Maxumum span for unsupported foam?

[CampsALot]

Woking on my ceiling / roof this evening and I started thinking...
There is a four foot section of the ceiling that is the full width of the trailer. (6 feet)
With two inch foam, sheet skin inside and canvas out, is this too far of an unsupported span?

I realize you could never get on the roof, but..
In the summer, camping, I think it would be fine. In the winter, do you think the 'snow load' would be too much and it cave in? Its not unusual to have a foot of snow and in the spring it becomes heavy.

The roof is glued up, (6x8 feet), but I could cut it up and put a few ribs across, then re glue. Or add another layer and make it thicker. ( more $$$, I thought I was done buying foam)

Ideas?
Am I over engineering/thinking?

Thanks
Warren

[GPW]

It’s been said a cubic foot of snow varies between 5 and 15 pounds ... Do the math ... see what you think ...

[ghcoe]

Is it a true 6' or are there walls or cabinets involved? That would make a difference too. Does it snow that much at one time or is it accumulative? You could brush the snow off between storms if it is accumulative.

[CampsALot]

George,
There are walls and cabinets at each end, the four foot section is over the kitchen counter where it is the full span of the trailer. (Floorplan in signature)

GPW
The weight of snow can be figured, I guess what I really don't know is what 'skinned' two inch foam can handle.
The winter secneario was just something that came to mind. It can be dealt with in various ways much like boat owners do with a peaked winter cover or some such set up.

The other thought is, would it oil can / vibrate up and down from highway speed wind?
Some 1 1/2'' interior parts that I precoverd with sheets amazed me at how rigid they became.

Good engineering is to do the most with the least.
Airplanes are built to fly, not crash. If they were built to crash, they wouldn't fly. (Wow, deep )

[GPW]

Skinned foam is pretty Strong ...but you can always err on the side of caution and add a couple spars ... Not like standing on top where the weight is concentrated in one spot.. The snow load is spread out out over the whole roof ...

You could just go out and brush it off the top so it doesn’t get really thick ... or use a “peaked cover”as you say ...

"The other thought is, would it oil can / vibrate up and down from highway speed wind?” ... We’ve never heard of that here...

[CampsALot]

Good info, Thanks !

I think I'll use the 'Chicken and Ribs' method.

I'm going to put it together as is and if I get chicken thinking it's to weak, I'll add ribs...

Warren

I need to stop thinking so much, it usually just costs me more money.

[GPW]

Thinking is GOOD .... just think Thrifty ...

[GPW]

You know you could go to the manufacturer’s website for more information on structural loads of their product , and they probably have a contact for more info , maybe even talk to a human ... If you really wanted to get into it ? ... Sounds like Math to me .... must run away immediately ...

[atahoekid]

I used two inch foam for the roof and had an unsupported area about 6' long and about 6'wide. I ended up putting in three 1 x 3 spars over that span. It would have been two, but I put in a fantastic vent fan and have a spar on either side of that to support the extra weight. I have my roof fiberglassed on top and have a thin (5mm) ply skin underneath. The panel is very rigid and I'm not afraid of it collapsing now. I just cut a groove just wide and deep enough for the spar in the foam panel, glued it in place and then glued the ply skin to the foam. I too live where we get snow and I can garage my Road Foamie but I also know from shoveling it, that even one foot is a lot of weight. I think the average weight of a cubic foot of snow is 15 lbs, so an area that is 6 X 6, covered in one foot of snow would need to support almost 550#. If the snow is wet and heavy, it could be a lot more, if the snow is deeper than a foot you'll obviously need a stronger roof to support the load. I used the principals of TLAR engineering. I probably went overboard, but I also knew that I could not afford to have a failure here.

[Wobbly Wheels]

Just a thought, can you incorporate a bit of camber into the roof ?
It wouldn't take much- maybe an inch or two - and it would give you a big increase in strength.
I wouldn't be as concerned dabout catastrophic failure as I would about a slight depression holding water.

[CampsALot]

Good idea,

I was thinking of trying something along those lines.
I'm almost ready to cover the ceiling , (tomorrow I hope), and was planning on adding a small amount of 'bow' with the inner skin ending up under a little tension.

Warren