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I am building a pergola with dimensions of 20x17. I am planning on using 4 6 x 6 posts, 4 2x 10 x 24 feet running along 20, 14 2 x 6x 20 Feet running along 17 feet(rafters), 14 2 x 2 x 24 on top for shade, 2 feet 6 x 6 knee braces to support 2 x 10 beams. The posts will installed in ground in concrete. The wing span is 20 with out knee braces. The knee braces might add 1 1/2 feet along both ends so that will bring the wing span to 17 feet for 2 x 10. Is this wing span ok or do I need to add another post ?

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  • I’m not sure I understand what “wingspan” is. Is it the clear span from knee brace to knee brace along the long side ?
    – Lee Sam
    Commented Aug 24, 2020 at 14:18
  • Yes the clear span from knee brace to knee brace. This the place where the 2 x 10 beam will be hanging with out support. Commented Aug 24, 2020 at 14:26

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Yes, your 2x10’s spanning about 20’ from post to post (or about 17’ from knee brace to knee brace) will easily support your 2x2’s on your 2x6’s.

I usually qualify my answer by saying “depending on the species and grade”. However, in this case, it’s not necessary because it is so far under-stressed.

You didn’t explain a couple of things so I’ll make some assumptions and you tell me if I’m wrong. First, your Pergola has an open air roof. That is to say, the roof materials are spaced out over the roof and you do not have a solid roof, so snow will “fall through” the roof.

Second, you’ll probably have better than average lumber because it will be exposed (not covered). That is to say, there will not be any loose or missing knotholes.

So, depending on the actual spacing the weight on the 2x10’s is:

  1. Spaced 2x2’s = 1.5 psf

  2. Spaced 2x6’s = 3 psf

  3. Miscellaneous (lights, chimes, hanging plants, etc.) = 1 psf

Total Dead Load = 5.5 psf

Therefore, the total load on each 2x10 is half the width of the pergola, times the total Dead Load....17’ / 2 x 5.5 psf = 46.75 plf.

The allowable load for a 2x10 spanning 17’ is about 80 plf > 46.75 plf. If you decide not to install the knee braces, a 2x10 spanning 20’ is about 70 plf > 46.75 plf. However, you’ll probably want the knee braces because it will sway if not braced.)

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