Advanced tab on Model Options dialog.
A component block is considered 'flat' when its' 2 largest surfaces make up most of the total surface area of the component block. The Fraction two largest areas data defines the meaning of 'most'. The default value of 0.8 means that when the two largest equal areas make up 80% or more of the total surface area of the component block then the block is considered to be 'flat' and only one of the 2 surfaces will be modelled. Which of these surfaces is used in the calculations defined by Flat component block surface selection.
Note: If you need all component block surfaces to be written out then enter 0.99 here.
For flat component blocks DesignBuilder uses the selection made here to decide which of the 2 largest surfaces of the component block is used in the simulation as a shading surface. Choose from: