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A couple of years ago I built a deck onto my house. It is 32' x 14'

I used a cantilever design at 12' with 2x8" floor joists, with 2x6" decking boards. The Cantilever beam was set on 6x6" posts that are on top of 30" deep footers.

I doubled up a 2x12" for the cantilever beam by notching the 6x6" on either side, and putting the 2x12 in each notch. These posts are spaced every 8 feet.

The question I have now is how to go about building a pergola on top of part of the existing deck - thinking something like 12x12', 8' high. The existing deck is only maybe 3 feet off the ground at the highest point. Can I build a freestanding pergola on top of the existing deck, or attached to the floor joists or something like that? I dont want to attach it to the house, and I dont really want to have to dig footers for it. The reason for this is because I had to rent a bobcat to dig the first footers because the ground is like solid shale about 3" under the top soil, and would be pretty hard to dig now that the deck is already there.

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So if I understand correctly, you have 32' long with 4 posts spaced 8' apart, its attached to the house on a 32' ledger board, only two beams and no other posts in the middle or on the sides? The overhang on either side is 2' because of only 12' beams? What is the overhang from the beam furthest from the house to the front of the deck? –  maple_shaft Jun 15 '12 at 16:33
    
the ledger attached to the house runs 32'. Parallel to that, the cantilever is 12' away from the house, and the overhang is 2'. This means my deck's entire depth is 14' from the edge to the wall of the house. The floor joists run perpendicular to the ledger. –  Derek Jun 15 '12 at 18:25
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This is quickly going to become too complicated to tell you for sure if your deck will be able to support a structure like a pergola, but if it is even half the weight of your typical hot tub then your deck will very likely not support it.

To accurately determine this you need to figure out the additional #'s/sq ft of load that a structure as large as a pergola will place on top of the deck. Doing this is simple with something like a spa where you have a clear square footage area and a relatively even weight distribution when filled with water and people. This is not so easy to determine with a structure where certain points of the structure are going to carry the weight of the structure differently. The weight is not as evenly disbursed.

Right off the bat there I would advise you to consult an engineer with your structure plans to determine if this is feasible and safe. This is completely disregarding the fact that your deck is only as strong as its weakest point. If your house is brick and your ledger board is attached with carriage bolts then even if the joists were on 12" centers then you are still likely putting entirely too much stress on the ledger board. You also could have a lot of problems if any part of your structure is going to sit over that 2' overhang.

In the end, an experienced engineer will probably be able to confirm pretty easily if this will be feasible or not.

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