Floor bottom material

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Floor bottom material

Postby JamesDixonLR » Tue Jan 02, 2018 10:13 pm

I've been thinking about the bottom layer of the floor. Thinking of but not committed to a foam sandwich floor. People have had success with that as well as a single sheet of say 3/4" plywood. There is probably no single right answer though many wrong ones.
Anyway, over the course of the evening I've considered the green treated plywood, marine grade plywood (which appears to only come in 3/4" at Lowe's), and even Hardiebacker.

If I go with the 3/4" marine grade I probably won't need to make a sandwich unless it is for insulation.
I don't know if treated lumber is safe in this situation, sure in an open air deck, but what about in a camper? Would arsenic seaping up be a danger? That said even the marine grade needs to be sealed according to Lowe's so if I seal the boards that could eliminate the risk.

Finally the Hardiebacker, it is certainly heavy at 39 pounds per 3'x5' sheet (0.42" thick) but that is not too much more than the weight per square foot as 3/4" plywood and would be very water resistant. Would it be brittle though when a piece of gravel is kicked up?

I'm planning to use Foamular on the walls and roof with 1/4" ply on the outside and 1/8" ply on the inside and have guesstimated that would come to about 350 lbs.

Any thoughts on the subject?

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Re: Floor bottom material

Postby tony.latham » Tue Jan 02, 2018 10:42 pm

Unless you're planning on immersing your teardrop in your local swamp, exterior grade plywood is fine. The difference between marine plywood and CDX or ACX plywood is the wood. The glue is the same. I built a couple of drift boats out of ACX thirty years ago.

That's my opinion --along with my belief that hardiebacker belongs under the tile in your bathroom, not in a teardrop build. Part of my belief is based on wood joinery.

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There are several reliable methods for sealing a plywood floor. Some like to use roofing tar. I prefer to seal with thinned epoxy. The Mix should work fine too.

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Re: Floor bottom material

Postby jsnbergman » Wed Jan 03, 2018 11:12 am

No on the treated, it will discharge gas. Remember unless your dancing on the thing it doesn't need to be that thick. I went 3/4 foam and 1/4 baltic birch. 2 5x5 sheets on a 5x10 frame worked out nice. Some 1x3s and 1x2s to span accross the trailer supports saved some weight over 3/4 ply. I have to move cautiously during construction, usually with a 1/2 sheet of insulation board to disperse the weight if I have to put a knee down but other than that once the mattress is in it will disperse the weight.
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Re: Floor bottom material

Postby Pinstriper » Wed Jan 03, 2018 12:30 pm

You’ll want to seal the bottom surface before installation. Spray-in bed liner should work. The interior surface can be whatever you like. Epoxy sealer, varnish, even exterior floor and deck paint, or even just a stain. Depends on what you’ll put over it, what you’ll be able to see.

Hardiboard or tile backer is pretty fragile stuff, I’d be concerned about screws pulling out due to vibration and the meterial crumbling. It really doesn’t want any dynamic strains like road vibration or even tiny trailer frame flexing.


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Re: Floor bottom material

Postby JamesDixonLR » Wed Jan 03, 2018 8:07 pm

Thanks, everyone. Sounds like I've been over thinking it. That is standard for me. Sounds like a plywood floor is the way to go, birch in particular. Probably a foam sandwich like jsnbergman did. Sealed on the bottom with thinned epoxy or bed liner.
Tony.Latham, what is that "the Mix" you referred to?
Pinstriper, now that I am thinking about it staining and sealing the top surface until it is shiny would certainly look good wouldn't it?
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Re: Floor bottom material

Postby tony.latham » Wed Jan 03, 2018 10:02 pm

Tony.Latham, what is that "the Mix" you referred to?


Do a search on it. It's highly-thinned oil based polyurethane finish. Lots of discussions concerning it's use.

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Re: Floor bottom material

Postby Pinstriper » Thu Jan 04, 2018 2:39 pm

JamesDixonLR wrote:Thanks, everyone. Sounds like I've been over thinking it. That is standard for me. Sounds like a plywood floor is the way to go, birch in particular. Probably a foam sandwich like jsnbergman did. Sealed on the bottom with thinned epoxy or bed liner.
Tony.Latham, what is that "the Mix" you referred to?
Pinstriper, now that I am thinking about it staining and sealing the top surface until it is shiny would certainly look good wouldn't it?


As much as I love good wood finishing, I’m the wrong guy to ask about this. My deck is bare wood with i/o carpet over it, then foam tiles from Harbor Freight over that.



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Re: Floor bottom material

Postby JamesDixonLR » Thu Jan 04, 2018 7:24 pm

I came across "the mix" in another thread today. 75% mineral spirits and 25% polyurethane. By all accounts an excellent sealant and very affordable, the best of both worlds.
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Re: Floor bottom material

Postby swoody126 » Fri Jan 05, 2018 10:25 am

justa a comment based on boatbuilding w/ plywood...

when using materials like ACX(sometimes found in FIR) or BCX(yellow pine) plywood if using fiberglass as an exterior component folks put the LEAST SURFACE on the outside which will be covered w/ the glass/epoxy layer and insulating the knot voids and footballs from the elements

this also leaves the best side to the interior where it can/will be visible to the eye once everything is finished...

if not glassing the best side goes to the outside where water intrusion is most likely and a good paint job will protect the wood w/ less concern about delamination due to the effects of the elements

"marine" plywood has way fewer and smaller irregularities in both the surface plies and particularly the interior plies

"marine fir" is likely to be plagued w/ footballs in all but one exterior layer

"marine" exotics such as Miranti & Okoume($$$$$+) are virtually completely made up out of voidless veneers and offer way less opportunity for delamination due to effects of the elements and/or water intrusion

all that said/realistically BCX-YP is a very sturdy product and if you have help(this stuff weighs a bunch) and are allowed you can find it w/ one smooth/sanded side and the flip side will have very few blemishes that need filling IF YOU PICK YOUR CHOICE FROM A STACK checking 1 sheet at a time(i can manage this task solo when 1/4" stock is my quest) in the big box stores

when picking a stack it is VERY WISE to allow a lot of extra time for NEATLY RE-STACKING your culls so you may be alowed back in the store ;-)

whichever way you go IMHO it is rarely ever wrong to insulate...

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Re: Floor bottom material

Postby breb » Sun Jan 07, 2018 4:48 pm

Hi Jim My floor is a sandwich construction. All ply is baltic birch 1/4" on the bottom 1" poplar frame for core and 1/2" ply for inside floor. because of the 10' length I had to scarf the 5 x 5 panels of ply to get my 10' . The bottom was coated first with 2 coats of the mix. I then used 2 coats of roof seal called Seal Best Duck Coat white. It covers about 70 square feet out of a .9 of a gal. the core was filled with 1" construction foam insulation and topped with a 1/2 ply. Not much going on with my project since it's been hovering around the 0 degree mark for the last 2 1/2 week's in Wisconsin. my TD is a 1/2 done and the discussion of inside flooring came up while I was shopping with wife in a box store over the weekend. I think I've decided to sand the inside baltic birch ply and coat clear and be done with it. I won't be looking at it unless I pull up the mattress .143864143860
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