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

submitted 9 months ago by [email protected] to c/[email protected]

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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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[–] [email protected] 12 points 8 months ago* (last edited 8 months ago) (2 children)

The advantage of LVLs are that

you can put them up 1 ply at a time
they hold nails/cut easily
the lumber yard will likely size them for you if you ask

The disadvantage is that the depth will be about 1/16th of the span when using 2-3 plies.

The advantage of steel is that an I beam (W shape is what you want, for "Wide Flange Beam") will be about 2/3 the depth of an LVL. The disadvantages are

An engineer will likely charge you about $600-800 to size this beam, but will also tell you how to top connect it
It will be one piece (fwiw it will weigh about the same as the LVL)
You will have to buy a 40' piece, or pay a premium to have it cut down from a 40' piece. (stock lengths of steel are 20'/40')

Note that nobody can properly answer your question from the data given (edit - just notice you mentioned 16' rafters below). You would need to include the span of the rafters and (at least) your location to determine the snow loads and wind loads (edit: and seismic, though it's unlikely to control for this design) for sizing the connections.

Disclaimer: I'm a structural engineer, but I'm not your structural engineer. For a long span like this I recommend contacting someone licensed in your jurisdiction to help you out.

[–] [email protected] 7 points 8 months ago (1 children)

Not their structural engineer yet!

[–] [email protected] 5 points 8 months ago

They're our structural engineer now, Dave.

[–] [email protected] 3 points 8 months ago

One option would be to make the beam a flush condition. To get a 16' span with rafters you're going to be using at least 2x8s. That's 7.25" deep. If you were set the top of the beam at the top of the rafters and hang them from the beam (simpson or USP hangers) that buys you some space. Now an 11.88" LVL would only stick down 5-5/8" below the bottom of the rafters. (okay, 5-3/4"-6" with the additional slope over the 5.25" of beam) I'm not saying that a 3 ply 11x88 LVL with a 2.1E, bearing in a BC6 cap on 6x6s would work for your application, but the height tolerance would seem to add up in your favor.