The posts I have seen on here recommend you do not use PVC for roots. it is supposed to be too slick for the chickens to grasp. The usual recommendation is wood. If someone who has tried PVC writes in, we could all learn more.
Most people use 2x4's. If frostbite is a concern, some people lay the 2x4's flat side up so the birds feathers cover their feet when they roost. It is supposed to be more comfortable for the chicken if the 2x4 is on edge with the corners are rounded off so the chicken can grip. That would make the prefered thickness about 1-1/2" to 2" but I'm sure you could go bigger. PVC that small would have to be braced a lot or it would sag badly.
I'm building my coop and already have branches from the recent ice storm drying to be used as roosts. You might find branches from where someone pruned trees over the winter. One downside to using branches is that mites can hide in the bark. I'll try sprinkling DE on it and leave the bark so the chickens can grip the rough bark better. If that does not work, I'll skin it off and oil the underside. Another problem with branches is to get them to lay flat and not roll. Properly applies nails or screws should solve that.
Good luck with what you decide.