Pergola span design

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Old 10-17-13, 12:25 PM
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Pergola span design

I am building a pergola over my existing concrete patio which is 20 feet wide x24 feet deep off the house. I'm thinking of using 8x8 posts with 6x8 boxed beams set on front and two sides with ledger board on house for fourth side. 4 posts, 2 on each side, one at 12 feet out from house and one at the end at 24ft. So I would have 24ft beams on each side and 1 20 ft beam on the front and one in the middle. Posts would sit inside patio, so it would be around 16-17 ft span on the front and middle beam. Now place 4x6x24 rafters across and topped with 2x2. I hope that's not to confusing. Anyone think this will work structurally
 
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Old 10-17-13, 12:33 PM
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Here is a pic of what I was thinking

Here is a pic of what I was thinking. It would be wider and longer though. Hope the pic shows up
 
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Old 10-17-13, 01:39 PM
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You don't want to attach this to your house. Too much chance of leaks. Make it free standing just like the one in the picture. You will need to core out the patio so you can dig footers to the frost depth for your area, probably 2 to 3', and 12" around. You can't sit such a weight on just the patio, as it will crack and fail. What size lumber and type do you propose to span 24'?
 
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Old 10-17-13, 10:31 PM
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Like I said I will be using 6x8 to span 17 feet at the widest, with 4x6 rafters on top. Was thinking about using redwood as I have a local supplier source. As far as the house, the way the door sits to the patio it wouldn't look right or be able to set a rear post to make it freestanding. Most all pergolas/patio covers I have looked at around here are attached to the house just like a deck.
 
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Old 10-18-13, 03:42 AM
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You can attach it to the house, but you need special precautions, such as flashing and fasteners to prevent water infiltration. The other specs stand, however, regarding the vertical support.
 
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Old 06-25-14, 06:47 AM
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Other than flooring/roof specs for loads, does anyone know where to lok to find span allowance for 2x6 cedar, 12" o.c. set on 2x8 cedar sandwiched beams? Right now I'm planning on 2x8's spanning 10'. And the 2x6's, spanning 11' 3 1/2". Because there is no sheathing/decking for people or snow load, it seems like there would be a different load/span calculation since its open rafters?
 
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