pergola
Building pergola with two rows of 3 posts. I'd have 2×10 beams mounted on the posts supporting the 2×8 rafters, but I'm curious how far apart I can push my rows of beams. Ideally I would love to have them 13' apart with the rafters cantilevered 18" on either end but I'm worried I will have sag.
I should note that I am planning on having 2×2 slats on top of rafters spaced every 12". Also, will be using PT lumber.
Thanks
You can easily span 12' with a 2x12 for a pergola as long as your load is not crazy. What is going on top of the 2x12's? 2x2's? I would set my posts on 12' centers and cantilever 4' on each side. I would also sandwich a 2x12 on each side of the post. There actually is enough information in your question but there are many online beam span charts. Here is one. Hope it helps. Make sure there is no earth to wood contact on your posts and you will enjoy your pergola for decades. http://www.decks.com/deckbuilding/beam_span_chart_table
Assuming the pergola is a “lattice” type roof and not a “solid” roof, (because you mentioned 2x2 slats,) the total load on any one side would be about 1.5 psf for 2x6’s 24” o.c. plus 2.0 psf for 2x2’s 6” o.c. For a total load of about 3.5 psf.
So, the total load on the 2x10’s would be about 3.5 psf x 17’/2 = about 30 plf.
Depending on the species and grade of 2x10’s, one 2x10 spanning 17’ will support about 60 plf. So, two 2x10’s spanning 17’ will support about 120 plf, which indicates your support beams are grossly oversized.
Connection of the 2x10’s to the 6x6’s can be with 3 - 16d, which will support about 100 lbs. each nail.
Of major concern is how the 6x6’s will be supported and stay plumb.
Best Answer
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)
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.