Addition to existing home. Addition to have a shed roof.

[Jesse O'Roscoee]

Hi Folks. I am a new user. I am trying to add an addition to an existing building. I can generate the existing building (has gable roof). When I add three walls to the existing building for the addition, I cannot generate a shed roof. Existing building has 10' foot walls, addition to have 8' walls. Foundation is same elevation. Addition is off the existing house gable end. Addition roof to be 2/12. Thank you.