Is it ok to use half inch plywood for sub-flooring under hardwood floors ?

Is it the original sub-floor or is there something under it?

you want 3/4". 1/2" over 16" centers will flex and cause more squeaks than normal.

What it is, we built a lean-to addition on the side of the house to create a hall with a bath and an extension to the kitchen. We put half inch plywood on the roof, then tin. Now we want to build the upstairs bath and hall on top of the existing bath and hall. It would just be so much easier, less wasteful, and it would save us some money to remove the tin, and lower the lean-to roof over the bath and hall to make it flat which would create the floor for the upstairs bath and hall. It already has the half inch plywood on it. I wonder if we got some quarter inch plywood and glued it to the half inch would that work?