Teardrops n Tiny Travel Trailers

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1/2 lap will work, but scarf joint is better................but more difficult to make. There was a very long thread on the subject. Do a Search and you'll get some links to some pretty neat information.

hey guys,
how search?

I agree with Chris and others that a scarf joint is much better and the preferred way among boat builders. However, I am using a half-lap joint system as you described because it is easier and faster. I will be covering the whole piece with another skin of plywood so the joint will be hidden and also both sheets will be glued together across the entire surface. I did a Google search on "scarf joint" and found several very good sites and also references to some jigs you can buy or make to help with making a scarf joint. Good luck either way you go... Dave

I'll have 3 vertical pices with two vertical joints. One will be at the top of the door with only a 4-5" seam. The other will be full length top to bottom of the side. I'm gonna just bevel the edges, butt joint the plywood with a 3/4" thick x 3" wide wood piece behind the joint as part of my framing. Both edges of the plywood will be attached to the strip with glue and countersunk flat head screws about every 6". Then I'll fill the screw holes and the seam between the beveled edges with bondo and sand smooth. I joined 1/4" luan plywood on the sides and bottom of a boat this way and it worked just fine.

My minde is made up...don't try to confuse me with facts...

A butt joint will work just fine. My trailer’s over 10’ long, with ¾” plywood walls: I had to ‘extend’ the 4’ x 8’ side walls. After gluing up the sides, using plain old exterior carpenter’s glue, before adding the backer block, the butt joint was strong enough that two goons could pick up the blank at each end and move it around, no worries. I added a backer block, placed at the galley wall, just because.

Scarf joints are used in boatbuilding because, more often than not, the builder is bending the plywood and a scarf joint won’t change the ‘flex’ of the ply as much as a backer block does. I agree that a scarf joint is stronger.

Worked for me, anyhow.

shil wrote:A
Scarf joints are used in boatbuilding because, more often than not, the builder is bending the plywood and a scarf joint won’t change the ‘flex’ of the ply as much as a backer block does. I agree that a scarf joint is stronger.

I failed to mention that my butt joints with the backing strip were placed where there was no curvature. It was john boat design, and I made sure that where the sides and bottom began to curve in and upward to the narrower flat bow, it was all one piece of plywood. And the side joints and bottom joints were staggered so that they did not join at the same location. That boat got a heckofalot more flexing and stress on the water, running over rough water under power, and with people walking in it, than a trailer will receive riding flat on a chassis...and there was never a problem with the joints separating. And I never used anything but Elmer's wood glue back then. It'l be Titebond III and Gorilla glue for me now.

Since the trailer walls are flat, I have absolutely no structural concerns with using the butt joint. On the top, the butt joints will be placed where the curvature is minimal.

If the butt joint is used, one should definitely put a backing strip/block or spar behind the entire length of the joint to keep it from "opening" from vibration or stress. My vertical backing piece will lap joint with a horizontal framing member that runs from front to rear for stiffening the sides.

As Shil said, it worked for me. If anyone else thinks they need more solid or secure joints, go for it. It's your trailer and you should build it the way that you feel is best. And I certainly am not saying my way is best, but, just the way I want to do it. Time will tell if it holds up well. That's one of the ways we learn and share our experiences.

I used a sheet of 1/2" plywood, then cut a 1/8"spine in it with one of these.

http://www.harborfreight.com/cpi/ctaf/D ... mber=42133

Glued then inserted the spine, and laminated a sheet of 1/4" finished material to the inside. Staggering the joints.

It may be overkill or lowering the river instead of raising the bridge, but it works. Of course with an item like a trailer bouncing around and all I don't think there is to much overkill.


Happy Trails

Len

thanks guys,
all the info relly helps,i forgot to say im using 1/2 in plywood & also i have a bisqcuit jointer,would it be stronger with biscuit and butt jointed or half laps,it will have a 2x2 backer after instaled.

I had considered biscuits also but was told by the pros at Rockler and also by pros on this board that after cutting the slots there is only 1/8 inch on each side and that the swelling of the biscuits would most likely crack the outer ply. Made sense to me so I scrapped the thought. Hardwood splines like Len mentioned above would be a better choice as they won't swell.

And I'm using 1/4" plywood, so there's no room for any kind of spline...so, butt joint it is.

Critter,

An epoxy-fiberglass sandwich is another way to join plywood panels. Check out the drawing and explanations at Michalak's site,
http://homepages.apci.net/~michalak/1Mar04.htm or at http://www.angelfire.com/nc3/davecarnell/epoxy.html

In general, the boat-building guys are who to ask, because they are dealing with the problem all the time, and they have developed a variety of ways to join plywood panels, all of which can be equally strong, but some of which require more expertise than others.

Charlie

Personally, I think it is overkill to compare some aspects of trailer construction to building a boat. Plywood joining is one of these. A boat is subjected to all kinds of stress that a trailer isn't. A boat running under power on waves or choppy water is prone to flexing and twisting from rough water that will really test the joints. If your trailer chassis is built correctly, it should be stiff enough that the body of the trailer isn't flexed or twisted like a boat can be.

As I stated earlier, from my experience, the boat that I built with butt joints on the sides and bottom had no problems with those joints separating or cracking. It was 12' long by 4' wide, and I ran it with an outboard motor on some pretty choppy water. The boat did flex some, but the joints remained solid. If the butt joints on a boat held up under that type of use, I am pretty darned sure that my trailer won't come apart.

This is JMHO, but I think I'll stick with the KISS format.

cracker39 wrote:Personally, I think it is overkill to compare some aspects of trailer construction to building a boat. Plywood joining is one of these. A boat is subjected to all kinds of stress that a trailer isn't. A boat running under power on waves or choppy water is prone to flexing and twisting from rough water that will really test the joints. If your trailer chassis is built correctly, it should be stiff enough that the body of the trailer isn't flexed or twisted like a boat can be.

Dale,

You misunderstand my intentions in referring to boat-builders. The reason I provided the links was to show that there are alternatives to doing a scarf joint, which earlier posters had suggested was the "better" option, though they didn't quantify how to measure "better".

You advocate butt joints, but there are butts joints and there are butt joints. Mostly likely you used the conventional treatment which makes use of a backing block. The boat guys have been experimenting with just fiberglass strapping, either applied to the surface of the plywood or else set into a shallow depression (so the the joint becomes relatively invisible). Also, some of them have done stress testing to determine just what weight of 'glass needs to be used and how many layers on each side.

Why disparage that information, especially when making judicious use of it could produce strong --aka, properly engineered-- panel joints, easily and quickly, which is the question which with the thread began, namely a would-be TD builder looking for an easy and good-enough way to join plywood panels?

Charlie

Charlie, I don't think Dale was disparaging anything...just laying out some personal thought on a process based on his personal experiences.....
madjack