Block tab on Model Options dialog under Block Transition header.
You can enter the angle which defines the surface tilt below which a wall becomes a roof. For example if the roof-wall transition angle for a particular block is 45°, a surface in this block having tilt 40° is modelled as being a roof (using roof constructions, glazing layout etc). And a surface having tilt 50° is modelled as being a wall (using wall constructions, glazing layout etc).
You can force all non-horizontal surfaces to be walls by unchecking the 'Allow pitched roof construction' option.