Spar height

[Greg M]

I have found that narrow strips of plywood will split open if screws are applied parallel to the grain.

The plywood is where the majority of the transverse strength comes from, not the spars. If the aluminum is attached directly to the spars, then the spars are much more structurally significant and need more careful design.

Jeff L.

Doesn't this contradict the "plywood spars are bad" idea though? The ply doesn't take much load as the skin provides most of the force joining the sides to each other; a shear joint being stonger than a pulling one. If you predrill for your screws, the ply shouldn't split.
I'm just thinking about ply to use up the inevitable left overs, rather than from any relegious belief in its superiority


-Greg

[madjack]

I have found that narrow strips of plywood will split open if screws are applied parallel to the grain.

The plywood is where the majority of the transverse strength comes from, not the spars. If the aluminum is attached directly to the spars, then the spars are much more structurally significant and need more careful design.

Jeff L.

Doesn't this contradict the "plywood spars are bad" idea though? The ply doesn't take much load as the skin provides most of the force joining the sides to each other; a shear joint being stonger than a pulling one. If you predrill for your screws, the ply shouldn't split.
I'm just thinking about ply to use up the inevitable left overs, rather than from any relegious belief in its superiority


-Greg

...there is no contradiction...spars are only 1.5 to 2" wide...thus a narrow strip of ply with the mounting screws coming thru the outer skin and into the ends...not a real good idea even if predrilled, there is just not enough "meat" there, especially in a flex condition to keep the ply from splitting...the second part of that statement applies to screw being put thru the side(face) or perpindecular to the plys...that is much stonger and way past acceptable. Having said all of that, I don't know how much diff it would make once everything is all screwed, glued and finished, especially if a plywood substrate is used to cover the top prior to putting on the AL...however for piece of mind and ultimate longevity's sake I would not use ply for my spars...just my thoughts
madjack

p.s. if you use a hanger of some sort the spars could certainly be made of ply since the hangers are connected to the spars thru the side of the spar...but I would still not use ply for my spars...but tha's just me

[JLaman]

I like the notion of consuming all the "waste" plywood and putting it to good use. Maybe consider another project for the extra plywood - a toy box for the kids or or shelves for the garage? Or maybe the cabinets, front storage box, or whatever. The spars are an important structural element and probably not a spot where you want to economize. Besides, consider that you need about 9 spars, ripping from premium 3 - 2x4s by 10'-0" costs about $15. Not a big investment for an important piece!

[cracker39]

Just my $.02 worth...for most TDs or TTTs, I think a spar of 1" x 1 1/2" is strong enough and you can rip 6 of those from one 10' 2x4 if they are no more than 5' long. Again, it depends on the roof shape. A flat roof won't be as strong as a curved one will. The curved plywood will resist a load placed on it better than a flat piece will. But, Greg, where you live in BC, you may have a heavy snow load on it during the winter and might want heavier spars for peace of mind.

[rooster]

I split the 2x4's also and they came out just fine for my spars. I did watch however for the slight bow in some of them, these I put on top of my tear with the bow up so the roof had a very slight cure to the sides so water will run off and not puddle.

Jim

[JLaman]

Don't accept any double curve at any location on the roof. If you do, the flat plywood and/or the aluminum will not fit correctly as it can only bend in a single curve. Your desire for water runoff is good, but there is little chance for water to pond on a teardrop shaped trailer anyway. Take it from me -- I had HUGE problems because the subsurface for the aluminum was not a single curved surface (my insulation made the plywood buldge out -- a double curve). Think how you will get the aluminum to bend in a double curve?! You can't do it and there will be big gaps. Hopefully you will dry fit everything first to see what you have. My recommendation is to make sure you have the flatest spars possible and plane they if you need to.

Jeff L.

[Greg M]

But, Greg, where you live in BC, you may have a heavy snow load on it during the winter and might want heavier spars for peace of mind.

No snow worries here, I live in Victoria. We had a really bad snow in '96 that shut down the whole region. It was equivalent to a middling blizzard back east

-Greg