Hi All,

We removed the carpet in our house this weekend, and found a 1/2" plywood subfloor with 1/2" or 9/16" particle board over top. I saw from reading here and other places that the particle board was no good for a subfloor, so we ripped it up. Plus most of it had glue-down carpeting pad which was unremovable anyway.

Now I am wondering what to put over top. I read the sticky post here, it looks like 3/4" plywood is reccomended. Well, I don't have that. So can I just add a 1/4" plywood? 1/4" of something else? Is 1/2" + 1/4" as good as 3/4"? Or should I build it up a bit more? It seems to me the existing plywood on the floor is pretty low grade stuff. It feels a bit "springy" when I walk on it with just the 1/2", some squeaks.

What should I fasten it down with? Screws?

I talked to a local wood flooring store/installer and they said that going right over the 1/2" is fine, they do it all the time. That conflicted with what I read here though. I want to do it right, but don't want to do a bunch of unneccessary work either.

Oh yeah, project details:
I will be installing 3/4" X 3 1/4" Solid maple
1100 square feet
Michigan climate
House is 1971 vintage

Thanks!

I would put down a sheet of 1/4 or 3/8 run a bead of urathane glue in between them and screw that down. I wouldnt go right over the 1/2 its going to sqweek like grandmas rocker if you do.

Kevin,

Thanks for the response!

You are saying it is going to squeak if we do the 3/4" flooring right over the existing plywood, right? When I read your response the first time I thought it was saying it would squeak if we don't put the urathane glue down between the "2 subfloors"

The 1/4" or 3/8" needs to be plywood, right? A specific grade? Or could we use OSB or something else?

Kevin or anyone else,

I need to do some "leveling" too, fill a couple sags. I was thinking I would try to do that between the 2 subfloors, so the surface the flooring goes on would be nice and flat. what should I use between the 2 layers to level? I have seen it done on top of the subfloor by using multiple layers of the 15 or 30 lb felt. I have also heard of "liquid" type leveling but never seen them. Any suggestions here?

Do your leaveling before you cap the floor with the new plywood , this as you guessed will produce a smother substrate in which to nail your flooring, use felt paper,asphalt shingles ect. To clarify yes the floor willl squeek if you go over the exsiting 1/2" subfloor with the hardwoood without beefing up the subfloor a bit. It may be overkill to glue the two sheets together just a good easy preventive measure if you are worried about squeeks.

I might add to Kevin's recommendations. 1/2 " ply with 1/4" over it isn't as strong as a 3/4" plywood subfloor. If it were me, I would use at least 3/8". CDX grade is what I always use with the C side up, which is usually the better side but not always. When installing the new plywood, don't marry the seams. It is prefered to run the new plywood at an opposing angle, either 45 or 90 degrees and stagger all the joints I'm a big fan of staples for underlayment. In fact, today I just finished installing 500 sq.ft. of 1/2" CDX plywood underlayment for a nail down floor. I stapled the snot out of it with 16 guage, 7/16" crown, 1&1/2" leg Senco staples. No squeeks at all and very fast. But gluing and screwing works well too if you don't get any bubbles in that thin plywood. Thin sheets aren't flat at all so I shoot the crap out of them with my staple gun and they go down real flat then. Just a suggestion as an alternative to screwing and gluing.

I would put another layer of ½" plywood or OSB.

The thinner ¼ is usually finish grade and just as costly if not more then ½" CDX or OSB.

So right! BTW, bought 1/2" CDX today. Cost $20.00 for a 4x8 sheet and it was 3 ply, not the better 5 ply. I measured the thickness. You know, they say it's actually 15/32". Well, the stuff I got, made by Georgia Pacific, was exactly 3/8" and they called it 15/32". It wasn't even 7/16"; it was 3/8". The 7/16" OSB was THICKER than the 15/32" CDX and cost $15.00 a sheet. I imagine the prices are going to go nothing but up due to the rebuilding after the hurricanes.

You posted as I was typing.

thanks guys. I think I will go with 3/8" or 1/2" depending on price difference. I would rather spend the money than have a squeak though. floor thickness is also an issue. I would match up perfectly to the adjoining room with 1/4" added, but will be over with the thicker boards. No big deal, I can get a transition piece but making up an 1/8" is probably less noticable than the 1/4".

Any opinions on whether plywood is better than osb or vice versa?

I like Kevin's suggestion of glueing, I will do that too. How much glue would you use? A line every foot? More?

Hi everyone,

Just doing some last minute research before I lay down my subfloor and came across this tidbit about installing the CDX with the C side up. How does one know which is the "C Side"?

I've also heard very opinions regarding the spacing of the subfloor. Some say 1/8". Some say 1/4". Others recommend spacing with a 6 penny nail or a thick washer. Any thoughts?

Best...Stan

Plywood is graded as A,B,C,D.

On CDX the C side should be the best side, the grade stamp will be on the D side. The X means exterior glue.

CDX will have some small voids and splits in the veneer. Just put the best side up in your opinion.

I allow 1/8" spacing in the field and maybe a 1/4" at the walls, and I hit the butt joints again with a belt sander.

The subfloor should be nailed down good before you add this underlayment over top.

Hey, wait a minute. Don't we live in the day of the screw, now?
I'm pretty sure Jerry meant "The subfloor should be screwed down good before you add this underlayment over top." Or he's had a flashback

He may well have meant screws. In my case, my subfloor is going on a slab with adhesive AND screws.

Thanks Jerry for the detailed explaination of CDX. Best description I've ever heard.

And if as Floorguy said, if you did have a flashback, next time ask me along.

We're off to the races.

Thanks to everyone for their help.

Best...Stan

No flashback Perry.

This thread's going to get discombobulated real quick, let me clarify.

The majority of my work would be altering an older home. Tearing out carpet, vinyl,ect. and installing either wood or tile. The floor joist are usually 20- 40 year old SYP joist. Hard as a rock.

I always re-nail the sub-flooring using 2" ring shanked underlayment nails in my Paslode gun. Even then... I will have to set some of them with a hammer. Screws will tend to break off going into that SYP.

Now on the "underlayment" going over the subfloor I do screw that......but I am going to change over to stapling like Gary does. We are not trying to hit any joist fastening down the underlayment.

I bought one of these when I was doing drywall. . .

http://www.amazon.com/gp/product/B0000D ... e&n=228013

It sure throws screws down fast.. . . really fast.

-'Chung