For the next step use 3 cubes that are more than the length of the house and the width that you would like the roof to be. Make the first one (the one touching the roof) 45 degrees. The second add a few degrees maybe 15, and the third make flat. These should all be intersecting each other.
With the first one selected and using the knife tool cut the section that intersects with the house and delete it. Next select the first and second and cut across there intersection point (directly where they intersect, anywhere else could mess up the roof) use an angle that is equal on both sides. Delete the extra after you cut it. Do this step to the second and third pieces as well.
Duplicate all of these pieces around the building and then select them all. Using the knife tool again cut across the house so you cut the corners delete the extra and the roof is finished.