If you model a roof as a solid object, and the pitch of the roof is less than 45 degrees, AccuRender can think that the model is actually a curved object. This is determined by the “Smoothing” value set in AccuRender Object Properties.
To solve the problem, select the roof object and bring up the Object Properties dialog. Switch to the “Meshing” tab, and set the “Smooth Shading” radio button to “None”.
Also see this PDF