Ceiling spar specs

[nrhmaine]

I’m seeing some variation in ceiling spar dimensions and species.

Being mindful of both weight and strength, I want to find the right balance.

I have maple and ash on hand, and given their strength (and ash’s moderate rot resistance. I’m inclined to use one of those.

Is 1.25”x1” too skinny? 1/8” ply glued in and out. 12” and 8 inch spacing, and I’ll mortise in a 2x2 hatch spar. Considering gluing in the foam as well, to add rigidity.

The roof is 5’ wide.

Thoughts? Am I going too skinny? It seems to me that for something that won’t get jumped on, this wouldn’t have any flex, with all the layers built up, but, of course, I’m a novice with campers.

Thanks, Nate


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[tony.latham]

I use 1x 2" popular for the spars. This is what I wrote in my book:

The rule of thumb for spar spacing is 12” on the nearly flat portions of the roof and 8” on the front curve. But that’s not written in stone.

On this build, I installed a solar panel on the left side of the roof. I attached it with screws through the panel’s grommets, so that’s what drove the setting of the first several spars. The other issue is the fan opening. It needs to be 14-1/4” square. Because of this larger gap, double the two spars next to the fan opening. (Use two spars on either side of the fan.)




And I do a 2" square hatch spar out of oak.

Tony

[nrhmaine]

I use 1x 2" popular for the spars. This is what I wrote in my book:

The rule of thumb for spar spacing is 12” on the nearly flat portions of the roof and 8” on the front curve. But that’s not written in stone.

On this build, I installed a solar panel on the left side of the roof. I attached it with screws through the panel’s grommets, so that’s what drove the setting of the first several spars. The other issue is the fan opening. It needs to be 14-1/4” square. Because of this larger gap, double the two spars next to the fan opening. (Use two spars on either side of the fan.)




And I do a 2" square hatch spar out of oak.

Tony

Thanks Tony - I was just reading that chapter, and noticing your 2” spars, which prompted the question. My roof is entirely curved, so as I’m laying things out, I’m leaning towards ~ 8” spacing throughout.

Is your 2” thickness driven by insulation space? A decision on deflection over that span? Fastener length? Something else?


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[tony.latham]

Is your 2” thickness driven by insulation space?

Structural. But that's not based on calculations. Just my gut so take it with a grain of salt. But 2 full inches of foam board really adds to the roof strength.



I think I could walk on my roof without damaging it.

Tony

[nrhmaine]

Thanks Tony.
I played around with the “Sagulator” at WoodBin, and decided on 1.5” x1” Ash on 8” centers.

https://woodbin.com/calcs/sagulator/




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[noseoil]

If possible, I would go with a depth of 1 1/2" for the roof. The smaller dimension may be a bit too thin for the 5' length. 1 1/2" with 1' spacing would be fine, since you will be fastening the roof material to the spars & the ceiling as well. Once the spars are glued in place & skinned (insulation between the skins), it should be plenty strong. I still don't recommend walking around on the roof. I did the poplar like Tony's builds (light weight & strong), but I used red oak for any place a fastener was holding something. The solar panel, roof vent & hatch spars were all done with hardwood for better fastener holding strength.

[nrhmaine]

Thanks for the input. I decided on 1.5” for the spars. Spacing isn’t fixed yet. I wasn’t sure if the curve would require spars more closely spaced than 12”, to keep the curve defined. But maybe I’m over thinking that?


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[noseoil]

If you go with something like 1/8" Baltic Birch, you can bend into a 2' radius without any trouble. Do a mock-up first, to see how the material responds to your curves (grain direction), but it looks like you will be fine with the lofting you have & curves.

[nrhmaine]

Thanks.

I’m using 1/8” okume outside, and 1/8” BB inside.


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[tony.latham]

If possible, I would go with a depth of 1 1/2" for the roof.

When Tim, AKA noseoil talks about roof strengths, you should listen. He did that stuff professionally.

Tony

[gudmund]

something that I did when I bought my trailer, was when I found out that the builder had pictures posted on his web-site of the assembly of trailers as he built them = I ended saving as many as possible in a file for future reference research = and yes, I use them all of the time. (and being he has since closed his business doors, it is nice now, in having them to look at when wondering 'what is behind here or there' when doing modifications/changes to my trailer.) this first picture here is of the spar build-up of my 'Orama brand' trailer's roof using what look's to be 2x2's.

this second picture is from a builder located in Libby. Montana "Sherpa Trailers" that builds their trailer's using 2x4 spars - they probably are just 'a-bit of overkill, but they swear by them. I stopped by their place in '21 when passing through Libby and they gave me a tour of their assembly shop = nice people........