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Pergola design help needed

Posted by cjb1 (My Page) on
Mon, Jul 12, 10 at 21:07

Hey everyone. This is my first post here and I hope someone can help me out with my little project.

I am building a paver patio in the back yard of my Southern California home and want to incorporate a freestanding pergola. However, due to the design layout of the back of the house, particularly the placement of the kitchen window and a sliding glass door, I was thinking about doing a rather large rectangular pergola using four 6 x 6 posts with faux Tuscan columns slipped over them. Two of the posts would be 20' apart (the long side of the rectangle along the back of the house) and the other two would extend out 15 feet from there. Accordingly, I was looking at essentially spanning 20 feet using either 2x10s or 2x12s bolted to either side of the 6x6 posts. 2x6's would form the rafters spanning the beams and then 2x2s on top of those.

From what I've read here and on other websites, it sounds like the 20 ft span is too large even for the 2x12s and I run a serious risk of sagging and I'd have to look at a 6 post design at a minimum. As stated above, the design of the house makes the six post style problematic because it would likely place a post right in front of a window or door. What's the largest recommended size for a rectangular four post pergola? If I do go with a 6 post design, I would end up with a narrow structure projecting away from the house as opposed to a wide structure along the width of the house (if that makes sense). The latter would be more usable given the layout of the yard. I'd like my large, 4 post design to work but I don't know if it is possible from an engineering standpoint short of going with steel beams. If I ran the beams across the 15ft posts, I would then have to span the 20 feet with the rafters which would likely sag given that they would be less substantial than the 4 2x10s (or 2x12s).

Any help would be appreciated.



Follow-Up Postings:

RE: Pergola design help needed

Forget the sandwhich beam, go with a solid beam, along with moving the post in a foot or two on each side. You could easily move the post in 2' on each side, then your span is only 16' that way you could use a 4x12 or 6x12 beam. For the 15' span on the rafters go up to 2x12's then the 2x2's on top, 16" o.c. not 24".

There's other ways to go about it too. A flitch plate inside 2 - 2x12's with a 1x wrap would work too.

Of course the right way would be hire a deck contractor & let him & his engineer work something up for you.

RE: Pergola design help needed

Thanks for responding. I'm thinking that I'll just reduce the dimensions to a square 16x16 and use the 4x12 as the beams as you have suggested. Going with a flitch plate sounds like a good idea too but would probably be a budget buster--which is why I'm doing this myself rather than going about it "the right way" as you suggest.

thanks again.

RE: Pergola design help needed


I just built a pergola that spanned 19' using 6"x12"x 24' western red cedar. I can post some pics and help with calcs if needed. I'll try to follow-up shortly.

long span pergola

This is a picture of it partially complete:

It now has a two rows of 2"x4"x12' that provide shade. Each 6" on center. There is a 1 foot gap where they meet in the center. I'll post a pic once I get one taken. It went up pretty easily. The hard part was cutting the angles. A 16" Makita beam saw (rental) was money well spent.

I thought through this for about 2 years, so fire away if you're interested in help.

RE: Pergola design help needed

Nice job bemis, I like the look of the big beams.

I did one a couple of years ago, 26' long 6x12. I moved the post in 3' on each side letting the beam cantilever past the post, so it spanned 20' no problem.

RE: Pergola design help needed

The knee braces in the corners basically have the property of shortening the distance between posts. So if the knee brace comes in on the beam 2' on each corner the post distance is more like 4' less. Not actually as much as 4' but it will help and add lateral support.


this message is to beemis, I really like your pergola design and i was hoping you could share the dimensions with me and maybe something of a building plan. I am wanting to build something very similar to this. also did you build the stone bases for the support beams yourself?

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