Wood Subfloor Waterproofing with Roofing Membrane

[merryweather]

I'll be replacing the subfloor on my 1975 Excella 500. I have seen a number of ideas for painting the plywood with epoxy or polyurethane and then I saw that Airstream uses only an "anti-wicking coat":
https://www.airstream.com/why-airstream/design/process/

One available material that is used for sealing plywood (for roofs) is 1 millimeter thick rubberized roofing membrane like this:
https://gcpat.com/en/solutions/high-...-underlayments

I searched the forum and couldn't find any posts about using this product on an Airstream subfloor. I plan on wrapping it around the plywood edge to protect the cut edge as well. Since this product is intended to seal around penetrating roofing nails, my thought is that it will seal around the fasteners that penetrate the subfloor too.

Is there any reason that I shouldn't do this?

Thanks in advance for your input and ideas.

[rugjenkins]

I have not heard of anyone doing this but it's worth a shot.

[rmkrum]

Consider that the floor will need some way for moisture to get out of it. I would not wrap it 100%.

[mayco]

Retired carpenter here. We refer to it as snow n ice shield and it is primarily used on the bottom 3 feet of roof sheating. I'm not a roofer but Ive watched our subcontractors install it hundreds of times. Self adhesive backing where you peel as you go when installing. Obviously good for water protection and may even prevent some air infiltration at the seams of the sub floor. Were you thinking to install it on both sides of the subfloor or just the top? Could get a bit tricky if you've never worked with it to get it to lay nice and flat with tight corners. But it's made for water protection so it should seal the plywood just like on a roof. I would guess a bit pricey but have no idea how it compares to a liquid product.
Might want to trim your plywood to allow for the membrane. I also wonder how a spray on bed liner would do for the same purpose.

[switz]

The “U” channels cup the edge of the ⅝” plywood all the way around the coach. Thus the membrane could not be inside the channel unless the edge of the plywood was reduced in thickness which would severely weaken the connection between the floor and the body shell.

A truly water proof flooring material thickness has to be sized for that channel.

[DBinSD]

The subfloor debate is long...

I built Custom Homes for 18 years. We had a very solid floor system but, that was in a home.

If your concern is for that much possible moisture from the top perhaps your effort would be better spent on the exterior shell?

If you Ice shield the top of regular Plywood or OSB for that matter. Without a fugal treatment you will eventually rot the top. Not to mention the overlap seams of the shield will drive you nuts trying to smooth out. Additionally, Shield and Tar paper as well, need the heat of the sun to warm the product AND fasteners in order to form the proper seal. You don't have that option.

I would recommend a top quality (#1) marine grade (treated) product. Then if you need a little more protection (if chemicals allow) lay on a layer of epoxy or other sealant. You will then have a quality product that will outlast you for sure (both sides).

Good luck !

Dan

[vintageracer]

Take a look at "Advantech" water resistant subflooring material from JM Huber Company to replace your trailer floor. It is an engineered wood product specifically for flooring in the building trade that is VERY water resistant.

Below are answer to questions about the Advantech Huber product. I have seen demonstrations from home builders who have left this product outside and underwater for several years with no change in the boards. Here is what Huber says about the product to cover their rear as far as water resistance:

"Can I leave AdvanTech panels exposed to the weather?"


"AdvanTech panels carry an Exposure 1 bond classification. This means that they are intended to withstand weather exposure during the construction phase of a project. However, the panels are not intended for permanent exposure to the weather. AdvanTech panels, like most other wood products, are susceptible to decay IF exposed to long-term high moisture conditions. Proper conditioning or ventilation of crawl and attic spaces, as well as correct installation of roof and wall coverings and application of flashing and caulking, will minimize problems due to excessive moisture."


Here is the technical website for the product that includes the above information and other technical information on the product. I have used this product for floor replacement in trailers and found it to be wonderful product for trailer. I also used the same product when we built our home many years ago. That's where I learned about the product.

http://www.huberwood.com/technical-library/15/205/207

[skyguyscott]

You might want to consider Coosa Board, a high density polyurethane foam with reinforced layers of fiberglass, used in the marine industry. The claimed advantages over plywood are:

30% - 40% lighter than plywood
NO water rot or insect infestation
Less than 1% water absorption
Mold, Mildew & Algae resistant
NO special maintenance required
Sanded for easy lamination
Easily fabricated with standard tools
Proven performance on thousands of boats

Main disadvantage is that it is more expensive that plywood.

Here is one link to a vendor:
https://www.boatoutfitters.com/mater...et/coosa-board

To think that a strong, no-mantenance, light weight, water proof material exists that is as easy to work with as plywood seems too good to be true, but there it is, evidently.

[bradt]

I second the Coosa board. It does not need to be wrapped, sealed or treated. You just cut it, install it, and you are done. It is lightweight and so easy to work with.
Coosa Bluewater 26 is the type they recommend for Airstreams.
Brad

[A W Warn]

Originally Posted by merryweather

<< snip>>

One available material that is used for sealing plywood (for roofs) is 1 millimeter thick rubberized roofing membrane like this:
https://gcpat.com/en/solutions/high-...-underlayments

I searched the forum and couldn't find any posts about using this product on an Airstream subfloor. I plan on wrapping it around the plywood edge to protect the cut edge as well. Since this product is intended to seal around penetrating roofing nails, my thought is that it will seal around the fasteners that penetrate the subfloor too.

Is there any reason that I shouldn't do this?

Thanks in advance for your input and ideas.

In my general contracting career I have use tons (literally) of this type of bituthene membrane. It is sold by several manufacturers and has several brand names. I would not recommend it in the application you are considering. Someone already mentioned the extra space it will take. Another reason it will move, bleed out at the edges, when placed in compression under load. If there is movement it is easy to rub a hole in.
I have it in my own house as masonry flashing, window perimeter flashing, and as a roof underlayment. Where it is in the brick joints under compression load, it will occasionally bubble out of the wall through the brick joint in warm weather.

[Garnco]

Our 2016 bambi19fc has marine plywood with the anti wicking painted around the edges with linoleum on top. Our unit is currently in the factory having the floor replaced. A leak got under the linoleum and apparently soaked into the top ply of the plywood. It must have frozen over the winter separating that plywood layer. I can feel the ripples through the linoleum and a pin-tip moisture meter reads maximum everywhere in the area between the shower vanity and bed. I would use the coosa board if I had the opportunity. The shell and plumbing have too many places that can develop a leak over the years that would not be detectable until damage has been done.

[Rzrbrn]

As previously mentioned, research Coosa 20 or 26 (lighter and less weight) Board or one of the other synthetic boards. Others may be a bit cheaper but perform as well or better than Coosa, depending on the application.

Expect to pay $200 to $350 for a 4x8 sheet, although they come in pretty much any size you might want. Just becomes difficult to handle. You may want to check out your locale yacht or boat club. If someone is renovating their boat they may have some left over and sell for a bit less.

There is a raging debate in the Wooden boat forums between Coosa (and similar synthetic board) and plywood.

For boaters, who have their decks exposed to the elements, the Coosa board is fiberglassed or epoxied, just as is the plywood.

While there are issues with the Coosa type boards, more and more people are using it in their boats, new and old.

[Isuzusweet]

As a person that replaced large portions of his sub-floor with Coosa 20, I can attest to its superior attributes.

As for strength, I took a 6" off cut and place it across two saw horses. I then very gingerly took my 6'3" 250lb frame, and stood in the middle. The Coosa bent, but didn't break.....so I started bouncing, slightly at first and then processively harder; the Coosa strip finally cracked, (didn't break), but it took at least twenty bounces to do it.

Coosa 20 is enough, especially if covering with another flooring material like engineered hardwood, (like I did). The marina that sold me the product stated that Coosa 20 is good to over 16" spans with little or no deflection, (I detected no deflections in my floor). I bought 3/4" material as that's all the distributor stocked; I just machined the 3/4 material down at the ends to fit the 5/8" C channel.

Pricey, yes; peace of mind knowing that your floor will never rot, priceless.

Cheers
Sidekick Tony

[CCWilson1]

We are gutting our 20' Safari now and plan to cover our existing (or new if necessary) floor with Denny Foil and Shurtape. (http://www.foustco.com/building.html) I plan to cove it up the side, sealing it with the 3" aluminum tape. We have used the Denny Foil extensively in home applications and it has proven very durable. I'm not sure how much play/movement there is between the subfloor and the walls during transport, but I guess I will find out. We will then lay down a thin hardwood floor with a coved stainless steel or aluminum floor in the bathroom. That way any inside spills or leaks will not pass through to be absorbed by the subfloor. Mold and chemical toxins are a serious problem for my family, so I must do everything possible to avoid hidden/trapped moisture and materials that outgas.