Originally Posted by screasy
Thanks for all the good advice! I am in Tennessee, and for plumbing, they use 18" to get below the frost line.
I think I misled about the height. It will be 4" high on the 12' ends. The monitor is on the long (16') ends, so it will go from 4'-5', and then the monitor section will go from 6'=8' peak. Does that make sense?
Our Lowe's lists the non-treated 2x8x12 as Top Choice #2 Prime Kiln-Dried Southern Yellow Pine S4S Dimensional Lumber (Common: 2 x 8 x 12; Actual: 1.5-in x 7.25-in x 12-ft)' It is listed as "#2"...is that OK? Also, they have 2x8x16 for about $13 each. Are you saying I should use a doubled 2x10 beam or a single 2x12 beam, or should it be doubled no matter what?
In my opinion, for an outbuilding, number 2 lumber is fine for the joists. It means that knots are allowed in the board, and some edges may be "live" meaning exposed bark. If you're selective when picking through the pile, you can minimize both. Make sure to observe the "crown" of each board (all boards will have a bit of an arch to them, called the crown) and set them crown up.
For a 16' span, doubled up 2x12's is what he's recommending and will be more than sufficient. If it's me, I also buy a sheet of 1/2" plywood to sandwich between them, if only so that the final thickness matches the width of the 4x4 post you'll be setting them on. Basically you cut the 4'x8' sheet of plywood into strips 11.25" wide x 8 feet long, and sandwich it between the 2x12's using liquid nails and screws or framing nails to hold it all together.
As for the question about "develop around the 4x4's" for the posts. You only need the sonotube form where the concrete extends above the ground - the hole works as the form below ground. I believe that is what he's saying - for the 12" above ground, use sonotube to maintain the pier's shape.
If frost depth in your area is 18", dig the holes for your piers to at least 20" to be safe. Personally I'd go 24" since it isn't that much more concrete and you only get to do this once. Keep the hole diameter between 8" and 10" and make sure there's no loose dirt in the bottom. For good measure, you flare the bottom out to 12" diameter or so to spread out the weight - especially since we've cut it down to 4 piers. Now, this might add confusion, and it is certainly a debatable subject, but my opinion is this: Don't set the 4x4 posts in the concrete piers. Eventually, water will soak in to them, and even though they're treated, they will soften up and rot away. It will take awhile but it will happen. Instead, use anchors set into the concrete, then fasten adjustable post bases to the anchors, and lag your 4x4's into those.
Egghead_jr, feel free to disagree, but an even more simple way to do this is to not use 4x4 posts at all. Bring the concrete piers to 12" or 16" above grade (depending on your final height decision) using the sonotubes, with anchors set in the centers. Then fasten on adjustable post bases, and those can be wrapped directly around the doubled 2x12 rim joists (especially if you sandwich the plywood in there, as I recommended). This avoids "stilts" holding up your structure.
Here's an example of the adjustable post base I'm talking about: adjustable post base. I don't see that Lowe's carries anything adjustable like that, but Home Depot does.
And the anchors you'll want to set into the wet concrete: anchors
Hope this helps!