Adding load support on 20ft span

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Topic by DBuckley posted 05-24-2011 02:50 AM 5926 views 0 times favorited 3 replies Add to Favorites Watch
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1 post in 3898 days

05-24-2011 02:50 AM

Topic tags/keywords: load support garage beams

I have a large space over the garage that I need to convert to a closet space. It is 2×8 16 on center and 20 foot span. I was told that the joists are over spanned and I am not sure how to add a support beam. The garage floor is concrete and has block 1 foot up on the walls. Any ideas?

View dfletcher's profile


128 posts in 4398 days

05-25-2011 12:22 AM

Well, 20’ span with a load on top is going to require something more than a couple of 2×12’s put together. You are looking at either steel or glulam style beams.

I suggest taking your question directly to a good lumberyard near you, (Not Lowes or Home Depot) and see if they can help you engineer the beam and give you some advice on how to install it.

My reason is this: (I used to work for a lumber yard)

They will be able to send all of the information to one of their vendors who are engineers. These guys actually do this for a living, so, they are usually pretty good. They want you to buy their material, so, they are more likely to bend over backwards to help you figure out the entire project.

Generally, putting a beam underneath your joists requires 3 1/2” of support at each end and, if the span is long enough, some support in between.

Your end support can be gotten just by putting several 2×4’s together under the beam to the sill plate. That should be fine.

Your center supports, however, may require you to cut out some concrete and re pour a thicker slab, usually about 16”x16” typically.

The beam height will depend on the material you choose.

Hope you have good luck with this.

View Dan Lyke's profile

Dan Lyke

331 posts in 5043 days

05-25-2011 11:04 PM

I second what dfletcher says. The AWC beam span tables only go to 13 feet, so even if your foundation can support the point loads you’re still looking at a glulam or steel beam.

Other possibility: Sister the existing ceiling joists with something beefier. I think typically ceiling joists are supposed to be 20 lbs per square foot live load, 10 lbs per square foot dead load, the AWC span options calculator says a 2×12 Doug Fir – South (the cheap stuff) will span that with 16” centers. But if you’re going to walk up there rather than just toss some stuff up there, then that won’t keep your ceiling from cracking.

Disclaimer: I am not an engineer.

-- Dan Lyke, Petaluma California, http://www.flutterby.net/

View Grandpa's profile


139 posts in 3893 days

06-03-2011 06:10 AM

Ditto on going to someone that knows how to design this. 2×8’s should span 14 feet when used as floor joists. You are way over this in your garage if you put a floor up there and walk on it. The lumber yard can have the beam built and tell you how to support it. I agree with the 3 1/2 inch support on the end. I am not educated as an engineer an you need one. I will say this can be done and it can be done safely so find the right people. Go to a lumber yard that builds engineered trusses and they should be able to help you.

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