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I've been having trouble coming to a conclusion on whether I can reasonably build/mount a pergola to my existing 8'x12' deck. The main issue is I can't attach it to my house because of the roof overhang and clearance. Its also unreasonable to dig to add external posts outside the deck area. e.g. build a posted pergola bigger and around my deck. The below pics don't show it great, but there is good room for a 8'x8' free standing roof/pergola at the end of the deck. I was inspecting the deck construction and I can notch 4"x4" posts to mount outside against the joints, flush with the railing. e.g. the 4x4s would side outside on the left and right of the deck, and be leg bolted to the deck joints, ideally as close to the deck posts at possible.

An umbrella is not reasonable either. It's only a 8x12 deck and sticking a table big enough to support an umbrella would make the deck too tight. I could go lighter on the material so I don't have 400lbs of wood overhead. I was considering the possibility of boxing the posts on all 4 sides with 45 degree top corner bracers using 2x6 lumber, then running only 2x4 joints. I was also trying to look into retractable canopy kits that you bolt into a wood frame, similar to what I'm thinking of. Only they're hard to come by where I live. (Canada) Everyone (Lowes, HD, Rona) sells gazebos, pre-made pergolas, awnings and umbrellas only.

Anyone have any suggestions? Thanks in advance!

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you can try this google.co.in/imgres?imgurl=http://www.enhancecompanies.com/… –  Akash Jul 21 '13 at 15:01
    
No actually. That pergola is attached to the house which I said I can't do for clearance reasons. Additionally my question is about whether I can get away with mounting it to my deck. In the pic its not clear what they did, they probably built the deck and pergola at the same time with dug posts. –  garlicman Jul 21 '13 at 15:16
    
you are trying to do something like this google.co.in/… –  Akash Jul 21 '13 at 15:24
    
And I've googled tons of pergola pics. The professional jobs are all big ones where the deck and pergola are planned together or the pergolas was first. I'm trying to add a pergola to a deck with railings. Googling pics alone won't be enough. –  garlicman Jul 21 '13 at 15:26
    
Its hard to tell from the 2nd pic, which is what I want to do, but again, I'm trying to figure out I'd I an mount it to my existing deck. In that page/pic you have no clue if they dug posts for the pergola then built the deck around it. Its not just sitting on top of the deck, that would be unstable. –  garlicman Jul 21 '13 at 15:29
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2 Answers

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It's impossible to suggest a proper structure without much more detail about the existing deck structure. A solution is very likely available, I just can't say what it is. Here's some structural issues that must be addressed to be successful.

The weight of the added structure must be properly transfered into the deck structure. Plus some live load for accumulated snow or simply added safety factor. The weight cannot overload any existing structure or joints, nor the foundation system.

The added structure must resist lateral loads from wind and possibly seismic factors without racking. The railing posts could be adequate, depending on how they tie into the rest of the structure. If not, employ knee braces at every post to resist lateral loads.

You are basically building a parachute. All connections must be tied together so that this thing cannot possibly be blown away in a wind storm. Ties must be employed to transfer uplift loads from the roof all the way down into the foundation. If the existing deck connections do not provide this, they must be reinforced.

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You will have to bolt the new 4x4 posts to your existing posts. Drill 2 holes 4" apart on the top and bottom. Your new posts will have to start at the floor of the existing deck and up to as high as you want it to be. On the inside or the outside, it doesn't matter. Or if you want them to be ground level go ahead. Just bolt them securely.

After that it will be a breeze. It's just getting the posts up first. If you're going to have 1" x 1" wood going on top, 1" apart, depending on where the sun rises, you can go above your eaves. Mine are just even with the eaves. Some people like a bit higher roof as they put in outdoor ceiling fans.

You can put the slatted roof or a solid roof. If it's solid, you're going to have to have it slanted a bit with the higher part coming from the house.

Have fun. It will look great, and the shade is amazing. No more burning feet.

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