I was up on the roof watching a movie tonight and it was great. I have an Epson 730hd projector and it works great - I 'beam' it onto a wall that is almost made for it and get an nice 75-80 inch image ...
Not sure what your budget is but many projection screens cater for rear projection. Obviously it is not as ideal as front projection but it does no harm really. Screens from Stewart and Avance worthy ...