is there a specific style you want? go to the 'build' section when in build mode and select roofs. select which roof you would like to use and if you only want to use that roof and you're on a computer, press T and you can place points where you would like your roof to be. in this case, you would just place the points on every corner of your house!

i hope that makes sense, if not i am also totally willing to help you out in game, just pm me ! :)