Plywood camper fabrication questions (thinking of idaSho)

[ReluctantTraveler]

@IdaSHO One thing that occurred to me today was around load bearing at the joints.

With your current build, the welded steel frame carries a majority of the load, and it looks solid as heck. With an all-wood frame (except the jacks and corner bracing), I'd be concerned about weight on the bottom corners.

Would the wooden frame look identical to the steel one?

I'm assuming you'd instead need two parallel 2x4's running the length, with cross sections lagged in or something? I'm having a really tough time visualizing how you'd frame this in a way that can actually support the load.

I could be overthinking this.

 

[ReluctantTraveler]

Or is this a situation where the plywood and glued-in foam insulation add enough structure that you're ok?

 

[rruff]

I'm having a really tough time visualizing how you'd frame this in a way that can actually support the load.

You are better off thinking of the wood as core reinforcement and hard points for attaching stuff... not framing. The plywood is the structure, and the core keeps the plywood skins rigidly in place. It will be very strong and stiff if you do it right. You can build it like a house if you want, but it will be weaker/heavier/less stiff. But that would probably hold up fine too, if you do it well.

When I built one like this I used cheap 2.7mm Luan ply with a layer of fiberglass on the outside, and it was like a brick shanty. Ply is great when it's reinforced with a hard layer that prevents penetrations and crushing.

You planning on using XPS foam? Big camper on a F550?

 

[ReluctantTraveler]

You are better off thinking of the wood as core reinforcement and hard points for attaching stuff... not framing. The plywood is the structure, and the core keeps the plywood skins rigidly in place. It will be very strong and stiff if you do it right. You can build it like a house if you want, but it will be weaker/heavier/less stiff. But that would probably hold up fine too, if you do it well.

When I built one like this I used cheap 2.7mm Luan ply with a layer of fiberglass on the outside, and it was like a brick shanty. Ply is great when it's reinforced with a hard layer that prevents penetrations and crushing.

You planning on using XPS foam? Big camper on a F550?

Yea, that makes sense, in the same way that fiberglass with foam core can be structural. Though I assumed the aluminum framing common in those builds provided structure.

My current plan is a big camper on an F550 (crew cab, 208" wheel base with an 11' flatbed, a 2' angled overhang in the back, and a king-sized cabover bunk).

My thinking was a wooden 2x2 frame, with 1.5" Owen's Corning XPS foam board glued with sikaflex to an exterior 1/4" plywood shell and a thinner plywood skin on the interior. I'd planned on having some vertical studs spaced 16" apart for mounting things, because I still feel like that'd be beneficial over just gluing things to the walls.

@IdaSHO suggested steel reinforcements in the corners and for mounting the jacks, which I think makes a lot of sense. I'm just not sure how well the whole thing will hold up if it's off the flatbed and resting on its jacks. I'm also concerned about wiggle and flex in the cabover while its occupied. I'm considering including a cabover rack to the flatbed, and using a big jack (like you see on 5th wheels) when it's off the camper.

I may be over-engineering this thing, though. ¯\_(ツ)_/¯

 

[ReluctantTraveler]

We just wrapped up a second 4k mile trip down and back up the US East Coast in our travel trailer (16' living habit with no slides, 4 people and a dog—104 sq feet), and all of us agreed it's just way too small for our needs.

Desperately want to avoid a bigger trailer or 5th wheel, class A's are way too big, and they no longer make crew cab class C's (the last company to make them stopped in 2009 for some reason).

 

[rruff]

No, if they have aluminum it's just to join the edges.

I don't remember what Kenny used, but PL Premium works well and isn't expensive. Sand the foam, then use a wallpaper roller to texture it, then really work the glue into it, then use a trowel to 3d it. Probably best to do that in two stages so you have more time. Lay the plywood down and pile it with concrete blocks. You have help, right... at least for moving big panels around?

1.5" thickness is probably ok for walls, but 2" would be better, and if your roof is long and flat I'd make that thicker.

Go buy a sheet of cheap thin ply, a sheet of 1.5" foam, some 2x2s, and some PLPremium. Make a piece 8' long and as wide as your wood spacing. See what you think of it. When you are done playing, see what it takes to break it.

 

[ReluctantTraveler]

No, if they have aluminum it's just to join the edges.

I don't remember what Kenny used, but PL Premium works well and isn't expensive. Sand the foam, then use a wallpaper roller to texture it, then really work the glue into it, then use a trowel to 3d it. Probably best to do that in two stages so you have more time. Lay the plywood down and pile it with concrete blocks. You have help, right... at least for moving big panels around?

1.5" thickness is probably ok for walls, but 2" would be better, and if your roof is long and flat I'd make that thicker.

Go buy a sheet of cheap thin ply, a sheet of 1.5" foam, some 2x2s, and some PLPremium. Make a piece 8' long and as wide as your wood spacing. See what you think of it. When you are done playing, see what it takes to break it.

I'd planned on using idaSHO's construction approach of frame, then glue and plywood. Mostly because it seems like it would be far easier to join walls and install doors and windows that way versus having to cut them out later.

With the approach you described, you glue the panels together after building, have to ensure they stay properly square, and then need to cut out holes for windows, doors, etc, right? I can see the advantages, but that seems more prone to error when not in a factory setting.

My thinking with the walls was that the 2x2 lumber for framing has an actual size of 1.5x1.5 inches. After adding plywood to either side, the actual thickness of the walls becomes closer to 2".

What thickness would you recommend for the roof, and would you make the floor thicker as well?

I don't doubt the strength of industrial glues for holding the plywood and foam together. Where I'm less confident is at the seams, where walls, the floor, and roof meet.

 

[ReluctantTraveler]

No, if they have aluminum it's just to join the edges.

? I am now realizing that what I’ve seen in those builds is usually just an aluminum channel that the panels fit into. For some reason, I was imagining an aluminum frame.

 

[rruff]

? I am now realizing that what I’ve seen in those builds is usually just an aluminum channel that the panels fit into. For some reason, I was imagining an aluminum frame.

Some might use an aluminum extruded channel that the panels fit into, but that would create a thermal short (aluminum thermal conductivity is very high). The simple way is just angle inside and outside of corners. You could get say 1/16" 5000 series aluminum and press it into any angle you want for more interesting shapes rather than 90 degree square corners.

I'd planned on using idaSHO's construction approach of frame, then glue and plywood. Mostly because it seems like it would be far easier to join walls and install doors and windows that way versus having to cut them out later.

Oh no, if you are making ply-foam-wood panels, you build all the openings in as you go. For instance you'd lay out and screw and glue the wood "framing" on your concrete floor first, or huge table if you want (plastic sheet underneath). Then cut and glue the foam pieces in place. Next glue and press one plywood side (use screws too where you can). Flip it over and do the plywood on the other side. If you are going to use fiberglass on the exterior (highly recommended), it's best to do that on a horizontal surface also.

You'll get a better bond this way. Bonding on the vertical is iffy because you can't apply pressure. Not to mention bonding the bottom sheet for the ceiling! But if you make panels and join them, you'll need to move them around occasionally, so you'll need room to do that and a helper. I made a thing in my garage where I could store panels upright and ~2' apart so each could be moved without messing with the others.

Basically, if you have room and a helper, I'd definitely make panels. If you are cramped for space and working solo, then framing is probably the way to go. Bonding gets way trickier... but maybe it's a case where the wood in the core supports the skins well enough even if the bond to foam is uneven?

1.5" core is fine, but 2" would be better R value and stiffness, and where I live 1.5" isn't available. People who build big campers with composite panels usually have at least 3" roofs for stiffness and R, but I'm sure 2" will work. The floor will be supported on a flatbed, yes? Not much is needed there, so it really depends on if you want to use it when raised up. It will be strong enough, but maybe a little bouncy.

Make that sample piece I mentioned. Also buy some epoxy (I like Ebond's cheap stuff, slow cure), and cloth (6oz? plain weave) and see how much more durable that makes the skin. Compare it to just an epoxy coating.

Definitely use at least 25 psi foam. I can't believe it's over double what I paid 4 years ago! In fact unless you are really poor I'd go ahead and get PVC foam from Carbon Core. It's about 3x the price still, but it's 5x as strong too, and it's easier to bond.

The seams? You should have solid pieces of wood there, screwed and glued together. Very solid joints.

 

[ReluctantTraveler]

Oh no, if you are making ply-foam-wood panels, you build all the openings in as you go.

My concern is that I'll potentially get the placement wrong. I suspect there's going to be a certain amount of "layout some stuff and see how it fits" for my build, as I plan to include a ~1.25' basement for all of my tanks and electrical.

You'll get a better bond this way. Bonding on the vertical is iffy because you can't apply pressure.

Not sure if you ever saw @IdaSHO's photo gallery from his build, but he had a bunch of boards and clamps tacked across the foamboard as the glue set to provide the required pressure.

I will have an occasional helper, but I have serious concerns about my ability to fabricate full sized panels properly.

1.5" core is fine, but 2" would be better R value and stiffness, and where I live 1.5" isn't available. People who build big campers with composite panels usually have at least 3" roofs for stiffness and R, but I'm sure 2" will work. The floor will be supported on a flatbed, yes? Not much is needed there, so it really depends on if you want to use it when raised up.

Yea, I do plan to occasionally use it off-bed (for example if we want to leave the habitat parked by drive the truck somewhere for errands or to explore a town).

That's actually a big part of where my "oh no will this be strong enough" has come from!

@IdaSHO mentioned that he used just brad nails and glue, but he has a steel frame for the bottom half of his camper. I'm wondering if I need to do something similar, or run some big lag bolts in a few areas. I'm particularly concerned about the cab-over, which will be about 6.5' x 8' in size.

 

[rruff]

It's easier to build panels on a flat horizontal surface. With the caveat that you will need occasional help. I don't understand your concern about being able to build them properly... what would be harder about it? They seem easier in every way to me.

I would not use any steel. You won't need it if you build the panels correctly.

The cabover is not an issue either if you build the panels correctly.

You really need to build a sample panel! Try different things until you are happy with the results. I have a 48" x 16" x 1" thick 15psi XPS, with 2.7mm luan skins. Made it a few years ago just to see what it was like. No wood in the core at all. Bonded with PLP. It weighs a little over 5 lb so ~ 1 lb/sq ft. I can support it at the ends and bounce up and down on it, and it's plenty strong. I also made a sample with 2" 25psi, 8' x 2', wood perimeter, and epoxy/FG skins. I can jump up and down on that one and it doesn't budge... over an 8' span!

I suggest you make a piece that you'd deem suitable for a floor, roof, or under your sleeping berth. Maybe... 80" long, 16" wide, with 1/4" ply, 2" thick 25 psi core, and wood perimeter? See what you think then.

Putting your door right at the front is not ideal IMO, not so much for strength but because that area is better used for storage. Plus if you can put your entrance right behind or in front of the wheel, you can extend it down a bit more internally which is nice. With a 15" basement it will be quite a hike from ground to floor.

 

[ReluctantTraveler]

what would be harder about it? They seem easier in every way to me.

my two concerns with the approach:

1. Attaching the panels to each other, and
2. Putting frames for the windows and doors in the correct place.

if I build a frame and then install the foam and plywood onto it, I have a bit of flexibility to play with the internal studs and frames for the door and window.

I feel like that would let me experiment with a couple of different options rather than having to have a final design in CAD, or some thing before building

 

[rruff]

Attaching the panels to each other is super easy. The edges will be solid wood. Just screw and glue.

Regarding the door and window placement, I guess I can't relate. That is pretty straightforward, and you really won't know for sure until it's all enclosed and built out. Plus I'd absolutely glue the wood pieces together, so they wouldn't be coming apart anyway.

But... you can do what you like... that's the beauty of DIY!

 

[ReluctantTraveler]

Attaching the panels to each other is super easy. The edges will be solid wood. Just screw and glue.

Regarding the door and window placement, I guess I can't relate. That is pretty straightforward, and you really won't know for sure until it's all enclosed and built out. Plus I'd absolutely glue the wood pieces together, so they wouldn't be coming apart anyway.

But... you can do what you like... that's the beauty of DIY!

for long runs of wall or roof where you need multiple panels, how do you attach those to each other? Are the edges angled so that you can screw and glue, or are you using some sort of channel to join them?

 

[rruff]

for long runs of wall or roof where you need multiple panels, how do you attach those to each other? Are the edges angled so that you can screw and glue, or are you using some sort of channel to join them?

You don't need multiple panels, you build each as one piece. You need multiple pieces of ply for each side, but these are butted together at the center of a wood rib. That isn't a real strong junction, but I never had an issue with them... and it would be the same if you framed it out first.

You can angle cut the wood on the edges all you like. If it's a 90 degree junction you don't need to.

Are you planning to use marine ply?