The first if a few home improvement posts.  This is advanced level carpentry.

So you want to tile a floor, and you remove whatever you had to find a thin layer of plywood.  Not the 1+1/8″ of wood you typically should have.  Backer boards work great, but the fact is, the thinner the subfloor is, the more it will flex and the more potential problems your tile will have.  So you decide to add a layer of plywood.  If you add plywood and backer board, you could quickly find your floor over an inch or more, higher than it was.  That’s quite a step.  You could put a transition strip between the new floor and the nearby floor but that’s less than ideal and in some cases, completely unacceptable.  Ideally the finished floor height is the same throughout the house.

The solution, as I have found and used in many instances, is to remove the existing plywood and put a strengthening layer of plywood UNDER the base layer.  I use a nice fat 3/4″ cdx and then I can either put the existing ply back down, or if it is damaged or rotten in any way I just replace.

Its easy and fun to do, and its always better to do it right.  First cut a 1+1/2″ strip of the 3/4″ plywood about 5 inches long.  This will be a guide or scribe.  Cut a pair of 2x4s a few inches shorter than every floor joist you have exposed, with the exception of the outer floor joists as they need only one.  One at a time, and using either a buddy or a quick clamp, line them up 3/4″ below the top edge of the joist.  Now ideally you will be using a 2+1/2 deck screws and an impact gun to connect the 2x4s to the joists.  At each screw location hold the scrap piece of plywood you cut above the 2×4 to make sure it is exactly the right space for the plywood.  Get the scrap flush with the joist and screw the 2×4 to the joist.  Do that approximately every foot.

Now crosscut the plywood in strips that will fit between the joists.  (You MUST cut the plywood perpendicular to the grain.  Plywood is substantially stronger when the grain is oriented to run perpendicular to the joists.  These will be strips that are 48″ long by around 14+1/4″ wide.)  Attach the plywood to the 2x4s with 1+5/8 deck screws. 

You now have an additional 3/4″ plywood layer and not only does it not raise the level of your floor at all, it is STRONGER than if you had just laid that same layer of plywood, uncut, over the top of the joists in the standard way.  (This is because the only weak part of the plywood is the span.  With the 2x4s added to your joists, the plywood is only spanning a gap of around 11+1/4″.  This is 3 inches less than the existing joist span, which is around 14+1/4″.  This same benefit is extended to any material above the plywood as well.  In other words and without getting into some advanced math it is MUCH stronger than if you had done things the “standard” way.)