On Friday, May 11, 2018 at 5:28:09 AM UTC-5, DerbyDad03 wrote:

Is 1/2" (5/8") drywall then same thickness as 1/2" (5/8") plywood or at least close enough for
this application?

I know that 1/2" (5/8") plywood really isn't 1/2" (5/8"). Is drywall?

No, but they are both similarly smaller, so the plan works. It is important though, if you install as I described that you put the cut side on top of the plywood. That way your molding will sit flat on the plywood "ledger", and the molding will cover the joint. If you put the bevel side from the factory you will have a difference in thickness immediately where the rock sits on the ledger, which in turn will cause your molding to sit at an angle when bridging or covering the joint of the rock and plywood.

If you are running your sheets parallel to the floor and you put the /factory/ edge on top of the ledger you will have the bevel edge which is thinner as it is made to receive tape/mud. Then you would see a big difference, and you would be forced to mud the joint to fill in the bevel.

Robert