I am building a pergola with dimensions of 20×17. 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 ?
Pergola wing span
pergola
Related Solutions
You could probably get away with two 4x4 posts on this, since the house would pick up a good amount of the load. I would run 2x6 trusses and cleat them to your house (someone will eventually want to hang something off it, even if that something isn't crazy heavy, so don't toenail this end).
I would laminate a 2x6 beam to go between your 4x4s. The problem you face here is that 25ft is too long for a single board to really span well. Ideally, I would want this beam to sit on top of the 4x4s, since it will be heavy as well. To make a laminate beam, buy the longest 2x6s you can carry (12ft sections should ensure you're not needing to do a lot of bolting). You'll want some lag bolts and construction adhesive that can work with pressure treat (even if you use cedar, the PT stuff is coated for outdoor use). Stagger the boards to make one 25ft length, gluing and bolting as you go along. You can then put the beam on top of the 4x4s and use screw plates to attach. You can then toenail the truss boards (with deck screws) to the top of the beam
I would think you'd be safe at 13'. Although it's not an exact comparison, 13' seems to be "in the ballpark" considering there won't be much load on them.
Here's what I'm basing my opinion on:
Max. Live Load 20 lbs/ft2 (956 N/m2)
https://www.engineeringtoolbox.com/horizontal-roof-rafter-span-d_1484.html
And also using this calculator to try various wood species and spans, etc:
https://www.awc.org/codes-standards/calculators-software/spancalc
I think how the "rafters" are attached might have some significance for you too.
Best Answer
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:
Spaced 2x2’s = 1.5 psf
Spaced 2x6’s = 3 psf
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.)