in my possession (its climatizing right now).

Thanks!

Daniel Holzman

The easiest place to get the correct information would be from the manufacturer or supplier of the hardwood. Usually they have a lot of specific information regarding proper installation for their product. If that is not possible, check with the oak flooring association (they have a website), that includes a lot of good information on proper installation of hardwood flooring.

When I installed my 3/4 inch prefinished oak flooring, the manufacturer (Bruce) recommended rosin paper rather than asphaltic felt, not sure it makes much difference, but I went with the paper (it was red, must have been treated with some sort of vapor barrier). I don't recall the Bruce instuctions requiring two layers of subfloor, in fact my house is mostly 3/4 inch oak over diagonal pine subfloor boards, has not been a problem. Tile is a totally different story, typically requires two layers of subfloor, but hardwood is more resilient and does not require the same L/D stiffness as tile.

Just Bill

Rosin paper is to isolate the layers and reduce squeaks. You could probably go right over the original subfloor, but if I were doing it, I would add a layer of 4-ply 1/2 CDX, glued and screwed. I hate callbacks.

Fingers

I'm not worried about squeaks (the rest of the house is beautiful original birch hardwood, and squeaks like a pig!). To have one room sans-squeak would simply be out of character . I'm more worried about misaligned planks, but i'll go over the joints with a palm sander.

Seriously though, i'm going to go around with 1.5" flooring screws and screw all the planks down, then test for squeaks. I already have 15# asphalt paper so I will use that instead of rosin. Hoping that will be sufficient for a good end-product.

If any squeaks arise after the fact, I'll fix them from below (unfinished)