Help on plywood over plywood

Adding another layer of plywood to an existing plywood subfloor should be relatively simple. Well, everyone agrees you don't line up new sheets on old, but overlap seams. But you can either nail/screw to joists or not, use glue or not, nail/screw every 3 or 4 or 5 or 6 inches at the edges, and nail/screw every 6 or 8 inches in the plywood. Too many choices for someone who just wants to do this once, and not have to go back because he's picked the wrong thing. I guess the more nails/screws the merrier (the more the sheets will be held together), but what's considered too few? Gluing would certainly give more strength. Nailing or not nailing to joists? Why or why not? And if, in one sheet (to another) you accidently do hit a joist and skip fixing it, how bad is that?
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I would overlap joints GLUE and SCREW to joists.
screwing plywood to plywood may leave some wiggle room and lead to squeaks..
so why are you adding another layer?
add floor leveler to joints too for snooth finish.
whats your finished floor going to be?
you want to avoid dissimiliar height floors or steps as they are a trip hazard
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I would glue and screw to joists. Rent or borrow a screw gun with a long shaft so you don't kill yourself cut the first sheet down both ways by 15" and it should keep things off the seams. use a snap line to mark the joists and go to town I would do every 12"
A driver like this would make things easy but any belt fed unit would be a big plus
http://www.toolking.com/productinfo.aspx?cid70&productidS62
It would help if you told us the reason and thickness of planned plywood?
W
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Current subfloor is 1/2in plywood which will not adequately support any ceramic tile. But another layer of 3/4in will cause height problems. Another 1/2in plywood followed by Ditra and tile will just do it.
Now here's something I found at http://www.terrylove.com/forums/archive/index.php?t-1222.html
"Adding the same thickness plywood doubles the stiffness. Gluing it increases the stiffness 8x! Depending on the tile you might want to install would depend on whether it is necessary to either just add another layer, or if it can be glued together. If you decide to glue, use something like Titebond II, use a squeegee or something similar to spread it evenly, then use deck screws to put them together. Avoid screwing the second layer into the joists (but make sure the first layer is solidly attached before adding the second)."
And in response to the question about not hitting the joists:
"The goal is to make the floor stiffer, but, to also still allow for independent expansion/contraction of the subfloor materials. If you attach the second layer to the joists, then it isn't independent. Now, if you are going to glue them together, some of this is a mute point, but, you still want to offset the end and side joints, since bridging them makes the floor stronger - i.e., you don't want the edge joints to align vertically between the panels."
I'm not in the floor business, so I don't get it. Comments?
--jim
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I would just use backerboard as it comes in 3x5 sheets to avoid seams and will cost less than the plywood and the ditra to boot!
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