If you have a mac try and get hold of 'vectorworks' by nemetschek.
it is a cad program that will automatically design any type of roof down to the last purlin and will even generate a cutting list and quantity prices. It does almost anything you could ever need for a building job including terain surveys, staircases, spiral too, hvac, plumbing, steels, the works.
not sure if it has a demo but well worth a look - I'd be lost without it. http://www.nemetschek.net/products/index.php