3/4 vs 2x4 Skeleton Frame

[MJWBuffalo]

I am committed to building with a skeleton frame but am a little confused as to the benefit of doubling up 3/4" plywood strips vs using 2x4's off the shelf for my frame. I have seen some of you do it both ways but I am completely baffled as to the benefit of the double 3/4" plywood. I need to end up at 2" thick so I'll be skinning with 1/4" plywood either way.

Is the 3/4" lighter? Stronger? Cheaper?

Otherwise it seems like a LOT more work to rip down sheets of 3/4" ply into 3.5" strips and glue it all together in order to make basically a perfectly true 2x4.

Color me confused...

Matt

[tony.latham]

a little confused as to the benefit of doubling up 3/4" plywood strips vs ...

Doubling up 3/4" plywood strips? I'm not sure what you are referencing.

Building a skeleton from 3/4" plywood is an easy task. No joinery, other than lengthening which is simple with a 1/4" slot cutting router bit.



I gang cut them. Two at a time.

Tony

[KTM_Guy]

Tony pointed out his way for the walls. But my question is why would you want 2” walls? I can’t see any advantage over a 3/4” or 1” wall. I went with a 1” wall but that was because I couldn’t find 3/4” rigid insulation in my area and was able to find 1” Baltic Birch.

While building keep in mind you want to build like an air plane not like a tank.

Todd

[siena]

a little confused as to the benefit of doubling up 3/4" plywood strips vs ...

Doubling up 3/4" plywood strips? I'm not sure what you are referencing.

Building a skeleton from 3/4" plywood is an easy task. No joinery, other than lengthening which is simple with a 1/4" slot cutting router bit.



I gang cut them. Two at a time.

Tony

Tony for the skeleton frame, to insulate that would you just cut foam board to fit in between the skeleton? And then skin in plywood or aluminum?


Sent from my SM-G973U using Tapatalk

[tony.latham]

a little confused as to the benefit of doubling up 3/4" plywood strips vs ...

Doubling up 3/4" plywood strips? I'm not sure what you are referencing.

Building a skeleton from 3/4" plywood is an easy task. No joinery, other than lengthening which is simple with a 1/4" slot cutting router bit.



I gang cut them. Two at a time.

Tony

Tony for the skeleton frame, to insulate that would you just cut foam board to fit in between the skeleton? And then skin in plywood or aluminum?


Sent from my SM-G973U using Tapatalk

Yes, 3/4” foam board which can be a challenge to find. Some use 1” and trim it down with a hot wire cutter.

On this last tear, I used 1/8” Baltic on the inside and 1/4” on the exterior. You might want to peruse my recent build log.

Tony

[MJWBuffalo]

All:

Thanks for the replies. I always forget that I need to explain my project a little before questions like this as it's a little different....

I am building a standie on a 5x9 trailer that will operate as a food trailer. While the footprint is 5x9, it widens at the top by a food on each side for a total of 11'. Because of all this, I will have a large awning window (around 60"w x 40"h) on one side with pneumatic lift supports. I am concerned that the added stress of that would necessitate the need for a beefier frame.

After reading your replies and doing some more searching, I think I'll stick with the thicker walls just to be safe but I like the idea of doubling up the 3/4" ply instead of 2x4's. I can cut out the middle of the panels like you've shown to cut down on the weight/add the insulation. Since I'll be using 3 sheets of ply per side, I can overlap the seams and glue/screw to make it a stronger more cohesive wall.

I'll cover the outside with 1/4" ply before aluminum and inside will have FRP to make it easy to wipe up.

Thanks again!

Matt

[noseoil]

You really don't need the extra beef of 1 1/2" material for walls. 3/4" plywood with 1/8" skins is plenty of strength. The 3/4" skeleton frame isn't that strong by itself, but once the 1/8" material is added to the inside & glued in place, it's pretty much bullet-proof as far as structural strength. Adding 1 1/2" insulation may be necessary, in which case 2x4 would be ok, but I would use 1x2's for the framing to save weight & cheapen the build cost. 3/4" is plenty strong with skeleton wall construction.

I'm with Tony on this one!

[MJWBuffalo]

Thank you for the reply!

I'm going to take the advice of all your wise tnttt vets and build it with a single layer of 3/4" plywood. I'll cut out the interior spaces and fill with foam to provide some insulation. Then I'll glue RFP on the inside and glue/screw 1/4" plywood on the outside before floating the aluminum skin.

Thanks again for the input. All of your expertise probably helped me shave another 150-200 lbs from the trailer!

If anyone else has input, I'd love to hear it

Matt

[saywhatthat]

make sure that you seal around foam .Any gaps will let cold/hot air pass

[MJWBuffalo]

I have seen lots of pics of everyone's skeleton'ized frames but the width of the supports seems to vary. Is there a minimum width each "bone" of the skeleton should be to provide enough structural support?

It's a delicate balance between loading it up with insulation and providing enough support so the whole thing doesn't get wobbly

Thanks everyone!

Matt

[tony.latham]

Is there a minimum width each "bone"...

I wish I knew that answer, but I have always thought that Tim's skeleton, aka noseoil, is all that is needed. He made a career of designing roof trusses so that should speak for itself.

This may explain my skeleton a bit since some of it is chunky:



The reason for the 4" along the outside edge in the galley is because 2" of it gets cut off for the hatch ends.

I don't know who this guy is but he sure looks cool in this video.



That'll show the hatch ends.

Tony

[saywhatthat]

will operate as a food trailer. Why would you use any wood There a reason food trucks use SS are alum. most food trailers are molded fiberglass Faster to clean . no place for bad bugs to live

[MJWBuffalo]

Thanks for the replies. I'll keep it beefy around the edges but it's good to know I can cut it back to 2" or so in the middle!

Matt

[Absinthe]

Is there a minimum width each "bone"...

I wish I knew that answer, but I have always thought that Tim's skeleton, aka noseoil, is all that is needed. He made a career of designing roof trusses so that should speak for itself.

This may explain my skeleton a bit since some of it is chunky:



The reason for the 4" along the outside edge in the galley is because 2" of it gets cut off for the hatch ends.

I don't know who this guy is but he sure looks cool in this video.



That'll show the hatch ends.

Tony

QQ; the benefits vs a full sheet are mainly for weight and if weight is not a concern it's fine to go with a full 3/4 sheet?

[tony.latham]

QQ; the benefits vs a full sheet are mainly for weight and if weight is not a concern it's fine to go with a full 3/4 sheet?

If you are asking about the benefit of a sandwiched wall vs a solid one, here's my opinion:

1. The resulting structure is a torsion box that is stiffer than a single sheet of plywood with close to the same weight.

2. This method makes it easy to "stretch" plywood beyond the normal 8' length using a simple spline joint in the skeleton cut with a router bit. The exterior/interior sheathing that is glued on acts as a gusset and strengthens the joint.

3. Insulation. I purchased my first teardrop in 2004. It had solid plywood walls. On cold mornings we would always get condensation on the walls. We kept a bath towel in the cabin just to wipe them down in the morning. This would happen even with the cabin well ventilated. Condensation doesn't form on insulated walls.

4. A thicker wall allows for deep dados (or slots) to be included in the wall for bulkheads and any other cross members. This joinery makes for a much stronger cabin and is easier to assemble since these parts just slip inside the walls. Think mortise and tenon. (And yes, this is easy to do with a simple router jig.)

5. The thicker wall allows for a lip to be formed on top of the wall so that the headliner can be installed prior to the spars. This creates a perfect interior wall/ceiling joint that doesn't need to be covered by trim. (Having done this a few times, I can't even imagine having to install the headliner after the spars are installed.)

6. The thicker wall allows for a Fredricks style hatch that has a seal system that is water and dustproof.

Sandwiched walls are more labor-intensive but you end up with a stronger cabin that is insulated with a galley that doesn't leak.

Tony