pergola structure

momento · 09-22-2012, 12:14 PM

Hi all

I'm planning to build a pergola in my backyard. It is 21L x 16W from end to end.
16L x 11W between posts

I already have: 4 6x6 posts installed in concrete.

1. My plan is to use 4 2x8 to connect 6x6 posts at the W
2. Then use, 6 2x6 to connect the L
3. On top, run 11 2x6 on the W again.

It's all redwood. And I live in California.
My questions is, is it gonna sag? I couldn't sleep last night feeling doubt about my design. Please help.

Thanks,
Kevin

Daniel Holzman · 09-22-2012, 04:23 PM

All wood structures sag eventually, even if they carry little load (such as your pergola). The only way to prevent sag is to use precambered beams that are specifically designed so that the eventual sag is approximately the same as the the camber, so after 10 years or so the beams are flat. This is expensive and totally unnecessary structurall, it is strictly an aesthetics thing, or in the case of bridges you want a flat deck, so the beams are precambered so they lay flat when the deck is attached.

You can compute the sag on your beams using standard beam theory and the dead weight of the beams, but they won't sag much, and since you are not putting any load on them (no roof, correct?) it is an aesthetics issue.

oodssoo · 09-22-2012, 05:59 PM

Quote:

Originally Posted by Daniel Holzman

All wood structures sag eventually, even if they carry little load (such as your pergola). The only way to prevent sag is to use precambered beams that are specifically designed so that the eventual sag is approximately the same as the the camber, so after 10 years or so the beams are flat. This is expensive and totally unnecessary structurall, it is strictly an aesthetics thing, or in the case of bridges you want a flat deck, so the beams are precambered so they lay flat when the deck is attached.

You can compute the sag on your beams using standard beam theory and the dead weight of the beams, but they won't sag much, and since you are not putting any load on them (no roof, correct?) it is an aesthetics issue.

All wood do sag. So you will need to add support in the middle. If you would rather have no post in the middle of the 21' or 16' sides, you could also sister the 2x8s.

Hope this helps.

Ninjaframer · 09-22-2012, 10:45 PM

I don't like to go much further than 12' on exterior exposed beams, on pergolas I usually use 4x6 for overhead Spans. If you already have 6x6 posts I'd use 6x8 or 6x10 for overhead just so the ratio looks right. You won't experience much sag either.

momento · 09-22-2012, 11:10 PM

thank you all very much for your input quickly

To prevent sagging, i decided to separate six 2x6 21' into 3 group.
So instead of each 2x6 stands by itself, each 2 stand together and connected by a small piece of 6x6

would that help?

do you have any other suggestions?

thanks all

oodssoo · 09-22-2012, 11:24 PM

Build it the way you like. The freedom to be creative is why many diy-ers do what we do.

No matter what, just make sure your main frame of this stricture is sound - from top to down.

Another thing to consider is how things will look once its done.

Have fun with it and good luck!

Ninjaframer · 09-22-2012, 11:38 PM

And post pictures when done!