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We want to install either laminate or hardwood flooring throughout most of our home. The home is about 35 years old. I pulled up all the carpet and found 3/4" particle board over 1/2" plywood board subfloor underneath the carpet. It is particle board and not OSB. The particle board does not run under the walls. The walls rest on the 1/2" wood board and the joists. Since carpet was laid over the particle board they were not so concerned about one board being level with the next so they are uneven. We've been told it was OK to put flooring over the particle board if we level it up and by another to remove the particle board and replace with 3/4" plywood. So I'm seeking a confirmation of one or the other or another suggestion.

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So, the builders used a 1/2 inch subfloor? WTF! –  Gunner Oct 10 '12 at 22:41
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2 Answers

Particle board is not an appropriate underlayment for hardwood of any type. In fact, it's hardly acceptable for most carpets anymore.

Here is Jeff Hosking on the subject:
http://www.hoskinghardwood.com/Department/Hardwood-Floors/All-About-Subfloors--installing-flooring-over-various-sub-floors.aspx?dId=7&pageId=2

Particle board underlayment Looks like oatmeal and consists of tiny wood particles glued and pressed into 4 x 8 foot sheets. It is NOT suitable to use as a Structural subfloor material. I have only included Particle Board in this article because a lot of people see it when they remove old carpet and think it is a structural subfloor material like plywood or OSB. Particle board is NOT a structural subfloor material and is only manufactured to be used as a cheap underlayment filler sheet on top of the main structural subfloor material to raise the height of carpet to a desired level. Particle board can absorb excessive moisture is very brittle and does not hold nails very well. If this type of underlayment is present, it will have to be removed when planning to install a nail or glue down hardwood floor.

Are you certain you have 3/4" particle board? That is a strange material to sit atop 1/2" ply.

Furthermore, 1/2" ply is not suitable as a subfloor....

Do you know your floor joist spacing? I would suggest that you get t&g ply at an appropriate thickness for your joist spacing. I would say 1/2" minimum depending on your spacing.

I would glue this to the existing subfloor, screw into the joists, and nail the entire field.

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I think there might be an error - you say "1/2" ply is not suitable as a subfloor" and then " I would say 1/2" minimum depending on your spacing." I imagine the second one should be 3/4"? –  Steven Oct 11 '12 at 16:52
    
@Steven, when I say "1/2" minimum" I'm talking about supplementing the existing (potentially inadequate) subfloor. When laminated to the existing boards, bringing the thickness to about an inch. –  Matthew Oct 11 '12 at 17:08
    
We were told that at the time the house was built 1/2" subfloor was code in the area. the joist are 2" x 10" on 16" centers. It is partical board not OSB, we had to remove it in a closet were the water heater had leaked. It was like mush. –  M Lee Oct 13 '12 at 23:20
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1/2" subfloor is definitely insufficient, so I get why it was reinforced.

3/4" particle board... that is not structural. It's paper dust and glue. A little water and it loses all cohesion.

Simply for peace of mind, if it were me, I'd remove the particle board and put down either OSB or plywood. Probably OSB, only because it's a little stronger (not much) but it's also more expensive so if budget were a concern, I'd go with equal thickness plywood. Either will do you.

But the particle board? Get rid! Its probably not gonna fall apart on you, unless you have a spill... but why even keep that worry?

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