Yeah I have to agree with the above - 45 degrees is the sort of angle where you are almost guaranteeing the craft will flip on landing and come tumbling off the roof when you kill the motors - or at least you are creating circumstances where that is quite likely to occur. People land on flat roofs all the time, but if the slope on the landing site is greater than about 20 degrees, then it's hand catch landing for me every time.
And depending on the shape of the building there is additional danger too in that when you land it might be that you lose controller LoS to the craft , which may result in loss of video, which you might be trying to use to land. In that case, you might be trying to do it blind as well !