# Outdoor plywood for RV subfloor



## willhime (Mar 9, 2014)

I'm renovating a 1960 Avion rv trailer into a tiny home for a friend. After pulling up the almost completely disintegrated edged plywood from the subfloor there are several options I'm considering. A high moisture resistant plywood like radiata pine, which runs about $56 per sheet. And I need 6 sheets. I'd cut to fit then take it and finish it all with either waterproofing poly or several layers of paint. Another idea I had was to cut 3/4 1×6 or 1×4 cedar and use it as sort of subfloor shiplap for the perimeter where most of the rot was then fill in the middle with plywood. An even cheaper approach was considering OSB as subfloor due to its moisture resistent nature although this is my least favorite option. Are these smart approaches or naive ones ?


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## John Smith_inFL (Dec 15, 2017)

*OSB,* contrary to popular belief, is NOT moisture proof
OSB is a roofing sheathing - which is covered with tar paper or
rubber membrane and shingles - that is the waterproofing.
what part of the country are you in ??
I don't know how far out Georgia Pacific ships their plywood,
but, they make a really good exterior B/C plywood which
will be very adequate when PRIMED and PAINTED within the
waterproofing regiment of application.
even the *MDO* (Medium Density Overlay) plywood would be a good choice.
(not to be confused with MDF, OSB or Particle Board).
some homemade campers that I have seen used .030" aluminum sheet
as the vapor barrier between the frame and the wood floor.
sounds like a fun project !!


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## Andybb (Sep 30, 2016)

Why is OSB your least favorite option? The thing is 60 years old so unless you're "restoring" it OSB should do the trick. OSB isn't really waterproof. I sometimes use Thompsons or some other inexpensive water proofing as an additional barrier.


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## willhime (Mar 9, 2014)

I'm in Austin, Tx. I'd like idea of MDO but it's $155 per sheet. I think I've seen Georgia pacific around so maybe that's a good choice.

OSB is my least favorite just because I'm not sure of its longevity properties especially as use for something as high traffic as a floor. I guess most of the apprehension with it comes from the fact that I've never heard of anyone using it so I figured there's a reason for that. And I guess the term restoring would apply. I've had to rip out every single piece of installation so now all it is is an aluminum shell with an iron floor chassis inside.


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## John Smith_inFL (Dec 15, 2017)

I think you mean $55.00. here in Central Florida it is around the same price.
I don't know how many companies are making MDO these days but the one
I use for making exterior signs is Douglas Fir with waterproof glue with almost
zero voids. there is not a mfg. mark on it so I don't know who makes it.
it is pre-primed with white latex primer and very easy to work with.
yes - there is a reason people don't use OSB for a lot of things.
think about it - OSB is "flake board" = big flakes laminated together
leaving lots and LOTS of voids and air tunnels. . . . it is not designed
for anything structural other than roof sheathing applications.
when I bought this house last year, it had a 10×10' metal shed in the back
that had P/T 2×4 joist braces with OSB floor . . . the shed is watertight with no leaks
and the OSB was laying on the ground like a wet carpet draped over the floor joists.
anyone that uses OSB for anything other than roof sheathing does so at their own risk.
why spend time and money weatherproofing something that was not designed to be weatherproof.
the vintage Avion and Airstreams are my favorite campers !!!
really looking forward to your finished project.


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## JCamp (Nov 22, 2016)

I recommend you totally avoid using OSB. It is not made to be drenched with mud or water. Even if it does last a few years it still swells pretty bad when it's soaked. I recommend a marine grade playwood. My great grandfather use to use it to build boats way back when. It's made to withstand water contact. Even with the boats tho they sealed it with fiberglass. You probably won't do that but as you already mentioned any wood must be sealed well. Even treated lumber will eventually rot if exposed to the elements long enough. I don't recommend cedar either. It'll absorb water as well. Anything that absorbs water will rot and/or mildew. If your doing it then do it RIGHT and do it ONCE


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## woodbutcherbynight (Oct 21, 2011)

What about Durock,,the concrete backer board they use for bathrooms? You lay it down, seal the edges and lay tar paper over it and install any flooring you want?


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## MrRon (Jul 9, 2009)

MDO is used for making signs, but that doesn't mean it is completely water proof. It is more water "resistant" due to it's resin overlay. The raw edges are what will cause water to be absorbed, so if using MDO, the edges have to be well sealed. If you do that, it should work OK for your trailer floor. Another option is marine grade plywood, but it is very expensive. If you choose to use MDO, there are 2 types available. One has a resin overlay on one side only and is used for concrete forms. The other has the resin overlay on both sides and that is the one that is used for highway signs. Here is a link to a waterproof MDF. http://www.sierrapine.com/index.php?pid=23


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## dhazelton (Feb 11, 2012)

^ Durock has zero strength - step on it if it were laid over those cross members and your foot would go through.

I have to believe that any plywood well sealed would hold up. Especially if this trailer will just sit and you can get a vapor barrier under it.


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## Tony_S (Dec 16, 2009)

> *OSB,* contrary to popular belief, is NOT moisture proof
> 
> - John Smith





> it is not designed
> for anything structural other than roof sheathing applications.
> 
> - John Smith


Not even remotely true John.
I guess a lot has changed since the last time you looked into OSB
OSB is used in hundreds of thousands of homes in applications other than roof sheathing including structural purposes. And it isn't water PROOF, no(but neither is exterior grade plywood), but it is highly water resistant as it's normally made with water proof plastic resin adhesives.
It has it's place, and this would be an ideal place IMO.

https://www.apawood.org/osb

http://www.tolko.com/products/osb/residential-flooring

https://www.weyerhaeuser.com/woodproducts/osb-panels/osb/

https://www.buildgp.com/plywood-osb


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## John Smith_inFL (Dec 15, 2017)

good information, Tony. (thank you for taking the time to find it and share).
but, working in the residential renovation trade for a few years,
I have pulled more rotten OSB out of structures than I have used.
I will admit that I am hard to change my mind when I am experiencing
the same issues with materials exhibiting the same symptoms with moisture
related issues over and over.
sort of like people wanting to use Helmsman Spar Varnish for exterior use
just because the manufacturer says it is highly UV and weather resistant.
here is a photo of some OSB that I used under the carpet in my work van.
I did not know there was a small door gasket leak that allowed rain water to run
under the OSB that is only 3 years old. I used it only because it was left over
from a roofing job. so tomorrow I will be replacing the door gasket and removing
a well rotted OSB subfloor - - - - I will not use OSB for anything ever again except 
for roofing. and I still stand by my statement:
"anyone that uses OSB for anything other than roof sheathing does so at their own risk." 
including myself.
again - you found great information for anyone wanting to make their own informed decisions.




























.


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## rwe2156 (May 7, 2014)

OSB is definitely not a good choice if for no other reson it is not rigid enough for a floor unless you double layer.

I would go with 3/4 ply or ideally, Advantek. Its waterproof and T&G and reasonable priced.

If the bottom of the floor is exposed, I would lay down a sheet of vinyl flooring, or very thick liner material first as a water barrier.


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## GR8HUNTER (Jun 13, 2016)

get marine grade and pay the bill then you will only have to replace it 1 time and it will outlast your life :<))


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## Tony_S (Dec 16, 2009)

.


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## bondogaposis (Dec 18, 2011)

If it was mine I'd use marine grade plywood.


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## 000 (Dec 9, 2015)

> .
> 
> - Tony_S


LOL, I agree.


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## Tony_S (Dec 16, 2009)

> .
> 
> - Tony_S
> 
> ...


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## r33tc0w (Jan 27, 2017)

Look for a sign supplier in your area, Reece Supply, grimco, Piedmont - there is a product called Omega Signboard. We generally seal the edges at our shop, I believe they have a 1/4" version if you want something thinner than 1/2"


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## tadmaz (Jul 18, 2018)

Another vote for marine plywood.


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## MrRon (Jul 9, 2009)

There is a big difference between waterproof and water resistant. Marine plywood, although used for boats, is not waterproof. No plywood is waterproof It is water resistant. Sealing it with paint makes it somewhat watertight, but left unsealed, it will fail. I think the MDO or ordinary exterior grade plywood is good as long as it is sealed. It is being used for flooring in a trailer, so it won't be exposed to the direct weather. Towing in wet weather would expose the floor to water and that's where you need to seal it. Avion is a top of the line trailer, so my choice would be 2-sided MDO or marine.


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## ArtMann (Mar 1, 2016)

Most new travel trailers are floored with OSB. It may not be good enough for you or the guys here, but it is good enough for modern manufacturers. You must seal the under side against water but that is true of any flooring. It says something about the material that probably over 95% of crawl space houses where I live have a 3/4" OSB sub floor.


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