Stiffening plywood subfloor

I'd appreciate advice on the best way to stiffen an exisiting 5/8" plywood subfloor on 2x10 joists.
I'm thinking of screwing down the existing plywood subfloor tightly to the joists and then topping it with sheets of 5/8" plywood subflooring laid perpendicular to the existing floor, and then I'd screw the new subfloor down into the joists too.
Would this stiffen the floor to any significant degree? Would screwing down the existing flooring stiffen it? Are there more effective (not necessarily cheaper) ways?
Incidentally, there is no access to the joists from below.
Thanks in advance for all helpful replies.
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Darro wrote:

How about if the reply isn't helpful? They still tried!
The second layer of plywood will do wonders. Screwing into the joists is a must, but you should also screw the layers together between the joists if you want the stiffest floor possible. Don't use drywall screws, use deck screws.
R
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Use construction or subfloor adhesive between the two layers in addition to the screws.
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It really depends on how far you think you'll need to go.
We're renovating a bathroom (5/8" ply on 2x10 12" OC joists), which had a lot of squeak and a fair amount of "play"/bounce.
Went over the plywood refastening it down with 2 1/2" screws, and the linoleum installer covered that with 1/4" ply (not the traditional birch 4x4's BTW), stapled it down and then filled the ply seams to make sure there was no grooves to "telegraph" thru the lino.
Then they glued the lino to that.
Only used construction adhesive under the 1/4" in two small areas where the T&G appeared to be non-existant in the original subfloor, and the plywood joint wasn't perfectly aligning. Glue was to keep the joint from moving/squeaking.
Zero squeak, floor's very solid now.
Might not be enough for a ceramic floor job, but it certainly was enough for the lino.
--
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
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Plywood should help alot unless joists are the issue then who knows. Why is there no access to them
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I did not see this in the other posts,
Make sure that the new joints are at least 2-4 feet away from the current joints.
I am not sure that perpendicular to the existing will do any good. Plywood spans the structure running with the structure does not span the weight out.
How about jacks under the structure at the half way point. That should stiffen up the floor.
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I wanted to tile a kitchen that had 1" boards as a subfloor covered by 1/2" plywood. Tiling contractors who looked at the job said the floor had too much deflection, and they would only do the job if they could add another layer of 3/4 plywood over the existing. That would have resulted in the kitchen floor being higher than the rest of the floors in the house.
I simply screwed down the floor to the joists with long deck screws, and then peppered all over the surface with 1 1/2 inch screws so that there were screws within 5-6 inches of each other at all points. The floor was then VERY rigid, and I did the tiling myself. 10 years later, nothing has cracked or failed in any way.
Commodore Joe Redcloud
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Yes it would. A house is being rehabbed next door and the owners are doing all their own work. They double layered the subfloor on the 2nd story they're adding for this very reason. I was able to walk up there pre and post 2nd layer, and the difference is quite noticeable.
Best Regards, -- Todd H. http://www.toddh.net /
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