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Old 05-23-2008, 11:44 AM   #1
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unloaded joist spans


Like many others, I am building a pergola using pressure treated lumber in my backyard. It will be roughly 13' x 19'. It is being build to cover a pond so I am placing three 4x6 posts on either side of the pond at 9' apart and I am sandwiching them with 2x6's to form two long beams to cover the 19' length. I have been looking to find a table that has the maximum recommended span between posts for the joists that will actually span the pond. the actual distance between the posts will be 12'8" but the distance between the inner 2x6 beams will be 12'5". We are anticipating roughly a 12" overhang on either side of the beams.

I have attached a not-to-scale drawing to help describe what I am planning.

Originally, I planned on using 2x6 for joists. I have not seen anywhere an "official" list that says 2x6's with 12" overhangs, can span 12'5" without sagging. 1) has anyone seen such a chart/table? 2) anyone have experience with 2x6 at this kind of span?

I figured I would ask the other DIY'ers out there who may have run into the same issue.



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Old 05-23-2008, 04:03 PM   #2
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unloaded joist spans


Since the only load the joists will be carrying is their own, it should be fine. I will warn you that without something in place to tie the joists together, they will warp and twist. You need at least 3 or 4 runs of something to lock them together. We replaced about 40 2x6 joists on a pair of pergolas that spanned 12' and they hadn't sagged, but they twisted and warped to the point of ruin. You need to either install blocking between or purlins on top.

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Old 05-23-2008, 06:39 PM   #3
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unloaded joist spans


Maintenance6's info is good info! There isn't a span chart for unloaded use, nor are there any applicable codes (aside from local ordinances perhaps). I think you'll be fine with 2x6's going 12', provided you follow the aforementioned advice.
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Old 05-28-2008, 11:34 AM   #4
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unloaded joist spans


Quote:
Originally Posted by Maintenance 6 View Post
Since the only load the joists will be carrying is their own, it should be fine. I will warn you that without something in place to tie the joists together, they will warp and twist. You need at least 3 or 4 runs of something to lock them together. We replaced about 40 2x6 joists on a pair of pergolas that spanned 12' and they hadn't sagged, but they twisted and warped to the point of ruin. You need to either install blocking between or purlins on top.
we plan on putting purlins on top (1x2 strips about every 3 feet?). I hate treated pine but that is about all we can get around here for any reasonable cost (southern Louisiana). How about 2x4's? One of the local hardware stores here recommended 2x4's to keep the top from looking SO top heavy which was a concern I had.
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Old 05-30-2008, 11:03 PM   #5
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unloaded joist spans


1x2s would probably work. 1x3s would be better. 2x4s would get pretty bulky looking.
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