Could use some guidance with wrapping PMF underneath

[yuppers]

Hey guys. I’m building a foamie trailer and I’m at the point where I need to figure out the details regarding the sidewall and floor PMF interface. I know you’re supposed to wrap it underneath and overlap by about 4 inches but I don’t know if I should seal the plywood first and then wrap the PMF or wrap the PMF and then seal the plywood.

 

[yuppers]

I was also wondering if could just PMF the bottom of my trailer too as I feel that could solve all my problems but I want to double check before I commit.

 

[GPW]

Yuppers ,Previously we wrapped the PMF under the edge , which in our humidity( morning dew) quickly rotted out ... Since then , we use a "drip edge" to channel all the water down and away from the underneath ...preventing any future rot or problems . . Something like this ...

If using a plywood floor best seal the ply THOROUGHLY with something like "the Mix"' ...

 

[tomhawk]

I did a variation on GPWs scheme. I did not like the idea of stooping down to glue flaps of canvas under the floor of the cabin; I am too old for that. Instead, I flipped the floor over and glued an 8" wide band of canvas around the bottom edge of the floor. Since the surface was facing up I could easily see what I was doing and did not have to glue anything upside down.

Then, when installing the walls, I wrapped the free edge of the canvas band onto the bottom of the wall and glued it. The bottom flap of canvas covering the wall was glued onto the wrapped canvas to provide a strong bond. I have towed my trailer many hours at 70 MPH. These walls are attached.

For illustration of this process see:
viewtopic.php?f=55&t=75248&p=1296691#p1296691

Tom

 

[ghcoe]

I lap the canvas 2" under the floor and glue it to the wood directly. Once it is dry, I paint the bottom of the floor with paint. I usually just roll the body onto it's side for this process to make it easy.

When I get the body onto the trailer, I add a drip edge using drywall end bead. I simply just screw it onto the bottom with pan screws. I start the screws, but leave a bit of a gap between the floor and the beading. then I add some Dap caulk between the floor and beading and then finish tightening the screw down. Clean up what caulk squishes out on the front and back. Then paint the same color as the body.

 

[Lizzie Bug]

This is my biggest area of “not understanding”!
I have read different posts and thought I had grasped it, but then I get lost again.
I should change my name to “XplainLike5” because that is how I need people to explain these kinds of things to me. (And I dream of making a Foamie. Go figure! )
Okay, so there is the metal of the trailer, and then presumably a wood floor? Do I wrap the canvas all the way under the metal of the trailer itself? Does the same glue hold the canvas to metal? If so, is that the only way that the Foamie is attached to the trailer, or do I need to bolt it somewhere? If so, where and how?
Or do I wrap the canvas under a plywood floor, then lag bolt that canvas-wrapped-plywood-floor to the metal of the trailer? If so, how do I waterproof those screwed-in places? Should I put something on the canvas that is now exposed to the road under the trailer? Should I paint or mineral oil soak the plywood? Will the glue for the canvas stick well to a painted or mineral-soaked plywood floor?

 

[Lizzie Bug]

Yuppers ,Previously we wrapped the PMF under the edge , which in our humidity( morning dew) quickly rotted out ... Since then , we use a "drip edge" to channel all the water down and away from the underneath ...preventing any future rot or problems . . Something like this ...

If using a plywood floor best seal the ply THOROUGHLY with something like "the Mix"' ...

Thank you for the illustrations!
I LOVE visuals!
What I understand from this picture is that you have the foam (blue)extending past the plywood floor on the bottom of the trailer. Is this correct?
And then you have the canvas (“outer skin”) extending by itself past the foam. Is that correct?
Is that canvas and foam painted or otherwise protected?
In the picture, is there something else attached where it is labeled “drip edge”?

 

[Lizzie Bug]

I lap the canvas 2" under the floor and glue it to the wood directly. Once it is dry, I paint the bottom of the floor with paint. I usually just roll the body onto it's side for this process to make it easy.

When I get the body onto the trailer, I add a drip edge using drywall end bead. I simply just screw it onto the bottom with pan screws. I start the screws, but leave a bit of a gap between the floor and the beading. then I add some Dap caulk between the floor and beading and then finish tightening the screw down. Clean up what caulk squishes out on the front and back. Then paint the same color as the body.

I looked up “drywall end bead”. There were several different things that popped up that were all called that.
Is there any way that you could post a picture of exactly what you used?
And what part did you put the screws into?
Did you screw it to the side at the bottom or on the actual underside of the trailer?

 

[Pmullen503]

If the walls end flush with the bottom of a plywood floor (unlike the drawing above) wrap the PMF from the side wall around the bottom of the wall and at least 2" inches onto the plywood floor.

If your walls more or less sit on top of the frame, you should unbolt the cab from the frame, lift it a couple inches (or move it sideways) and wrap the canvas under and glue it to the floor plywood using whatever you glue you used to adhere it to the side wall. It's a good idea to give it a few coats of paint (at least the part that's in contact with the frame) at this point while it's off the frame. Raw canvas will rot, soaking it with paint is important.

It's a pain if you didn't plan for it but the canvas is what holds the foam walls to the floor. The foam is weak and can break just beyond the glue line.

Don't glue the canvas to the metal frame.

 

[philpom]

I think the absolute best solution for a foamy would be to fully cover the bottom just like you would do the roof.

 

[Pmullen503]

I think the absolute best solution for a foamy would be to fully cover the bottom just like you would do the roof.

That certainly wouldn't hurt.

My 13 Y.O. foamy has logged thousands of miles with just roofing tar on the bottom and it still looks great.

But wrapping the canvas (or fiberglass) around the bottom edge is what holds the cab to the floor. It's not just for waterproofing or aesthetics.

 

[Lizzie Bug]

Thank you very much!
Should I pre-paint or pre-mineral soak the plywood floor before I glue the canvas to it, then paint it all again after I glue the canvas to it? Or, use roofing tar?
With the roofing tar, did you put it on the bottom of the canvassed plywood and then bolt it to the frame, or did you bolt the canvassed plywood floor to the frame and then tar it? Did you put tar on the metal as well? Have you had to reapply the tar?
Did you do anything special to where you bolted the floor to the trailer to waterproof the holes?
Did you do any special kind of “drip edge” system?

 

[Lizzie Bug]

I just reread and reread your post and realized you said to paint the whole bottom after you overlap and glue the canvas to the plywood, before you bolt it to the trailer.
The rest of my questions still stand.

Oh! And does the foam wall sit on top of the plywood floor, or does it get glued to the edge of the plywood floor?
Either way, should the plywood be raw to assure adhesion of the foam and plywood? Would any paint or mineral oil affect the adhesion?

 

[Lizzie Bug]

Is Figure A or Figure B the correct placement to glue the foam onto the plywood?

 

[Pmullen503]

B is much easier to build but you end up with only 44" inside if you are trying to stick with standard dimension materials.

As far as treating the plywood, I believe that sealing the edges of the plywood with "the mix" (thinned polyurethane) is a good idea.

On mine, I brushed on thinned roofing tar, leaving a 3" band around the underside of the floor of bare plywood to adhere the canvas to. Then painted it with several coats of paint after adhering the canvas. Then bolted it down, no tar coating.