I'm going to install tiger wood 3/4 inch hardwood floors in my house. The manual says you need 3/4 inch CDX plywood subfloor. I have 1/2 CDX plywood subfloor. Is the extra 1/4 inch really needed? Some people have told me it is not needed, but I want more opinions.
If you are interested in the science behind your question, the answer lies hidden in your question. You mentioned "is the extra 1/4 inch really needed".
As do most people, you translated this in your mind to "I know how little stiffness there is in a wimpy piece of 1/4 inch plywood, so how could it really make any difference".
The stiffness of a piece of material is only determined by simple addition in a case where the two layers are free to slide against each other. If just laid one on top of the other with no connection between them, two 1/4 inch thick pieces of plywood form a combination that is merely twice as stiff.
In the more common case of well attached or glued layers (or in this case just a thicker single sheet of plywood), doubling the thickness of a sheet MUCH more than doubles its stiffness. The 3/4 plywood is more than twice as stiff as 1/2" plywood.
If you are installing the new flooring perpendicular to the joists you will be ok with 1/2" subfloor but if you are running in the same direction the joists you will need to replace the subfloor with 3/4" t&g plywood