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Need beam for a 22-foot span. It's a carport. What to use? (beehaw.org)

submitted 7 months ago by TheBaldness@beehaw.org to c/diy@beehaw.org

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I'd like to build a carport. It will basically just be a shed roof extending off the front of my garage. I'll need a beam, as thin as possible, to span 22 feet from side to side.

It looks to me like steel is the only thing that could easily do that, but the cost is high, and how thick does it have to be? I priced out some 6" i-beams. $500 each. How do I attach wood to those? Can't exactly nail into them.

That brings me to LVLs and such. Less pricey, but very thick. I suppose I could double up on a couple of thinner ones (like 6") but they don't seem to be rated for exterior use, and I'm pretty sure a carport would be considered exterior use, even though they'd be under a roof.

Any guidance?

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[–] oo1@kbin.social 4 points 7 months ago (1 children)

22 ft unsupported seems like a very long span to me, what's that nearly 7 metres?
Sounds like it's getting into the realm of structural enginneering not diy for me.

If you want to save costs you might think aout a "flitch beam", that's 2 wood beams with a steel plate sandwiched in between - the three components are bolted together every few feet. Easier to join to the timbers then.
https://www.youtube.com/watch?v=QWUNd559UQY

I still think you might be more like 10"x2 or even 12"x2 timbers to cover that span if totally unsupported. But might still come in a little cheaper than the i-beam.
Maybe the roof will be very lightweight and no snow weight is expected - but I'm no structural engineer so don't take my word for it.

Other features like corner bracing or canti-leverage, or some other support structure or other feature (like is it the bottom side of a framed gable triangle) might also help.

LVLmight not be suitable, but i think you can get treated "glulam" beams suitable for exterior (covered) use.
https://en.k2-builders.com/what-type-of-glulam-can-be-used-for-exterior/

[–] TheBaldness@beehaw.org 2 points 7 months ago* (last edited 7 months ago) (1 children)

A flitch beam sounds interesting. The problem with 2-by timbers, like 10x2 or 12x2, is their thickness. A 12x2 would drop down 12 inches, and the highest point of my carport is only 8-feet high. In its 16-foot run, it has to drop at least 4 inches in order for water to run off. I can't be using material as thick as 12x2. That's why I was considering doubling or tripling some engineered wood. But at that point it seems like steel would be the same cost and better performing. I dunno. Like you said, I may have to talk to an engineer.

[–] oo1@kbin.social 4 points 7 months ago

yeah, glulam is probably a non-starter too if there's no height available.
I think you can do triple flitch.

So like : [w]|[w]|[w]
maybe that'd work with 3 2x6s , 2 steels, all boted together.