# Plywood sheathing vs subfloor vs underlayment



## acerunner

What's the difference? I see plywood available specified as either sheathing, subfloor or underlayment. But other than thickness, I don't see a difference. And even then, some thicknesses overlap in the categories.


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## oh'mike

Google Plywoods grade The explanations there are good and easy to understand

Exposure 1 (or X for exterior) means water resistant glue--that's key for your bath floor

You don't want the wood delaminating.

The letters have to do with the quality of the faces--D-ugly with open knots--C less ugly-patched knots--B not bad A--smooth and sanded

The other numbers are for the core---weather voids are in there or not--number of plys.

3/4" CDX tongue and groove is the most common subfloor---This is not the preferred subfloor for tile--it has open knots and some voids--BCX is the better choice---

I've never had an issue with CDX---it what the builders use most--

Google ---Mike---


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## Just Bill

What he said, but to be less specific........sheathing and subfloor are generally the same stuff, CDX(1 side C/1 side D/exterior grade glue). OSB is more common now than plywood, less expensive. Subflooring is usually T&G, sheathing is not. Underlayment bridges the large imperfections of subflooring, as a prep for finish flooring.


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## acerunner

so underlayment would typically be the thinner stuff?

i asked in another thread about laying hardwood flooring parallel to plank t&g subflooring (although that makes it perpendicular to joists), and was told I need at least an additional 1/2" subfloor. So I went to HD and they had the 3 types of plywood. not sure which to use. Almost sounds like underlayment is what I need since the existing plank is the true structural support...


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## oh'mike

You need structure--in this case --nail holding ability--1/2" BC will support and hold nails for that wood floor.


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## Gary in WA

http://bct.nrc.umass.edu/index.php/publications/by-title/wood-underlayments-for-resilient-flooring/

http://www.apawood.org/pablog/index...tion-of-Plywood-Underlayment-in-Floor-Systems

http://www.apa-europe.org/Languages/English/PDF/R340G.pdf

Gary


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## joeyr

Very easy, 

sub floor is always tongue and groove and minimum 5/8's (in our days at least) 

sheathing is plywood that butts together with no groove and minimum thickness for walls is 3/8's

underlay is a product to deaden sound under laminate flooring..

hope that helps


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