As possums cannot be relocated off a property, they will try to find another way to access the roof. This can be via an open subfloor and wall cavities. Rats can access the roof via this method as well. For a subfloor to be secured properly, a strong aviary type mesh needs to be put around the perimeter of the house and dug into the ground at least two inches. Chicken wire unfortunately is too flimsy and possums will push their way back through.