To par-boil or not to par-boil root vegetables before roasting?
I think this depends on how big the pieces are, and of course which ingredients. Some veg cook faster than others.

If you are using 1 inch or smaller pieces, you may not need to parboil, but if you are using larger pieces you may wish to par boil for longer as the pieces get bigger.

This lets you cook the interior without burning the exterior when you roast.

Of course if you add things like squash or broccoli to the veg roast, add those last and they don't need par boiling.