I'd like to ask quite a few questions & maybe this one post will handle it. I'm redoing a 425 sq ft family room that was carpeted. I pulled out the carpet & padding and removed 5/8" particle board, and finally felt paper. All that is now remaining is 1/2" plywood sub floor. This is what I propose doing, with some questions intermixed.
1) Fix local spots of existing 1/2" sub floor where some spots (edges?) are low. Do this by bracing up on joists. Then SCREW down existing sub floor to joists. Screw recommendations?
2) Install underlayment sheets of 3/8" plywood. Questions.. Install 90 degrees to existing plywood. Is tar paper required between 1/2" sub floor & 3/8" underlayment? For what reason? Install 3/8" underlayment with screws, recommendation for screws & spacings?

I'm not sure if I will be doing the actual hardwood flooting or contract the job, but I do want to have a good, secure, & strong sub floor/underlayment base. I'll be probably going with 3/4" 3" plank, prefinished HWF.

Thanks for any guidance, FJK

If possible, I'd prefer 1/2" ply instead of 3/8". Felt is not required between the two. Use 1&1/2" screws 6" oc for underlayment. Gluing underlayment to subfloor is a big plus. Same size screws for subfloor to joists.

FJK

A 1/2" inch plywood sub floor with 3/8" under layment may not be the greatest base for hardwood flooring.... The MFG of BR-111 recommends the following:

STEP 3: RECOMMENDED SUB-FLOORING
Staple-Down or Glue-Down Application
Preferred – 3/4" (19mm) CDX Grade Plywood or 3/4" (23/32") OSB PS2 Rated
Underlayment with a minimum 40 lbs. density
Minimum – 5/8" (15mm) CDX Grade Plywood with a minimum 40 lbs. density

Just curious, If you already removed a 5/8" particle board underlayment... why not replace it with 5/8" plywood?

Felt is used between hardwood and the underlayment.

Run whatever underlayment you add the same direction the 1/2" sub floor is currently running.... assuming it was installed properly at a 90 degree angle to the joists.

Don't allign underlayment edges and ends with the sub floor and don't align underlayment ends with itself.

Hope this helps

Dave,
With all due respect, where do you read that asphalt felt is required between subfloors and underlayments? And what would be the advantage? Greater vapor transmission dispersal? Many prefer to glue and nail; the felt would prevent that. Since felt is required on top of the underlayment and under the hardwood flooring, is this not sufficient? 1/2" ply over 1/2" ply would in effect be a 1" thick plywood subfloor, especially if glued as well as mechanically fastened. This will exceed manufacturers requirements.

Your right Gary, 1/2" ply sub + 1/2"ply underlayment is great but....

JFK said nothing about the support under his subfloor...

5/8"ply subfloor would bring his floor back to origional hight (may be a big on-site consideration or him)....

and my reply does indicate felt between sub and hardwood... not sub and underlayment :--).

Thanks

Dave T

Sorry Dave, I guess I misread that about the felt. And since the orig. subfloor is 1/2", we can be pretty sure the joists are spaced 16" oc. While 5/8" p. board was used originally, it isn't mandatory to use the same to achieve stiffness in the subfloor, which is what he is trying to do. And since he isn't mentioning the thickness of the finished wood floor, we can't tell what thickness of underlayment would be best from that perspective. 1/2" or 5/8" would deliver equal results in terms of subfloor/underlayment stiffness and deflection. The variable is the thickness of the hardwood flooring. Once that is determined, then the optimal underlayment thickness could be decided.

Hi guys.. thanks for your help so far... The reason I would like to use 3/8" underlayment:
First, it meets & slightly excedes 3/4" min total sub floor (1/2 + 3/8 =7/8)

Secondly, the total finished floor with 3/4" plank would meet the tiled finished floors in adjacent rooms (those are under construction as well).

The interesting comment made is that I DO NOT have to run the underlayment 90 degrees to existing sub floor, just stagger the joints to the subfloor & stagger the underlayment joints as well.

BTW, floor joists are 16 oc, 2x? (? = they are fine, I just don't remember)

I know it is over kill, but I run the top layer of plywood(underlayment) at a 45º to the finished flooring. It is peace of mind for me and the chance of panelization years later.

My signature stays on that floor, long after I'm dead.