There's no way to mesh or join brushes, unfortunately. However, a smooth diagonal edge could be created using the clipping tool, which seems like it may be what you're looking for. If you're going for a more unconventional roof shape, then try using a displacement.
weeellll... kinda although it's not really what youre looking for in this case. View: https://www.youtube.com/watch?v=_oLIhII0vHs