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Afraid Knot
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# Posted: 6 Mar 2014 10:34pm
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Love this site, and need some advice. I'm building a 12x24 lean to with 11' and 7' walls (24' long), and want suggestions on footer spacing. Right now I have two rows of 4 posts, which are 4x6's and will be about 24" out of the ground, and sitting on top of each row will be a 24' 4x6. I have the posts spaced 7' in the rows and the rows are spaced 9' apart, and 2x6 joists (16" OC) will be on top of the beams with about 12" of over hang and the 24' beams will also have about 12" over hang (cantilevered). The concern is do I have enough posts so the floor is not spongy? I could put in 5 posts (1 extra per row), and the spacing would go from 7' to about 5.5'.
Using the floor joist table at https://www.auroragov.org/cs/groups/public/documents/document/010609.pdf it looks like I'm within specs, but does anyone have any experience with this layout?
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Afraid Knot
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# Posted: 6 Mar 2014 10:36pm
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Lets see if this file attaches.
Thanks for any comments, suggestions or advice!
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Don_P
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# Posted: 7 Mar 2014 12:09am
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First you're forgetting the roof load when using that table which is the self weight of the roof plus whatever the snow load is in that area.
Next, the cantilever typically shouldn't exceed the joist depth although there is a table of cantilevers in the IRC, although it doesn't go down to 2x6.
Then, for the joist span in the beam table they are really trying to collect the entire joist load so the 12' span would be the correct span... but it still is not accounting for the roof load, it's just a floor load table.
A better approach is building more like a post frame building (pole barn). There the post extends from the footing to the top of the wall and the wall braces the post. If you read the leaning telephone pole pier thread you'll see the problem with a short unbraced pier under a building. With the post braced by the wall sheathing, it is locked. For this building 14' 6x6's, 4 spaced evenly along the 24' would work for the tall side and 10'ers would work for the low side. Using the IRC girder tables a double 2x10 hung between the posts on simpson HUCQ hangers with 2x8x12' joists would create a strong, well braced structure, for snow loads below about 30psf.
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Afraid Knot
Member
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# Posted: 7 Mar 2014 06:39am
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I will be doing the work myself so ballon framing is out. I could always add an extra row down the middle and eliminate the joist cantilevers. I really want 2x6 joists and I know I can use them safely which I think I am, I was just curious if the floor would be soft?
Thanks.
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OwenChristensen
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# Posted: 7 Mar 2014 07:32am
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I've built lots of 12' floors with white wood 2x6's on 12'' centers. I made them with 1' overhang beyond the beams, so 9' span. Decked with 3/4 t&g underlay plywood. They are solid. I don't like the 4x6 beams though. I assume you're talking treated wood. They have little strength, and will warp. I'd use laminated 2x8's at least with a span between them no more than 7'. I'd also prefer 6x6 posts, but I don't know what kind of soil or drainage you have.
Good luck in you build. Have fun.
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MtnDon
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# Posted: 7 Mar 2014 10:44am - Edited by: MtnDon
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I saw those charts last night and couldn't figure out what their purpose was. They looked off; did not make sense to me. Reading Don_P's comment about no roof loads being considered made something click. Most likely those charts were made for planning the construction of a deck. Decks have no roofs. (General FYI note: Porches are not decks, as porches have roofs.)
Do not build a cabin using specs for a deck.
Use the IRC tables for floor girders; the tables take the number of floors and the roof into account along with span of floor joists. Read footnotes. Spans may be interpolated.
FWIW, good deck info ... includes updated data/specs for southern pine.
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Afraid Knot
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# Posted: 7 Mar 2014 12:40pm - Edited by: Afraid Knot
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I think I will decrease 4x6 post spacing down to 5.5' and spread the rows out to full 10.75' and have 8" cantilever using 2x8's.
Thanks for the link OC, that is piece of mind!
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Borrego
Member
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# Posted: 7 Mar 2014 06:35pm
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Quoting: Don_P A better approach is building more like a post frame building (pole barn). There the post extends from the footing to the top of the wall and the wall braces the post.
Quoting: Afraid Knot I will be doing the work myself so ballon framing is out.
He's not talking about balloon framing here if that's what you thought. Merely a different concept and stronger way to build your place. Look it up and consider it......
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Don_P
Member
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# Posted: 7 Mar 2014 11:10pm
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I sketched this for someone on another forum as a concept of how a postframe would work. The perimeter posts run from footing to top plate and are well braced by the exterior wall sheathing.
This is much bigger than a 6x6x14'post but gives an idea of what you can do with simple machines used by the ancients. I saw basically the same setup in a drawing of a barn raising from the 1500's. My wife did help with that lift but if she hadn't been around I would have simply put longer levers on the windlass. That's about a ton and 20' tall. One of my bosses described our job one time as "outsmarting inanimate objects"
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