Looking for waterproof alternatives to OSB for subfloors

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SUMMARY

The discussion focuses on alternatives to oriented strand board (OSB) for subfloors in RVs, particularly in wet conditions. Participants recommend using pressure-treated Southern Yellow Pine plywood for its durability against decay and rot, as well as marine plywood for its water resistance. Starboard, a high-density polyethylene (HDPE) material, is also suggested but noted for its high cost. Additionally, moisture barrier fabric is mentioned as a cost-effective solution for protecting the underside of the flooring.

PREREQUISITES
  • Understanding of RV flooring materials and their properties
  • Knowledge of pressure-treated plywood and its applications
  • Familiarity with marine plywood specifications
  • Awareness of moisture barrier fabrics and their uses
NEXT STEPS
  • Research the properties and applications of pressure-treated Southern Yellow Pine plywood
  • Investigate the specifications and benefits of marine plywood for RV use
  • Explore the cost and performance of Starboard as a flooring alternative
  • Learn about moisture barrier fabrics and their installation techniques
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RV owners, builders, and maintenance professionals looking to improve subfloor durability and water resistance in recreational vehicles.

johnsonte1
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TL;DR
OSB is used for rv subflooring, when road conditions are wet water kicks up from the wheels to exposed OSB, over time this makes the OSB sag.
OSB is used as a subfloor in my RV, it's supported by a steel frame but the bottom of the OSB is exposed. The floor is sagging and I believe it's from repeated towing in wet conditions. I think the splash from the wheels is kicking water up at the underside.

I'm considering my options for waterproofing short of adding a waterproofing layer below the OSB. I would prefer to replace the OSB with something lighter and waterproof (weight is worth consideration because of towing capacity). I would appreciate any help in finding the right materials.
 
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Search pressure treated plywood. Here is a quote from one of the hits:

Pressure-Treated Southern Yellow Pine meets the highest grading standards for strength and appearance. Treated for protection against fungal decay, rot and termites, it is ideal for ground contact and a variety of general uses including exposed structures, sill plates, decks, docks, ramps and other outdoor applications. Responsibly manufactured from renewable southern yellow pine, this plywood is both safe and environmentally friendly when used as directed in appropriate applications.

It's also a good idea to paint the plywood. I once built a plywood boat using standard grade AC plywood. I applied a sealer (don't remember exactly what), a primer coat, and a topcoat. That boat was stored outside for nine years, at which time the wood was still in perfect condition. Except for the unpainted area inside the dagger board well.
 
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Pressure treated plywood would be a good upgrade, thank you for the suggestion. Funny I was thinking in terms of boats too, but along the lines of fiberglass and foam. Just brainstorming
 
Marine plywood is a specific grade, not necessarily identical to pressure treated wood. There is also "water resistant" plywood, that is still not the same as marine plywood.

There is also a plastic board material called Starboard that would probably fix your problems, but it is very expensive.

A moisture barrier fabric, tear and puncture resistant, may be a less expensive solution. This vendor offers several brands. https://www.mcmaster.com/moisture-barrier-fabric/
 
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OSB = oriented strand board
 
Cool! Looks like Starboard is a sheet of hdpe, it would cost over $1000 for a 4x8 sheet that is 3/4 in thick. Very pricey, it would be a bit heavier than OSB, but it would definitely solve the problem. I wonder if it's rigid enough to reduce the thickness to bring the price and weight down.
 
johnsonte1 said:
Cool! Looks like Starboard is a sheet of hdpe, it would cost over $1000 for a 4x8 sheet that is 3/4 in thick. Very pricey, it would be a bit heavier than OSB, but it would definitely solve the problem. I wonder if it's rigid enough to reduce the thickness to bring the price and weight down.
Maybe you could get away with 1/2 inch thick board, but maybe not. Here is the website with physical properties. What does the steel frame under your RV look like? I'm wondering how big a distance between steel supports that the floor must span.

https://www.kingplastic.com/products/king-starboard/

Maybe a mechanical engineers can tell us how thick a HDPE floor should be.
 
  • #10
Sounds like marine plywood is the way to go, thank you folks!
 
  • #11
johnsonte1 said:
Sounds like marine plywood is the way to go, thank you folks!
I get price, weight and performance advantages from Formply over ordinary marine ply. "A durable A-Type Marine bond, Specrite Formply can withstand the rigours of construction moisture. Its hard phenolic surface and high-quality outer veneers make it durable and reusable".
 
  • #12
Baluncore said:
Its hard phenolic surface and high-quality outer veneers make it durable and reusable
That phenolic surface would be somewhat brittle. Probably best to avoid it for that high vibration and flexing environment.
 
  • #13
Tom.G said:
That phenolic surface would be somewhat brittle.
FUD. I have never had a problem, and I am pretty rough on it. Since the surface of the ply is impregnated with phenolic before it is baked in a press, I would expect it to be tougher than ordinary plywood.
 
  • #14
(painted) marine ply is good for constant immersion, that's a bit over the top, and replacing the floor of an RV in situ is not going to be easy. I'd just paint it with bituminous paint.
 
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  • #15
pbuk said:
(painted) marine ply is good for constant immersion, that's a bit over the top, and replacing the floor of an RV in situ is not going to be easy. I'd just paint it with bituminous paint.

That works only if the existing floor is not already in bad condition.

I've seen a professional crew replace a rotted 12x10 section of RV floor in less than 2 hours.
 
  • #16
anorlunda said:
That works only if the existing floor is not already in bad condition.
Nah, the rubber will hold it together :wink:

anorlunda said:
I've seen a professional crew replace a rotted 12x10 section of RV floor in less than 2 hours.
I was imagining all the partitioning, furniture and plumbing mounted to it making this a much bigger job.
 
  • #17
pbuk said:
I was imagining all the partitioning, furniture and plumbing mounted to it making this a much bigger job.
It depends on which section. Floor under the "living room" is easier than floor under "the kitchen".

I can't fully explain it, but RV flooring problems tend to appear more on either end, not in the fore-aft middle, and many RV's place the kitchen in the middle.
 
  • #18
pbuk said:
(painted) marine ply is good for constant immersion, that's a bit over the top, and replacing the floor of an RV in situ is not going to be easy. I'd just paint it with bituminous paint.
I like this solution, tbh the condition of the floor isn't horrible. I've gutted the thing down to the floors once already, and it does take a while. I'm sure a team of professionals could make quick work of it, but I'm just one dude.
 

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