Pergola idea: looking for guidance (spans)

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Old 06-06-20, 11:46 AM
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Pergola idea: looking for guidance (spans)

Looking to build a roughly 16'x 14' pergola in San Antonio, TX, so no snow to consider, and no wisterias (or other heavy vines) will come near my project. The only load will be the pergola itself and perhaps some string lights.

I'm initially thinking of the following (all pressure treated pine):

8' 6x6 posts, spaced 12' apart back-to-front and 14' side-to-side
14' beams (made using 3 sandwiched 2x10s) sitting atop the posts using column cap hardware. Span will be 12'.
16' rafters (using 2x8s) spaced ~24" oc (I understand this might not be to code - if any roof would be needed I'd have to adjust) with a span of 14'.
14' purlins on top (using 2x2s) spaced ~12" oc.

So in short, can I span 12' between the posts using 6x8 beams (3 2x8s together)? And can the rafters span 14' if they're 2x8s as well? Moreover, with certain hardware, would this be possible to do without angle/knee braces?

Ideally looking to reproduce the look in the photo.

Thanks for the help!

 
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Old 06-06-20, 12:26 PM
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These are things you need to hash out with your local building inspector. We have no way of knowing exactly what the codes are in your location, since everyplace is different. Just ask them what the code requirements are for a pergola.

If it is attached to the house, they will likely require post footings similar to what the house is on.

That being said, three 2x10s sounds completely overbuilt for a beam that is supporting nothing more than an open, undecked pergola roof. You will just need to ask them what span tables you need to use for a pergola.... or what deflection is allowable. L/240 would be reasonable to me. But they have the final say so. I would notch the 6x6s for a doubled beam that could be bolted to the notched 6x6, if they allow it.

And you say "beams". Your photo shows one beam, the one in front. The other posts in that photo are there to allow for fencing in the ends. They don't each have beams on them... only the pair in front does.

As for the bracing, some custom steel L brackets bolted to the face of the beam and post could be designed to resist lateral movement in lieu of gussets. But they may require a structural engineer to specify that.

Do you have an existing deck this is going over?
 
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Old 06-06-20, 05:24 PM
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Checked my city of San Antonio "No Permit Required" list and pergolas of the size I'm discussing seem to fall under that (one-story detached accessory structures, provided the floor area does not exceed 300 square feet). And yes, I plan to have a freestanding 4-post pergola unlike the photo, which I failed to mention earlier.

I've attached a photo of my deck for reference. It's really quite overbuilt itself, with 2 2x6s for each joist on the foundation, and 2x6 boards for the decking. Planning on using these anchors from Ozco for the posts: https://ozcobp.com/product/6x6-post-base/

Understood regarding that initial idea being overkill. Perhaps I could use 10' 6x6 posts, then cut notches for a couple 2x8s for each set of beams? Then use 2x6s for the main rafters, and 2x2s for the purlins. I know Ozco sells some 2" rafter clips I could use for the rafters (https://ozcobp.com/product/2-rafter-clips-iw/), and the 2x2s I would just use deck screws from the top. Ozco also sell some flush inside 90 brackets (https://ozcobp.com/product/6-flush-inside-90/) that would would match the rest of the hardware. Think those would be big enough to be worth trying, for dealing with potential lateral movement?

 
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Old 06-06-20, 06:53 PM
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No, little clips like that will do nothing for lateral support. Knee bracing that is installed at a 45 degree angle at all 4 corners is what is standard if you are trying to prevent something from swaying. Most smallish pergolas dont have that because they often rely on the bolts through the post and the beam itself help to firm it up.

(What I was referring to by iron brackets was something like the largest 12x12 flat L bracket in this picture. You would need something like that to take the place of a knee brace. It would go flat on a post, then flat onto your beam. Something like that would keep the post and beam connection square. Flat T braces can be put on center beam-post connections to square them up. The larger the better.) But I don't know why you wouldn't just use some decorative wood knee braces on your corners instead.

Yes, 2x6s could span 14' for a pergola roof. This is using the span tables for ceiling joists, as a basis for your span. But you might be more satisfied with 2x8s... if you dont want them to sag. They will also provide some size to the structure. And you could cut some fancy design on the end without making them look anemic.

And depending on what your local inspectors say, they may not care whether or not you notch the posts for the beams or not. Pergolas are purely decorative, so they may allow you to make your beam by bolting your beam to opposite sides of the same post. That method used to be used for some deck beams but it's now prohibited in most places. But it actually looks better on a pergola. So you will just have to ask questions. They will probably love it that you are asking first rather than asking for forgiveness later. ;-)
 
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