A contractor weight trash bag as a liner inside of your pack will keep the contents dry, but not the pack itself. Since most packs have zippers the makers don't worry about using waterproof fabrics, which means that the pack itself can get saturated as KMT mentioned during a prolonged or heavy rain. You can buy a pack cover for your pack, or you can use a poncho over you and the pack to keep it dry. The advantage of the poncho is that you stay dry and the shoulder straps and waist belt also stay dry. Plus, you don't have water running down between your back and the pack frame or pack body.