I have a 70's rancher with a 3 ft crawl space and copper plumping.over the last couple of years I have started to experience leaks in the copper tubing.my solution was to replace the leaking section with pex using shark bite fittings.while this works fine I expect that this condition will continue to exist and as everyone knows, leaks occurs at the worst time. So my plan is the replace all the copper hot and cold water lines with pex. The lines are all 1/2", The plan is to cut off the copper pipe that goes to all the fixtures in the crawl space and solder on pex adapters. I will NOT go right to the fixtures with pex but instead leave the copper to the tub,sink etc. I am trying to decide whether to use parallel runs for hot and cold from manifolds near the hot water tank or do zone runs where a manifold will be used in 3 areas of the house . I would prefer the parallel runs but that also requires a lot of Pex. I also plan to have shut offs at the manifold. I plan to have all the pex be 1/2" as the copper is all 1/2" now and ,except for leaking, works fine. Does anyone have any recommendations or warnings for what I plan to do. I have some experience with pex as I replumbed a small house on our property but it was considerably smaller with only 2 sinks and one toilet and tub and runs of about 20 ft. Thx Fred