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Old 03-02-2013, 04:35 AM   #1
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Two-post freestanding pergola


I'm planning on building a two-post freestanding pergola in my back yard. I designed it myself with some input from my engineer father, and here are the basics:

Two 6x6 PT posts extending to 7'6" above ground, placed with 12'8" between them.
2 2x8x6' sideways beams, one at the top of each post
4 2x8x16' longway beams (the span between supports is 12'8")
16 2x2x6' rafters
2x4 angle braces at all the corners

I've attached a few pictures describing the design. Note that these are from a slightly out of date rendition -- the long beams will actually extend an extra six inches on each end than what's depicted here.

I guess my main questions are:

1. How deep do I need to sink my posts? I have 10' posts but I'm a little worried that I should have gotten 12', since I've seen some articles online saying the posts should be sunk 1/3 into the ground. With this current lumber they'll be 2'6" into the ground, which is 1/4 the total length (or 1/3 of the above-ground length). Is this sufficient? If it makes a difference, I live in the LA area and there's no danger of the ground freezing any more than occasional surface frost.

2. All my lumber other than the posts is heartwood redwood. Are my beams sufficiently big, or will 2x8s sag significantly over that distance? I'm planning on growing grapes on this, so there will be more weight to support beyond the weight of the wood.

3. Does anything else jump out as being weird or incorrect about my design?

I would really appreciate any and all feedback on this. Thanks!
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Two-post freestanding pergola-pergola-perspective.jpg   Two-post freestanding pergola-pergola-side.jpg  
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Old 03-02-2013, 05:43 AM   #2
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Two-post freestanding pergola


I wont comment on the engineering part, except to say that if theres any question about 2x8s being too small then i'd go 2x10. I realize redwood isnt cheap, but should be a reasonably inexpensive upgrade.

As for the design itself, I'd like to see a longer cantilever at the ends. Maybe bring the posts in to 9' or 10'.

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Old 03-02-2013, 06:11 AM   #3
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Two-post freestanding pergola


You have virtually no load on the roof, so any sag that occurs is a cosmetic issue, not a structural issue. I would not be concerned about it. As for the posts, a reasonable rule of thumb is 1/3 total length embedment where you want full lateral resistance in a reasonably strong soil (sand, gravel, well graded mix). You have 1/4 embedment, so there is some potential for sway, but this is a pergola, not a habitable structure, so I would not worry about it too much. Pack well draining soil (sand, gravel, crushed stone) around the posts, and you should be using ground contact rated PT, not the usual PT (which is not rated for ground contact).
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Old 03-02-2013, 07:20 AM   #4
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Two-post freestanding pergola


as 12penny mentions, cantilever the ends more. are you planning to make the notch cut in the (2 2x8x6' sideways beams, one at the top of each post) or are you putting the notch into the long beams to set onto the sideways beams. the (2 2x8x6' sideways beams ) might need to be doubled up also.
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Old 03-02-2013, 09:19 AM   #5
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Two-post freestanding pergola


I can't comment on the engineering but how about using steel instead of wood for the diagonal supports? It would be a smaller dimension than the 2x4 and if done properly would add some mixed material design elements. I don't have anything in mind particularly. Just thought I would throw it out there. Nothing wrong with the wood.

Pics when the project is complete. I would like to do something like this eventually.
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Old 03-02-2013, 12:14 PM   #6
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Two-post freestanding pergola


Something I forgot to mention is that we plan to hang a hammock between the two posts. So it'll have a little bit of side-to-side force on it from the sway of the hammock, but mostly it'll be inward force on the posts, which I imagine won't be a problem.
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Old 03-02-2013, 12:23 PM   #7
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Two-post freestanding pergola


Quote:
Originally Posted by hand drive View Post
as 12penny mentions, cantilever the ends more. are you planning to make the notch cut in the (2 2x8x6' sideways beams, one at the top of each post) or are you putting the notch into the long beams to set onto the sideways beams. the (2 2x8x6' sideways beams ) might need to be doubled up also.
I had been planning to make a double notch, though I hadn't made any final decisions on that yet. I figured a double notch would aid stability and placement. One problem with that is that I don't have a table saw onsite (I'm not sure how to cut the notches other than using a dado on a table saw), and I don't have a way to transport the 16' beams to my stepfather's house (he has a table saw).

As for the cantilevers, I agree. In my original design I had been trying to keep the boards down to 15', under the misapprehension that lumber could be gotten in single-foot increments, and trying to keep costs down (seems kinda silly when I think about it now). As I mentioned in my original post, the boards are actually 16 feet, so they'll stick out an extra 6 inches on each end, and I'll canti them for a greater distance as a result.
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Old 03-02-2013, 12:31 PM   #8
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Two-post freestanding pergola


With only 2 posts, the embedment length of 2-8" is not enough. The hammock will add to the defection/movement. The top foot of soil or so does really not add much stability in the long term. Any deflection over time will loosen the post anchors.

With only 2 posts and little extra depth and wider holes for the concrete are dirt cheap end the end if you want to keep things in place.

Dick
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Old 04-09-2013, 03:13 PM   #9
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Two-post freestanding pergola


In case anyone didn't see it, I posted an update here: Two-post freestanding pergola -- finished!

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