sub-floor

[jimmy]

Is it true that we need to replace the particle board sub-floor in order to install a hard floor? ply wood sub floor will be much better? give some advice please.thanks.

[jadnashua]

Don't confuse particle board with OSB. Particle board is essentially ground up wood dust, glued together. OSB is much larger wood chips glued together. A quality OSB can be used as a subfloor for many things. Particle board is useless as a subfloor as it has no strength and will fall apart. It also swells up like a sponge if it gets wet. Less than quality OSB can swell up and delaminate, too. If yours is OSB, and it is in good condition, you are probably okay. If it is particle board, trash it, replace it with some good plywood, and go for it.

It makes a difference exactly what you are planning to install over it.

[Nornee]

I am planning to reno 2 upstairs bathrooms, including installing floor tile in both and a tile shower in one. The sub floor appears to be 1/2" OSB. Is this sufficient?

Thanks so much!

[cacher_chick]

1/2" OSB by itself is not an appropriate subfloor for ANY flooring material.

[Terry]

I like to use ACX plywood.

[jadnashua]

Assuming 16" OC joists, 5/8" osb or plywood is the minimum that meets current USA codes as I understand it. Now, keep in mind what they call 5/8" is no longer the full 5/8" as it was maybe 20 years ago or so. Personally, I can feel the flex in-between the joists on 5/8" stuff and prefer 3/4". If you have larger spacing on your joists, then you'd probably want at least the 3/4" sheets. Preferably T&G, as the joints along the short edge, especially on the thinner stuff, will rub against each other. I've not looked for 5/8" T&G, and even if it was available, not sure if the tongue would be strong enough to really do anything...choose 3/4" (or thicker) if you can. Ideally, the stuff is installed with construction adhesive on the joists and ring-shank nails or screws.

[Nornee]

My mistake...it's actually 1/2" OSB on top of what looks like 3/4 OSB...so this should be fine for tiling?

[jadnashua]

Probably...but, there are two components of deflection that must be met: the stiffness of the subflooring AND the stiffness of the joists - they are two different things and both must be adequate to ensure tile will survive. Also note that, ideally, the second layer is offset from the first, still runs across the joists (like the first layer) and is fastened well. If all those things are good, the subfloor (assuming joist deflection is good) is enough subfloor to install stone tile (ceramic can handle a single layer, but two is always better).

While there is an approved technique that would allow you to install tile directly to the two layers of ply (if they are properly installed), you are probably better off using either cbu or a decoupling membrane on top of it. The thinnest would be something like Ditra from www.schluter.com. CBU would be over twice as thick (1/4" cbu is the thinnest you can buy, then add a layer of thinset underneath it). For expert help on tiling things, check out www.johnbridge.com.