Newbe question, sorry if this has been covered, tried searching this forum but came up empty.
I am installing 3/4 acacia over a somewhat gappy and loose subfloor of what appears to be 1" fir t&g over 4'OC joists. I will secure subfloor with screws and level it, but I still want the added stability and height of a plywood underlayment. 3/8 would give me the right height, but would 1/2 be substantially better for some reason?

Thanks!

Added:
The 3/4 hardwood is the narrower ~3 1/2" variety.

OK, so I got the 1/2" (CDX) and I've seen staples recommended here to fasten it down, but I'm not sure what size to use or if I need to fill in the expansion gaps between sheets.

Wow, 4' OC is huge. You better go with 3/4 ply and do some work on the floor joists. How far off is the floor flatness?

I dont believe it.

I think I over reacted newbie style to the condition of the sub floor, and I was wrong about the fir T&G, it is 1 1/2" thick and only dips a little at one inside corner, so it's pretty flat. Just has some loose nails or it is otherwise fairly tight as well.
But the joists, which are probably around 6x8? 4x6? ( will have to check), are definitely 4' OC.
I figure some screws in the T&G, and the 1/2" will do the trick, unless I get talked out of it.
I'm trying to figure out what kind of staples to use on the 1/2" underlayment but so far, no luck. I will keep searching.

Thanks for the help!

Added: The joists appear to be 4x6", and there's a little sub floor bounciness in the space between. What can I do for the joist situation?

I'm asking the same question on another post. I'll be laying 1/2" plywood over 3/4" solid wood plank subfloor. From what I've read, it is recommended that whatever fastener you use to attach the underlayment, it should not penetrate through the subfloor to the joists. You screw down the subfloor to the joists, as you stated, and then attach the underlayment to the subfloor (but not to the joists). In my case, I'll use 1-1/4" length fasteners.

I had originally planned to use ring-shank nails, but can't seem to find 1-1/4" ring shank nails that can be used in a pneumatic framing nailer. Instead of manually hammering or screwing down with deck screws, I've decided to use 18-gauge 1-1/4" x 1/4" crown flooring staples that have the adhesive coating. I bought the Porta-Nails brand that can be found on Amazon or at Home Depot. They're the same staples used for attaching 1/2" hardwood flooring, so I would think they could hold 1/2" plywood.

By the way, I was also planning on using 3/8" plywood to even out the flooring with the existing kitchen plywood. Everyone recommended 1/2" or 5/8" at a minimum, so I've decided to rip out the kitchen floor and do 1/2" plywood everywhere instead.

Good luck!

Thanks! Very helpful post. I just happen to have a 1 1/4" 18 ga. stapler and I didn't know about adhesive coating. I'll give that a try.
Cheers!

floormeintucson:
I really appreciate the advice!
I added this to my original reply above, but just in case you missed it:

The joists appear to be 4x6", and there's a little sub floor bounciness in the space between. What can I do for the joist situation?

Thanks again!

Our floor has 26-29" oc full-dimension 2x6". I added 2 layers of 3/4" glued+screwed sheet goods(across the joists, staggered joints), and it's pretty firm. You might want 2 layers of 3/4", or 1" or thicker plywood/advantec, glued to the subfloor, screwed into the joists with long screws, and then screwed to the existing subfloor between the joists with shorter screws (a collated screw gun works great for shorter screws). I think that would firm it up enough, and then the flooring is pretty stiff on top of all that.

Thanks for the advice! I talked to a carpenter here at work, and he said the 4' joist spacing is fairly common for post / beam construction(I think that's what he called it) with "car deck" (1-1/2"T&G) sub floor. He confirmed my conclusion that it's pretty stable once you lock it down with screws / nails. The plywood on top of that ( like you recommend) should make it even better.

Shimmed up subfloor over one beam, sanded down over another, power chiseled high spots around fireplace and along outside wall, stapled down 1/2" Plywood underlayment and glued with 13 large tubes of subfloor adhesive. At some point you just have to say it's as flat as it's going to get, and start nailing down the hardwood.
Really transformed the room, getting lots of praise. Hardest DIY I've done, and I've done five bathrooms, two kitchens and some tile work, but definitely worth the effort.
Lessons learned: get a good nailer, and spring for the Bostich cleats, they are a couple of bucks more for a 1000 box, but worth much more.