"Springy" subfloor and tile

Thank you for reading my post.
I want to know if I should pull-up the entire sub-floor of my guest bathroom.
My house is a 1982 tri-level (kit?). 50 foot x 25 foot. Most floor & ceiling joist are 24" OC. The joist run the width of the house.
The guest bathroom (full bath) had a tile floor on a mortar bed, that I removed.
The sub-floor that's under this tile floor is 1/2" plywood with some sag on the outer ends and very springy between joists. Is this normal after removing a mud-tile?
I ran a "stud-finder" along the floor, and found that the joists are NOT 24" OC and more like 16"OC it a hit-and-miss pattern... (guest bath is in center of the 3rd floor/ Master bath is on opposite wall of guest bath.) Is this ALSO normal?
~BUT~ my question IS .... Should I set my circular-saw at just under 1/2 inch, and THEN replace the entire floor.... or should I just add exterior-grade plywood over the exisisting sub-floor?
For more information... the "basement" (1st floor) Den, 1/2 bath and office are 8 foot ceiling, but the basement hallway is 7 foot. (Bunch of duct-work, plumbing. ect.)
Thanks, Dave
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1/2" plywood is too thin for a floor, also since it's a bathroom floor it may get moisture, which may have further weakened the 1/2" plywood
would go with minimum 3/4" thick plywood for floors

sounds like existing 1/2" subfloor is unstable, removing it and replacing it with 3/4" exterior grade plywood will be more stable

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Should I rip-up the 1/2" plywood and THEN install two sets of 5/8" exterior grade plywood?
OR should I just install 3/4 exterior grade plywood OVER the 1/2" that I already have?
And what about "blocking" inbetween the joist if I remove the old 1/2" plywood? A waste on time/material?
Deep felt thanks to everyone, Dave
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I guess I should have stated in my original post that I am going to be adding 1/4" cement backer/hari-board ONTOP of the subfloor BEFORE the tile.
Thanks, Dave
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1 1/4" is the recommended thickness of the sub floor under ceramic tile. That could be 1/2 +3/4. Ripping up the existing 1/2" is not a good idea. There will always be an edge with no support under it. If there is no damage to it leave it, vacuum it real good, screw (make sure screws are long enough and go into the joists) and glue 3/4 right over the top. Screw and thinset the cement board to the new 3/4 ply. Hardest part can be raising the toilet flange to the new level.

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Thanks Calhoun/Paul !!!
I'm a little confussed on the "gluing" part.
Should I first put down #15 roofing felt over the 1/2" plywood, and THEN 3/4" exterior grade plwood over that --- with glue inbeteen each layer?
Help! David
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If you are using the felt than no glue. The felt prevents squeaks from the two floors rubbing against each other and compensates for any grit you didn't get cleaned up. If you glue you don't use felt. One or the other not both. I always have preferred glue (construction adhesive in the caulk gun tubes) on a real clean floor. It locks the two plywood pieces together so they don't move against each other.

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On Tue, 18 Jan 2005 03:53:47 -0500, snipped-for-privacy@webtv.net (DaddyMonkey) wrote:

Put down tile for my fireplace hearth. The floor 'deflected' a litte even in the short run. I was told, and found it worked, that if you morter a cement board(I used hardbacker) down, and properly screw it down, like every 6", and 1" from the edge, the floor became rock solid.
Check into this, they have 1/4" and 1/2" versions. Easier than replacing a subfloor, and a step you need if you want to lay down more tile.
hth,
tom @ www.FindMeShelter.com
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