Spars for the roof question

[ioan]

I installed the beams for the roof and window (in front) yesterday, and today, while trying to figure out what to do next I saw that in the Generic Benroy Plans the spars are installed different. I suppose the way in the Benroy Plans give the roof more strength, should I take them out and install them on the side? Should I leave them like they are?

Here is what I did:




and here is from Benroy Plans:




Thanks.

[KCStudly]

Looks like you are at least 5 ft wide? If so I personally would go with the spars on edge.

Are you adding insulation, and if so, how thick? That might help you decide which way to put the spars. Flat, as you have done, will allow for 3/4 inch thk insulation. On edge will provide more strength and room for 1-1/2 thk insulation (assuming that those are 1 x 2 dimensional spars).

[ioan]

Looks like you are at least 5 ft wide? If so I personally would go with the spars on edge.

Are you adding insulation, and if so, how thick? That might help you decide which way to put the spars. Flat, as you have done, will allow for 3/4 inch thk insulation. On edge will provide more strength and room for 1-1/2 thk insulation (assuming that those are 1 x 2 dimensional spars).

The trailer is 4 ft wide.
I was thinking to use 3/4 insulation, is that not thick enough? I have 3/4 inch thick insulation in the side walls.

[mikeschn]

I think you are okay with the spars the way you have them.

While it's true, you can only get 3/4" of insulation in there, I think that is plenty for 3 seasons camping. And it's certainly enough room to get some wires in there.

So run with it, and keep the sawdust flying!

Mike...

[linuxmanxxx]

Running them flat like that will cause sag in the middle of the roof usually. I did my first like that and it sagged dead center and even tried to brace it on the inside and took some out but not all of the sag.

[ioan]

Running them flat like that will cause sag in the middle of the roof usually. I did my first like that and it sagged dead center and even tried to brace it on the inside and took some out but not all of the sag.

I didn't think of that. I'm going to change them, at least on the roof, to prevent sagging.

[mckenney56]

My two cents is that if there is plywood glued to both sides of the spars and it starts out flat it will stay flat. The whole torsion box thing, where the plywood ceiling would have to stretch and/or the roof plywood compress for it to sag.

[grant whipp]

Why go to all the trouble of taking them out? Just build them up in place by adding a 1/2" x 1-1/2" lath, glued & tacked into place ... it'll almost double the strength of a typical 2x2 (1-1/2" x 1-1/2").

FYI, on a 4' wide trailer, I typically use 1" x 1-1/2" beams laid sideways ... never had a problem with sagging.

Good Luck, and as always ...

CHEERS!

Grant

[markhusbands]

What if you were to use the full 1.5 - 2" dimension on the top, flatter part, but then taper it down where the walls start to roll more vertical? Just to have a tiny bit of weight? Foam thickness might get odd...

[CarlLaFong]

My two cents is that if there is plywood glued to both sides of the spars and it starts out flat it will stay flat. The whole torsion box thing, where the plywood ceiling would have to stretch and/or the roof plywood compress for it to sag.

What he said. ^^^^^^
If the inner and outer skins are securely glued to the spars and everything starts out flat there should be no problems. If the spars have some sort of severe internal stresses that cause them to warp as they dry and age, there could be some movement, but I would think it would be very minor. Hollow core doors have a lattice of cardboard glued to the inner sides of the skins and they're pretty stable

[linuxmanxxx]

It depends on how far the span is if it sags or not. I did 6 ft laid flat and the center sagged more than an inch and I had to run inner beams to lift the ceiling back almost level.

Instead of pulling them, just add one every other span up and down and double layer the foam. You get peace of mind and minimal effort to do it.

[KCStudly]

The moment of inertia of a structure (its ability to resist bending) is greatly affected by how far apart the outer fibers of the member are, the span that the member is bridging, and the load that is applied. So your 4 ft wide (short span) is in your favor. If you don't expect a big pile of snow to sit on your camper all winter and you don't need to stand on top (less load), that is in your favor.

In bending it is always the outer fibers that will fail first. So when you put the outer fibers of a member (like the skins in the composite structure of your roof) further apart you are essentially creating a longer lever; a lever where the outer fibers are at the long end of the lever and are strained less by the amount of force they are trying to resist. Moment of inertia is expressed in inches to the forth power and the relation of how far away from the centroid the cross sectional area (inches to the second power) of the material doing the work is has a dramatic effect on how well a given mass will do. Example: If you took a 1 inch wide x 3/16 thick piece of steel flat bar and placed it over your knee on the flat, you could easily bend it. Take that same flat bar and try to bend it on edge and good luck to you.

What I'm trying to say is, if you put the skins further apart using the same weight of material in spars to separate them, the strength will go up exponentially (which is to say a lot).

So while the answer may be that you might be okay with the spars flat, it would be much better with them on edge, much much better.

If they are just screwed in I would unscrew and turn them. If they are screwed and glued I would add another member to each forming an 'L' shape. But then TPCE is 5 wide, and will likely get snowed on at some point in its life, and has relatively few spars.

$0.02.

[pohukai]

Great and accurate analysis! You must be either an ME or CE... nice job.

[48Rob]

Could be my eyes...but in the illustration it appears that once you reach the flat area of the roof, the 1 x 2's change to 2 x 2's?

On my 7' wide trailer, I used 1 x 2"s (3/4" x 2" Oak) in the vertical position.
There are two layers of 1/8" ply on top, none on the bottom.
No problems with sagging.
If I had laid them on the flat, they would have.
On your 4' wide trailer, you may be fine.

It looks like you're using Oak too, a very good choice.
Oak is strong, as are many clear straight grained species.
I would however be concerned about the pieces in your picture that have knots in them, for they have only a small fraction of the strength of a straight grained piece.
It can be hard to find off the shelf wood that is free of knots, the easiest way to get it if you don't live next to a sawmill is to buy wider boards and rip clear sections from them.


Rob

[KCStudly]

Great and accurate analysis! You must be either an ME or CE... nice job.

ASME (2 year ME degree with many years of experience in mechanical design and maintenance/trouble-failure analysis).

Thank you for the generous compliment.