Supporting a 2-story deck

[old_guy]

We are almost ready to install siding on our Tennessee soon-to-be retirement home. Before starting the siding I want to be sure that I have properly prepared for the gable-end decks.  The main building is 52'x32', gambrel roofed with a full second story.  We are planning for a 22' wide x 12' deep deck on both the main and second floor, with the roof extended over the second floor.  First floor to second floor is 12 feet.

Current plans are for the joists to run parallel to the building's wall, and so have the decking (composite) perpendicular to the wall.  Joists would be supported by a beam on each side and one in the middle.  Current plans are for 8x8 treated posts set on concrete footings supporting the first floor deck, 8x8 posts extending up to support the second floor deck, and another set of 8x8 posts extending up to support the roof.  We are planning to install ledger boards for the deck at both the first and second story, and diagonal bracing to prevent racking.

I am wondering now if I should switch things around, so that the joists run perpendicular to the building.  I would then need a beam only at the outer edge of the deck, with 3 posts supporting it, rather than 3 pairs of posts (one at/near the house, one away) supporting the 3 beams.  With this change the ledgers would be supporting the inner end of the joists, rather than just keeping the deck from moving.  I have never build a deck supported by the house, but in this case it could save a LOT of work.

What do you think makes the most sense?  Why?  Anything else I should be worried about?

Thanks.

[Don_P]

Go to awc.org and under their publications tab download DCA6, the residential deck guide, this will give you some basic guidance.

[old_guy]

Thank you, Don.  I had seen several references to the DCA, but have never seen it before now.  It is a great guide and source of information.

I had already decided that I would rotate the deck to have joists perpendicular to the house.  All of my earlier decks have been for the first floor only and have been free standing, not attached, next to the house.  For this new deck I need the ledger connection to keep the second floor deck in place, and cannot easily transfer the strength of that attachment to the remaining framing without rotating it.

I already see post footing specs in the DCA, as well as post attachment and sizing info.  Good stuff.

Thanks again.

[Don_P]

It is a good guide. The footing specs would be undersized for your use. They are for the load of one deck, posts no more than 8' apart. You are proposing a load consisting of the combination of 2 deck levels plus a roof and at 16' post spacing (which I would reconsider). The footing size is based on the load divided by the bearing capacity of the soil. It needs to be at least as thick as the distance from the edge of the post to the edge of the footing.