create new roofplane at ridge of a roofplane

[wolfie415]

 hi! I wonder if the following is possible (in home designer pro): I manually create a roof plane and turn its shape into a triangle. At one side of the triangle  I line up the baseline of a new roof plane. looks perfect in 2d, but seen in 3d it becomes obvious that the 2 roof planes are angled against each other. i have tried adjusting the baseline angle to the pitch of the first plane, but this is a lot of trial and error and they never seem to meet perfectly ...