Picture from http://thewebsiteofeverything.com/img/silver-gray_brushtail_possum.jpgI saw a nifty show on TV once about how to make your roof so that the possum can get out but not get back in and when combining this with supplying a nearby nestbox can help to keep possums out of your roof. I can not remember what show it was on but the information I found on the Living with brushtails webpage of the Tasmanian Department of Primary Industries, Parks, Water and Environment has the same approach.
Their suggestion is to firstly find where the possums are coming in and out of your roof, to make tempory repairs to your roof to prevent re-entry to the roof by the possum, to trim trees that may provide acces to your roof and to put up a possum nest box.
The NSW Department of Environment, Climate Change and Water also has a fact sheet on removing a possum from your roof and making and installing a possum nest box.
So one of my new jobs around the house is to build a possum nest box and put it up in one of our trees in the backyard and then try to get the possum out of our roof. I think the kids will love this building project.