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