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Feb 24, 2014, 02:19 PM
Subfloor Material
I'm planning my first hardwood floor installation on my home. Currently I have worn out carpet that I want to replace with 3/4" oak hardwood floors. I took up some of the carpet, and found the dreadful particle board underneath. I drilled a small hole through, and as far as I can tell it looks like 1/2" plywood with 3/4" particle board over top. My question is, everyone says not to put hardwood over particle board, but why? I've read that the nails won't fasten into particle board, but what if I used a 3" cleat to fasten my hardwood? Is the problem that hardwood can swell over time, and can I just use a vapor barrier to help protect the hardwood?

I'm getting the feeling that my week long project has just turned into a several week project, and I'm trying to avoid having to remove the particle board.

Thanks Smile
Feb 24, 2014, 06:16 PM
You've got the problem exactly right - You can't nail into the 3/4" particle board as the nails will just hollow out a chunk of the material and not get a decent bite.

Several problems with the 3" nail idea:

1. Not sure if they even have them to fit a flooring nailer.
2. Just a guess but if they were available a 3" nail will likely be slightly larger in diameter than the typical 2-1/4" - this could split the tongue on the flooring.
3. The nail would still be going through the particle board and the particle board will still chip out. This would cause some slight floor movement and squeaks.

The overall problem is that particle board is crap. Any moisture in it and it is gone. It's only good as an underlayment for carpet. The 1/2" plywood and 3/4" particle board combination is typical 1970's construction. There are better materials out now - like OSB.

Feb 25, 2014, 12:03 AM
Thanks Jay,

I took up the carpet in my hallway tonight, and found that I have 1/2" particle board over 3/8" plywood. It took me about 6 hours to get 120 sq. ft. of the particle board up, so I've sure got my hands full, but I'm glad to be doing it the right way.

From what I'm reading on the DIY site is that I want a minimum 1/2" plywood to fasten my hardwood floors to, so I'm thinking of getting 3/8" plywood to put over my existing subfloor. I'm going to use the glue and screw method, but I'm wondering what kind of glue I should use, and how long should I let it dry?

Feb 25, 2014, 05:01 AM
Do you know how far apart your floor framing is? Most floors are framed 16" OC - but at that 1/2" plywood with a 3/4" particle board overlay is a bare minimum. Unless your framing is set 12" OC or less, your existing 3/8" ply with the 1/2" particle board is really too light-duty.

Some suggestions:

1. To remove the 1/2" particle board - Set your circular saw to exactly 1/2" of depth. Make wall to wall cuts across the entire room in both directions about 18" to 24" apart. This will give you a bunch of smaller squares that will be much easier to pry up. Don't worry about those last few inches where the saw cannot cut close to the wall - you can tear those parts out as you go.

2. You can leave the 3/8" ply in place but it's really not doing much for you. Instead of all the work involved with gluing and screwing another 3/8" layer - install a full 3/4" thick plywood or Advantec layer directly on top of the 3/8" ply. Nail the 3/4" through the 3/8 and into the joists below - basically treat it like that 3/8" layer isn't even there.

It will be easier, cheaper and make for a better subfloor to install the 3/4" layer vs another 3/8". The only glue needed will be typical tubes of subfloor glue - more than enough to just glue in lines directly over the joists. And really the glue is only needed if you nail the subfloor down. Put it down with screws and no glue is needed at all. The nailing / screw pattern on 3/4" is 8" OC and can be limited to just the rows above the joists. That makes for 49 screws per sheet. OTOH, if you add a 3/8" thick layer it will need screws set 4" apart in all directions - that's 325 screws per 4x8 sheet. Lots of work, lots of time and lots of money. Also, a moot point anyway since the existing 3/8" plywood is not thick enough for the screws to really bite well.

So adding a 3/8" layer with glue and screws is a lot of work to give you subfloor that is now 3/4" thick (the minimum for a nail-down hardwood). Installing a new 3/4" layer will make for a much better subfloor with less time and expense to do so.

This message has been edited. Last edited by: Jaybee,

Feb 25, 2014, 09:38 AM
Thanks Jay,

I think I will go with the thicker 3/4" board to reinforce my subfloor. By doing a quick look, and sound test, my joists are 16" apart running perpendicular to the long axis of the house (I have a 1200 sqft ranch). I'll use my circular saw, and hopefully the removal of the rest of the particleboard will go much faster. The good thing is that I'm picking up my floor planks tomorrow, so that gives me at least a week to get the subfloor prepped.

Thanks again.
Feb 26, 2014, 02:39 PM
Excellent info by Jaybee.

Be sure to acclimate the hardwood exactly as the directions tell you and to use a moisture meter on both the hardwood and the subfloor before you begin. I'm concerned the new subfloor could be too wet to cover.

Can you tell us exactly which hardwood you're installing?


Tile 4 You llc - Troy, MI
Kerdi shower specialist - Ditra installs - Product-Method suitability consultation. I have NEVER made a mistake, I thought I did once...........but I was wrong!
Feb 27, 2014, 02:21 AM
I've began laying my new subfloor. I'm putting in a layer of 5/8" over my 3/8", and using standard indoor 2.5 inch wood screws every 6" over the joists. I've got the first 100 sq. ft. in, and wow I can tell a difference on the feel when I walk over it.

I'm planning on picking up my flooring planks tomorrow, and letting them sit inside until at least Sunday, and maybe even Tuesday before installing. I'm getting Bruce 3/4" x 3.25" pre-finished from Lowes. Where can I get a hydrometer from, and can it be rented? If the plywood I'm putting down is too wet, what can I do to dry it out?

I'll try to get pictures up.

Feb 27, 2014, 05:49 AM
You can buy a moisture meter from Lowe's:

If your subfloor is wet/damp the fastest way is to run a dehumidifier in the house. However, even just running the regular heat and leaving the subfloor exposed should dry it out. Once you get a meter you can check it periodically to see if it's dry.

Feb 27, 2014, 11:36 AM
Just a reminder 5/8" OSB is not listed. and don't forget your 15lb. felt..

Subfloor panels should conform to U.S. Voluntary Product
Standard PS1-95, Construction and Industrial Plywood and/or
US Voluntary PS 2-04 and/or Canadian performance standard
CAN/CSA 0325.0-92 Construction Sheathing. Other CSA
standards also apply.
Acceptable Panel Subfloors: Truss/joist spacing will
determine the minimum acceptable thickness of the panel

• On truss/joist spacing of 16” o/c or less the industry
standard for single panel subflooring is nominal 5/8”
19/32”, 15.1 mm) CD Exposure 1 subfloor panels, 4x8
• On truss/joist spacing of more than 16”, up to 19.2”
(488mm) o/c, the standard is nominal ¾” (23/32”, 18.3
mm) T&G CD Exposure 1 Plywood subfloor panels,
(Exposure 1) or nominal ¾” 23/32”, (18.3mm) OSB
Exposure 1 subfloor panels, 4’x8’ sheets, glued and
mechanically fastened.
Feb 27, 2014, 08:36 PM
Hey guys,

Thanks for all the great advice. I picked up my wood planks, and a moister meter tonight. I took several moister readings of my subfloor, and the new hardwood, and they are both reading between 4-5.5% I also bought a few rolls of #15 felt that I am laying down between my 3/8" and 5/8" plywood layers, and I will also lay it down between the plywood and hardwood layers.
Feb 27, 2014, 10:14 PM
5% is perfect. At least that part is OK.

Putting the felt between the two plywood layers is a bit of overkill. Considering that low 5% moisture rating, it's not going to hurt anything but there is no real reason to do it either.. The two plywoods should be similar enough so that there is little expansion / contraction movement between them - even less so if they are screwed together.

Do use the felt between the top plywood and the hardwood. Between the hardwood being in plank form and by being a more dense wood, the felt can eliminate some potential squeaking areas. It will also be a barrier to protect the hardwood from any moisture from underneath which could cause buckling.