Wall construction questions

[wandererr]

For my first post I figure I might as well try and make it a good one

I'm looking to build a trailer on a 6x10 base that allows a person to stand up on one end, sleeps 3 and has room for a portajohn inside (in it's own enclosed space). In any case, I have most of the design drawn out and now I'm starting to ponder the construction materials and methods. It seems that for walls there are couple schools of though
1 wooden (or aluminum) frame with outer and inner skin and insulation in between)
2 solid plywood walls with no insulation or insulation glued in from the inside
3 plywood skeleton (3/4" or so) with insulation filling in the skeleton cutouts

As I'm evaluating the various choices and trying to balance cost, weight and functionality (in no particular order) I'm leaning towards option number 3. It seems that it is going to offer the most rigidity with best weight savings. Only thing that I'm concerned with is that the walls will have to be two or three pieces of plywood however I lay them out as height (at highest end) will be 6' and length will be 10 ft. If I can find 4x10 plywood - I would lay them lengthwise and stack two of them on top of each other - if I can only get 4x8 then I would stack them vertical and put them side by side.

Anything specifically that I need to keep in mind and consider as I am planning things out???

[sincere01]

I haven't built my teardrop yet, so take my advice with a grain of salt. But I believe when connecting two pieces of plywood most people do a halflap joint. For strength you want to make sure that the skin you apply on either side spans over the joint so as to hold it together.

Not sure about 4'x10' sheets but someone else on tnttt (can't remember who) posted about 5'x10' sheets from menards. A bit spendy but might be worth it. https://www.menards.com/main/building-materials/panel-products/construction-panels/sanded-plywood/roseburg-3-4-23-32-x-5-x-10-acx-fir-plywood/p-2450452.htm

[wandererr]

Hmmm 5x10 would work as well... Though no Menard's in my neck of the wood (SoCal). In any case I started looking for raw materials - maybe I'll get lucky.

[Graniterich]

Where are you located? There is some available in Orange County

[wandererr]

Where are you located? There is some available in Orange County

I'm in Westminster

[KCStudly]

Biscuit or spline joints are also popular when joining ply. Another good practice is to stagger the joints. In other words, if your outer skin has the big sheet low and to the rear, you want the inner core to have the big piece high and to the front, then go back to low and to the rear for the inner skin. This is just an example; depending on where your doors and bulkheads are you may want to tailor your joint staggers to take advantage of those features.

What you don't want is for the joints in all of the panels to be aligned with each other.

From what I can tell, the big advantage to skeleton wall cores is speed and it is easier to leave larger areas of blocking for attachments, especially if you are unsure of the exact locations for things to come, like galley layout or fender locations. You can leave larger areas of uncut ply and still know that you will hit solid with screws later.

OTOH, some people will say that the ply costs more, they have a lot more waste, and it ends up being heavier than a well planned stick built wall. So on your lighter, thriftier scale, I don't think skeleton cores take first place, but there are other advantages.

[tony.latham]

I've spliced plywood for skeleton walls using the lapped joint and the splined joint. Both have their plusses and both have their problems.

In the wooden boat building world –and those guys are the masters– they stick to a scarf joint most of the time. Historically, those guys used a block plane for plywood and used the plywood laminations –like map countour lines– to guide their progress. I've used that method, and although intimidating, as long as your plane is sharp, it works well. It does take a bit of elbow grease, but a well done scarf joint is stronger than splined or lapped joints.

These days, a lot of the boat guys are scarfing plywood using a jig mounted on a circular saw. Watching the video, it works amazingly well.

Here's a quick look:

West System (http://www.jamestowndistributors.com/us ... viewHeader) makes a screw on jig that appears to be similar. At $90, I'm sure I'd make my own.

I hope this helps.

Tony

[KCStudly]

I tried the scarf jig deal and did not have good luck, at least not the first try... there was no second try. I haven't tried just planing, but I think the scarfed joints are a lot more critical when forming a curve over something that you are trying to make 'extremely' light and extremely fair. For most of our purposes a butt joint with backer will be more than adequate.

Wandererr, you would be well advised to take a look at Tony's posts. He has built a few and always manages to strike a good balance between functional and practical, while at the same time using proven techniques.

[noseoil]

I just used 3/8" dowel pins to join the 3/4" plywood walls where I needed to join the edges for a longer panel. I know this joint isn't too strong, it can break easily with careless handling and just the weight of the panel, but it's plenty strong as soon as the frame is cut out & the 1/8" skin is attached to one side with glue. The dowel jig I used was $12.00 at harbor freight & an electric drill is all that's needed for this system. There's nothing high-tech about it at all. I drew out the pattern for a skeletonized frame first on one side, then located the pins at each web, the top & the bottom. In my case there are only three pins per side. I was adding about a foot to the 4X8 sheet to make a 4X9 panel back at the galley. Here's a shot of the process.

When the pins & glue have set, the skeleton webbing & outside edge are traced or drawn on the panel. I used a router & flush-cut bit to transfer the cuts from one panel to the other, so both are exactly the same size. Just rough-cut the second panel with a jig saw to size, so the router passes were clean-up cuts, not the full cutter bite (too much horse power & heats up the bit & router too much).



Before gluing the inner skin to the side, I traced the outline for the insulation on some 3/4" sheets of foam & cut it out. There was a stack of insulation sitting around for 6 months before I got back to it, but it made insulating the walls very quick since all of it was already cut & ready to go in place. No measuring of the strange shapes & sizes to fit.



Once the skin is glued on the inside surface, it becomes a strong, lightweight wall system which can be insulated and moved easily by one person (27# for the 4'X9' frame & panel on mine).



Pros: Insulated panel, lightweight & strong, attachment points where you need them for fasteners (and can make more for the ones you forgot at first, see galley upper area for an example), easy to do wire runs in the panel where you need them just by drilling the frame.

Cons: Takes longer than a simple solid panel, costs more than a simple solid sheet due to inner & outer skinning (less than an oversize sheet of 3/4" plywood), more waste of plywood in skeleton areas. Doors & windows need different trim rings for a thicker wall than the standard 3/4" wall.

There's no right or wrong answer to wall-panel construction, just what your tools, time, money & skills will allow. After thinking it through, I decided on a skeleton panel, but.......

[wandererr]

Hmmm didn't think about the thickness issue with doors and windows.... Maybe doing 1/2" plywood skeleton with skins would work out better. That should get me to more or less 3/4".

I'm SoCal based and while we would take the trailer out into colder weather we are not going to be in crazy low temps.

[len19070]

I kind of hit the middle ground on insulating sidewalls. (I always do the floor & ceiling)

I use a good 1/2" plywood.

And cover all my interior walls with a Foam backed rug.











Even though "Rug" has no "R" value it still has insulating qualities. (your winter coat doesn't have an "R" value either, but it works)

Its quiet, a nice interior finish, does an nice job insulating, takes up little or no interior space, I get no condensation and the best part is, it can be added at any time.

Happy Trails

Len

[len19070]

BTW, big sheets of Plywood are not hard to make.















Happy Trails

Len

[Alan_H]

I see the advantage to the skeletonized plywood for a curved design like most 'drops are.

OTOH, for a more squared design, and economy of supplies, I think a stick-frame design would be more efficient in relating to time building, and ease of cutting insulation.

What do you all do with all those scraps from puttin' holes in your plywood?

[VijayGupta]

Forgive a question from a newbie, but experienced woodworker:

Wouldn't the "skeletonized" construction essentially be a "torsion box?" (assuming the skins are glued on) If so, why would you not just construct the inner layer with solid wood and insulation?

[noseoil]

To me, the skeletonized frame is a way of adding time, cost & labor to a build. It isn't cheaper than plain plywood or really that much better for strength either. The advantages have more to do with weight savings, the insulation factor & quiet, than an actual ease of construction & money. It does cost more than a plain sheet of plywood & takes a lot more labor to make a wall this way.

To answer about the left over pieces, I used it up as best I could for blocking on the roof rabbet, hatch corners, interior fastening points, etc. There's a lot of waste this way, as about 3/4 of the sheet is discarded if a lot of weight is removed. Plywood is heavy but strong, solid wood like white pine or poplar is lighter, softer & weaker.

Vijay, the wall panel itself becomes a type of "truss" or structural member in and of itself. The stresses are distributed through the inner & outer skins, through the glue joints, into the skeleton and then back out. Stresses (both internal & external) are reduced by allowing them to migrate throughout the panel and be absorbed. As these panels are joined to a deck (also hollow but a similar type), cabinets, bulkheads and an exterior skin, the entire structure of the body becomes a large torsion box. That's why the frame isn't necessarily too heavy for a teardrop (except in my case, where I found a used one for a good price). The body itself adds strength to the frame once the fasteners are in place. There's no reason you couldn't use a solid panel with insulation and be done in a couple of hours with both walls.

One advantage of hollow walls is the ease of adding wiring runs wherever they're needed.

There isn't a right or wrong answer here, just what a person's needs, wants, the skills, tools available & money dictate in the build. I guess that's the fun part, dreaming it up & then making it.