How wide is too wide

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How wide is too wide

Postby Merv » Tue May 10, 2022 5:51 pm

I'm wanting to build my first foamie. I've seen several build that are 48 inches wide, outside to outside, which would give in inside dimension of 44 inches. At 48 inches no stringers are required, just foam canvas. I'm sure there is a width limit for structural strength of foam and canvas. How wide can you go before stringers are required?

I would like a little more room inside. If I built one 64 inches wide outside, 60 inches inside, will the foam an canvas be enough to span the extra 16 inches width? Or will I need to adds some stringers?
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Re: How wide is too wide

Postby TimC » Tue May 10, 2022 7:55 pm

On my son's foamie, which is 64" wide and 60" inside (2" foam walls), we used 1/8" lauan for the ceiling and 1 1/2" foam over the top. That was covered in PMF (10oz canvas). No stringers/spars needed. A bulkhead at 80" and cabinet frames at the foot of the bed plus shelving for the headboard is all the additional structure we installed.

Edit... I should mention that the curvature of the lauan as well as the bulkhead, footboard cabinets and headboard cabinets provide a good amount of support. The flat portion of my son's foamie roof is maybe 24" without framing. The lauan was braced on the flat section during foam install and when set the assembly became rigid. There's little air pressure on the flat surface while towing. However, if you add spars in that area all the better. We haven't had any issues with it so far and it is stored indoors so snow load isn't a factor.
Last edited by TimC on Wed May 11, 2022 1:45 pm, edited 1 time in total.
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Re: How wide is too wide

Postby KCStudly » Wed May 11, 2022 11:41 am

I built from the inside out with a slight crown/arched radius in my roof profile, and in the beginning planned to use 5mm Okoume ply ceiling without spars. 64 wide with cabinet face frames front and rear, plus 2x cedar spars and bridging around the roof fan. When I put the ceiling ply up on the cabin for a dry fit I noticed that the ply sagged in the middle, under it's own weight, and did not maintain the desired profile.

I decided to glue the fan spars/frame and additional spars to the top of the ceiling prior to installing it on the cabin, which solved the sagging issue and gave me additional peace of mind for potential snow loading here in the northeast.

Now if you have a more pronounced roof arch and you apply canvas to the outside of your roof foam it would likely act as a "strecher" helping the foam bend around your profile and there would likely be no sagging because the foam is so light. Applying PMF to the inside after the fact would be a total pain, IMO. The load carrying ability would depend on several factors including: the thickness of the foam; weight of canvas; and radius of roof arch.

The one sided skin stretcher would strengthen the tension side, while allowing the compression side to compress during the bending process, possibly precluding the need for kerfs that would otherwise weaken the roof panel. If you were to skin both sides prior to installation it would resist bending too much, I think.

For a flat roof, even with 3 inch thick foam, I don't think I'd risk it. There is bound to be some, even a small amount of sagging (especially with even a modest amount if snow load) and the resulting puddling/pooling of rain/melt water would challenge most all methods of sealing.
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