When I was bulding my house, I run into some problems with roof and my wall in at 45° angle.

See picture below:

I guess no-one had this problem before

Do you want the one that is farther back to be as long as the front one? It needs support, put floor tiles on top of the house to square off the wall, then use the manual roof tools to extend the roof yourself.

I don't the roof on top of the ground. I would rather have a diagonal roof or something

Right now, there is no way to create a diagonal roof. One either has to do some creative "roofing" so the roof isn't so far over the diagonal wall, or Maxis has to include this type of roof in an expansion.

Unless - someone who knows how the scripting works and figures how to edit the roof script to have a diagonal option (nudge, nudge to anyone who may know how to do this), we will be without diagonal roofing.

maxis is going to put diagonal pools into nightlife, so there iss a good chance of having diagonal roofs soon

NRJ - A gable roof won't run at an angle ..

Try changing to a hipped roof style.