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I have an existing deck on the back of my house, and I’d like to add a pergola. The deck is 17’ x 16’, and I’d like to build the pergola to roughly match the size and shape.

The attached picture shows a view of the deck from above, along with measurements and placements of the existing support posts and beams. The posts are 6”x6” and the beams are paired 2”x10”. The joists on top of that are 2”x8”, and there’s deck flooring on top of that. There is also a ledger that runs the width of the deck, so it is attached to the house. I don’t want to attach the pergola to the house though.

My idea for the pergola is this:

  • Place 4x4x10’ posts at the same locations as the deck posts. These would pass thru the deck floor and rest on the beams below.
  • For the 4 posts that are at the outer edges of the deck (A, B, D, E), lag-bolt the new posts to the adjacent joists.
  • The 5th post (C) would be 2 feet in from the edges of the deck, and about right in the middle between 2 joists, so I’m still wondering how to secure it. Maybe a post bracket (?) attached to the bottom of the post and to the beam?
  • Run 2x6 beams between the new posts, either sitting on top of the posts or attached to the sides of the posts with joist hangers, to form the outer edges of the pergola.
  • Run 2-3 additional 2x6 beams across the middle, parallel to the house.
  • Run some number of boards on edge, perpendicular to the house, across the top.

Questions:

  • Could the side beams that run perpendicular to the house extend almost all the way to the house? That would provide an additional 3.5-4’ of coverage along the house side, where we typically place deck furniture.
  • Are 4x4 posts sufficient for the pergola?
  • Is the placement of those posts OK to carry/support the load to the structure below?
  • Any other suggestions or questions I should consider?

Thanks for any info you can provide.

Deck diagram

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  • Forgot to mention that the deck is attached to the house by a ledger. I don’t want to attach the pergola to the house though. – DJR May 9 '18 at 3:18
  • How are the posts supported? Are there footers under them? I'd be tempted to replace two corner posts with long ones which will support both deck and pergola and two new ones (at house) to support the end you do not want to attach to the house. – peinal Oct 27 '19 at 21:05
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First answer! (On an old question.) Could the side beams that run perpendicular to the house extend almost all the way to the house? That would provide an additional 3.5-4’ of coverage along the house side, where we typically place deck furniture. I'm assuming you mean the rafters (that rest on the beam) would cantilever over the 17' beam by 4'? That will be fine, as there won't be any weight bearing on the rafters other than their own weight.

Are 4x4 posts sufficient for the pergola? In my estimation, yes. Check out load tables to be sure (capacity varies by species, length, wet or dry conditions, etc.) They will look a bit spindly though.

Is the placement of those posts OK to carry/support the load to the structure below? Should be fine, in fact, you could place the posts almost anywhere. Each post will be carrying less weight than the average American male.

Any other suggestions or questions I should consider? Pergolas don't weigh that much, but they do catch a lot of wind so lateral load and rotation of the posts is a real concern. I understand you're bolting some the posts to the joists so that should solve that problem.

The 17' beam span will likely cause the 2x6 beams to sag. I'd recommend bumping up to 2x8 or 2x10.

By this point, you've probably built your pergola. How'd it turn out?

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    Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, props for taking our tour before posting; few newbies do. – Daniel Griscom Jan 31 '20 at 17:11

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