Sub-floor - How thick?

[Darren S W]

Hello; I'm at wits end here I am trying to replace the sub-floor in my 1979 PP it was completely rotten when I picked it up and had very little floor left. It was OSB strand (I think its called) I could see the fiberglass on 85 percent of the exposed floor, what was under the front bench was so swollen (about 2 inches thick) and soaked with water it scraped right off. It was the same under the back setting area. The gap on the bottom of the back seating area measures just about 3/4 of an inch (5/8ths). But on the front bench it measures anywhere from a 1/4 to 1-1/4 inch. (were there 2 different sized floors? 3/4 On the upper and 1/4 on the lower?) Where the measurement is widest I assume its from the floor sagging, but where it measures smallest, is by the front bench nearest the stove cabinet. The floor does not sag at all there. If anyone has ever put in a new sub-floor please help me out here.

Questions:
1) How thick and what type of wood (I do not care for the OS if plywood what grade?
2) How do I secure it to the fiberglass floor? I read some place to use fiberglass resin?
3) Should this be done while the trailer is level or does it matter? I was thinking that when I do install the new floor I was going to put a jack under the trailer with a 18x18 piece of wood against the underside of the floor to raise the sag up any other ideas would be appreciated.

[Robert Johans]

Hi Darren,

If you can remove the FG furniture modules, get rid of every bit of subfloor above the fiberglass belly. I found mine to be affixed with resin, but given how you've described the condition of yours, I'm guessing you'll be able to pull it off relatively easily. A "multi-tool" like the Fein MultiMaster will make the job go even quicker. In any case, get rid of all the old material, and sand away residual resin or debris as necessary.

Ironically, because the PP is built with two fully molded "hemispheres"—the bottom shell creates a bathtub, trapping any water inside. (It's possible that the added weight of water and the soggy subfloor in yours distorted the fiberglass belly over the years.) So as you make your repairs, you must ensure that there are no leaks from above or penetrations from below. There should be 8-10 fasteners holding your shell onto your frame. These are no doubt rusted beyond recognition but you'll want to cut any remnants of these away. (Obviously, once free, you have the option of jacking the shell up and off the frame. The frame is most likely to be pretty rusty too.)

I would use a decent 5/8" plywood throughout to replace your original floor. (Definitely, jack up the sagging underbelly to make it as flat as possible.) When I do a restoration, after any necessary repairs and a thorough prep, I coat the fiberglass interior with fresh resin or a two-part epoxy paint. I do the same thing to the replacement floor, coating all sides and the edges. (This is, admittedly, a bit of overkill since presumably there's no chance of water saturating the floor again. Right?)

If you manage to get the FG belly flat enough, you can use a good adhesive (I've had luck with Liquid Nails, for example) and glue your plywood down to the FG. Your 8-10 replacement hold-down fasteners should be enough to squeeze everything together. (You can coat your newly drilled holes with resin or epoxy before winding.) If you can't eliminate the sag, you may have to run screws from below, up through the FG and into your floor, to make things snug. Of course, you'll want to seal these holes and use stainless fasteners. If you are really ambitious, you could now "glass in" the floor along the edges to the shell walls.

Once your new subfloor is in and secure, it should be ready to re-install the furniture modules, as well as your new top flooring material.

RJ

[peterh]

Robert, what do you think of using MDO (medium density overlay) plywood, which is plywood that has a solid epoxy-fiber layer bonded to one (or both) sides? It's used extensively for products like traffic signs that are left outside and has been suggested as a lighter-weight alternative to marine plywood.

Also, what do you think of cutting out the bottom of the "bathtub" when you replace the floor?

[Robert Johans]

Robert, what do you think of using MDO (medium density overlay) plywood, which is plywood that has a solid epoxy-fiber layer bonded to one (or both) sides? It's used extensively for products like traffic signs that are left outside and has been suggested as a lighter-weight alternative to marine plywood.

Also, what do you think of cutting out the bottom of the "bathtub" when you replace the floor?

Hi Peter,

I think using MDO is a [b]great idea. It's engineered for moisture remediation, extremely stable and nearly dead-flat in all thicknesses. However, it too performs best (that is, resists moisture) when coated with paint, epoxy or resin. Certainly along any exposed edge. I found a couple online resources describing the product for those unfamiliar with it.

http://www.swansongroupinc.com/pdf/TruPaint-MDO.pdf
http://www.alleghenyplywood.com/index.php?...5&Itemid=72

I have some trepidation as to "cutting out the bottom." I'm no engineer, but for those eggs that were built using the "monocoque" style, I'd guess that the structural integrity of the mold could somehow be compromised by something that radical. Having said this, I know that my Scamp has a glassed-in plywood floor and no "bathtub" fiberglass belly. So I suppose, if there was a good enough reason, one could remove the FG belly — but only if it could be "rebuilt" in a way as to recover any potential lost structural strength.

RJ