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?
I would overlap joints GLUE and SCREW to joists.
screwing plywood to plywood may leave some wiggle room and lead to
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
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
It would help if you told us the reason and thickness of planned
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
"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.
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