1/4 ply and canvas for minimalist sidewalls?

[andreuther]

Just wandering if building an average sized TD (4x8 or 5x8) using minimal framing and 1/4 ply all around with a full canvas wrap would work? I understand that the foamies are using canvas to add to the structural integrity of the build and it would seem that the same could be applied to an ultralight wood build. Any objections?

[KCStudly]

So are you asking about minimum framing with an inner skin of some type and foam insulated walls (a composite structural panel) or just some studs, the plywood outer skin and the canvas?

If it is the later, my experience with 5 mm ply suggests that you will need more support to keep the panels flat, especially in the roof where it will tend to sag.

On the other hand, with a minimalist approach, you will gain a lot a leeway on weight and can afford a few more sticks. I would plan on at least 1x2's on edge around the door. For the wall studs that are just stiffening the wall panels you could probably get away with 1x1 (3/4 square), but I would go back to 1x2's where there will be shelves or cabinets, and at the bulkhead/galley shelf. I like to use #6x3/4 FH screws, and anything smaller than 3/4 inch wide gets a little sketchy, IMO. If you have to butt join any panels where they land on a spar or stud I would back the joint up with a strip of the same ply, and shave the stud down to back that up (rather than going to a wider stud for the ply to land on), similar to how I did one of my ceiling panel/roof spar joints. From my experience, if you try to butt a panel on the edge of a 1x you have to staple too close to the edge of the ply and it can blow out the fibers along the edge leaving divots.

Something else to consider is how you will attach the walls to the floor and the joint between the walls and roof. The canvas will wrap the corner of the profile much easier with a generous radius, so you might want to plan for a full perimeter cleat along the profile.

For the roof, depending on how tight your profile curves are, you will probably need 1x2 spars on edge every 7 or 8 inches (at least in the flatter areas) just to keep the ply from sagging under it's own weight; it won't follow the profile in the middle of the span without adequate support. As a point of reference my roof is 64 inches wide and has a 520 inch profile radius at the ceiling. At first I thought I could get away with just two 2x2's at the vent, using the cabinet face frames at the front and back, but I was wrong. Then as a test I laid the ceiling over with additional spars at 12 inches, but it still sagged. Finally I settled on every 7 to 8 inches and it is holding shape well.

Another way to save weight is to pick a lighter species of wood. I used a lot of clear select cedar, which is very light and rot resistant, and some poplar for my framing, only resorting to heavier woods like oak for the hatch hinge spars and Red Grandis for the hatch ribs (... and a bunch of soft maple for cabinet frames ... not light but looks really good )... but fir is a lot less spendy. The trade off is that you can almost always use all of the better wood. Whereas if you try to rip your structural stuff from cheap box store lumber you will end up loosing a lot to knots and other defects.

[Woodbutcher]

You might find this useful. This is my build from 2012 . Basically a skeleton with 1/4" plywood on the sides. Roof is 2 layers of 1/8th inch baltic birch. The roof will support a person laying on it.

viewtopic.php?f=50&t=48321

[andreuther]

KC yes I was thinking of the 1/4 ply with just some inside sticks and an outer layer of canvas. I was figuring I would have to spar the roof like any other build as that is what will hold it all together. I'm thinking close to what Woodbutcher did but with a canvas finish.I am more curious about the added strength that canvas might add to the build. It is my understanding that if you wrap the entire build in canvas with overlapping seems then you end up with a very stiff outer shell. I much more comfortable working with wood then I am with foam but I like the Idea of canvas as a supporting outer layer.If can vas makes such a strong shell then it is feasible that once it has set then some support sticks could be removed without affecting the sag problem of using 1/4 ply. So many ideas and only one trailer to build.

Woodbutcher- WOW....... Nice build, and it is by far the closest to what I have in mind that I have seen. Did you get a weight on it? I'm also curious to know how the no insulation worked out for you. The beauty of it is you could always add insulation if you needed to and that is something I was considering in my design. Masterful work!

[andreuther]

I have also been considering sandwiching a vey thin peace of foam (1/8'' if I can find it) between two 1/8'' peaces of ply and using that as my wall. Kind of a downsized version of SIPs (structural insulated panels). That with a layer of canvas could be very strong and have some insulation. I could even insert 1/8'' thick oak on the inside around the doors and bulkheads for support and eliminate having to add supports on the inside. Hmmmmmm

Has anyone done this?

[Woodbutcher]

I have not weighed my Slowmobile. But my guess is around 800# I have camped when the temps went below freezing and found no problem. Dry camping I add an extra wool blanket and when I have shore power I use a 1500 watt small heater and mostly run it when I first get in the trailer. Personally I would look at the canvas over plywood mostly as a means of sealing the trailer before I would look at it to add strength to the wood. Good luck with your build, it's a big part of the fun.

[KCStudly]

The strength of the foamie method comes from the separation of the inner and outer skins. By moving the inner and outer fibers further apart you gain strength. Since you are proposing very little separation (just 1/4 inch; canvas one side, ply on the other) you will not benefit from this effect. Think about the difference between a flat sheet of paper (or thin sheet of ply) and a box or beam that has a lot more depth to its shape. That is the difference between one layer of canvas on one layer of thin ply, and a much thicker wall with a layer of something on each side. It's the same reason that spars are stronger on edge than they are on the flat. The farther away you put the inner and outer wall fibers (or upper and lower roof fibers), the stronger it will be. That is why a foamie works so well. It uses a relatively incompressible material that is light weight to add separation between the inner and outer skins. This places two relatively high tensile strength layers of fibers far apart with little weight penalty. When the panel tries to bend the foam keeps the fibers separated; the farther apart the fibers are the more "leverage" they have to oppose the forces being applied, so for a give strength of material it will be able to resist more force. This is the same reason that a foam filled wall with inner and outer skins will be stronger than a hollow wall. In the hollow wall the skins can still buckle.

For what you are proposing the strength to stiffen the panels will mostly come from the framing. The canvas will just be a water proof membrane that also helps to stabilize the grain of the ply and keep water from penetrating. I doubt you will be able to remove any structure after the canvas goes on.

The Slowmobile is a fabulous camper, but a lot of its strength comes from having inner and outer frames. The maple outer frame is very lovely to look at, adds a lot of strength, but is not as light as some other options.

[andreuther]

KC- I was afraid of that. The thing that gives foam its strength is the very thing I don't like about it, it's bulky depth. Any input on the 1/8'' sandwich idea? Correct me if I'm wrong but I would think that two 1/8 ply's sandwiching even a thin sheet of foam would be stronger than plain 1/4'' ply. And the ability to insert a thin piece of hardwood without having to stick frame also seems appealing.

Woodbutcher- Good information, I think people are hesitant to not add insulation and it would seem that you have stepped outside the average comfort level with great success.

Thanks guys!

[Forrest747]

this would be the profile with a .12 thick roof

[KCStudly]

I don't think the payoff in weight savings would be worth the cost, or strength difference compared to a single sheet 1/4 or 3/8 ply. The insulation value would be better, but not by much, and certainly not for the level of effort.

I haven't ever handled a big sheet of 1/8 inch ply, but I expect them to be fairly fragile (relative to 5 mm (3/16 inch) (I'm pretty sure what most people are referring to as 1/4 inch, such as underlay, is actually 5 mm). Have you handled any of these yet?

Yes, a lot of people say that the would go with 1/8 outside skins "next time", but I am pretty sure those are the same folks who have built up walls with at least 3/4 inch foam cores. 1/8 inch ply + 3/4 or 1/2 inch foam + 1/8 inch ply might work okay for walls with minimal framing, but you probably want to go at least 5 mm for the top skin of the floor, or just use a single layer of 1/2 inch ply with 1x cleats around the perimeter.

[Breytie]

I believe I have learned a few tough lessons when it comes to trying to build very light in wood. With my weekender build being single layer 1/4" ply with true 1" sticks doing most of the joining and stiffening, I am in a continuous battle.
I found that tight fitting joins, fully glued and screwed, is not negotiable. Preferably on both sides. The galley is the main structural stiffening element with the headboard storage not far behind. Doors can be made to fit these very thin walls, but is extremely difficult to get a good fit and seal all round. as is finding windows and locks for those doors. The cleats with the worst bend is used to stiffen the roof and hopefully keep it from sagging to badly.
In all it is very difficult to join things effectively, very few fittings and accessories can fit on these thin walls and there is no room for error.
Good quality 1/4" ply is actually quite strong and surprisingly stiff. I never saw or heard signs of distress when handling a 6'x4' piece on the flat by one edge or corner.
I still believe 1/4" (6mm) walls is doable. But if I have to do it again, I would probably go with 1/4" outside, 3/4" to 1" insulation and 1/8" inside.
How much the canvass and paint will change to all the above, I do not know, but I do not believe it is worth the trouble.

Greetings and happy building
Andre

[andreuther]

My final thought would be to go with a hybrid foam/wood board. 1/4 ply on the inside glued to 1" foam and then covered in canvas. This avoids using 2" foam and it seems to meet in the middle of the benefits of foam and wood. Bring on the pros and cons.

[KCStudly]

Other than the fact that 1 inch foam does not match stock lumber sizes (is it even available in that thickness?), that is basically what I am doing with 1-1/2 inch foam. My reasoning is: 1) that my wife does not like the cold and we are going to the mountains where it gets cold overnight, even in August; 2) 1-1/2 matches 1x2 standard lumber on edge (3/4 x 1-1/2 inch actual), as well as 2x2 (1-1/2 x 1-1/2 actual); and 3) I want the benefit of the added strength for forest road type adventure. An added bonus is that my doors (which are homemade using the same materials and techniques) are nice and rigid (hopefully will result in no leaks), and they match the wall thickness; so except for the seal flange around the door opening, they are essentially flush on both sides (an aesthetic that was important to me).

3/4 inch foam is less common, but is available (I ordered several 2 ft wide sticks to do a double layer over my curved roof) and will match 1x lumber's actual 3/4 inch thickness; or you will be doing a bunch of ripping on the table saw to get true 1 inch wood (wasteful and time consuming).

Truthfully, if you plan on camping in mild climates and/or only where there are electrical hookups, I think you were on the right track just doing an internal frame with 5mm (or 1/4 nominal) ply exterior covered in canvas. Boiling down my long winded commit on that option was just that you will probably be using more frame work than you initially imagined.

Have you looked at any of the basic foamies that don't have as much wood in them as mine? How about Laura's Superlegerra? It is a larger build, but she is following more of a pure foamie style of building on a budget.

[Woodbutcher]

Just to clarify a couple things with my build. The skeleton frame work is all glued and pocket screwed together. With no skins on it anywhere it can be pulled around by grabbing it from any part of the frame. There is no movement at all. Add to that a glued and screwed 1/4" skin and it is rock solid. The woody frame is one piece and screwed on only as a decorative look. But it really does little for adding extra strength. If you make a good skeleton the rest takes care of it's self.

[andreuther]

Looks like foam is not an option again, I called ABC supply and they said that the companies that make blue and pink foam are no longer allowed to sell to California. I could use the white styrofoam but I really hate that stuff. I am leaning more towards Woodbutcher's style frame at this point.The beauty of that build is if I ever want insulation all I have to do is add it and then do an interior skin. If mine looks half as good as Woodbutchers I will be a very happy man. Maybe when I leave CA I will do a foamie.