Did you draw the wall (roof-to-be) at an angle in SH3D, e.g. 45 degrees from 0,0 to 500,500?

Hint, you can create connected walls both sides to control the angle of roof ends, but export only the selected wall (with window).

More on this in these links:

Tutorial for sloping ceilings, roofs, dormers and more
Create a Simple Roof in SweetHome3D (Tutorial )