Aluminum span tale?

I haven't played around with this but it might do what you want:

deflection calc

The correct engineering term for what you want is 'deflection'

I would use at least 2" angle X 3/8's steel. Any aluminum that is only 1.5 angle will continue to sag more and more over that long of span.

Maybe use square tubing instead of angle? Just a thought.

Extruded aluminum tube will deflect much less than an angle piece. There is also "U"channel that has three sides, a bit more improvement than angle.
deflection calculator

rexlion wrote:
Maybe use square tubing instead of angle? Just a thought.

westend wrote:
There is also "U"channel that has three sides, a bit more improvement than angle.
deflection calculator

Both are correct ! Much better than angle.

Thanks for the input, greatly appreciated. The calculators are above my pay grade and of little use. I did find someone on another forum that used 3X3X1/4 aluminum and still got some deflection. With the cost of aluminum and strength, it is looking like I will have to use steel for this.
Jim

Our MH has a little sag problem across the cabover at the roof vent. I suspect a wooden crosspiece broke when someone stood on the roof. I intended to open the ceiling and use a steel crosspiece as discussed here, probably screwed onto the wood crosspiece, but somehow never got around to it in 8 years.

I am still pursuing this, hopefully I will do better than 8 years.

Do we have some fabricators around?

As I said I am not knowledgeable enough to use the online calculators, so have been trying to test what I can find and aluminum is no longer in the cards. 1 1/2X1/8 steel angle has too much flex for a 7 foot span. The only other fair size I have access to is 1 1/4X 1/16 square steel tubing and it has too much flex or deflection.

So anyone experienced enough to take a guess at what size I would need for this? I'm thinking 1 3/4X3/4X1/8 rectangular steel tubing, might do it.

I need to span 7 feet with little weight, I am guessing at the most 100 pounds where the roof air is, it would not be carrying the full weight of the roof air.

Any thoughts, ideas, suggestions, advice or wise cracks greatly appreciated. Jim

How much height do you have inside the ceiling?

If you have the space and some fabrication skills (or can hire some), some sort of a truss build up from say 1" square Al tubing could work well. One straightforward design would be top and bottom chords with zigzag diagonals between them; the diagonals could be rather lighter than the chords. Think of, for example, stage lighting trusses (or rather one side of them as they're usually triangular or square rather than flat).

For that matter, you could also build up a wooden truss or beam if there's sufficient space. Something like a miniature I-joist should work well and not be especially difficult to construct.

One of the easier ways to do this if you are not going to remove any roof or ceiling panels, is to install aluminum or steel square tube against the ceiling. I'd suggest to use jack posts to remove the deflection from the roof, install the box tube with brackets at the wall.