Block tab on Model Options dialog under Transition Angles header.
By default only roofs that are horizontal are treated as a flat roofs, but this block level setting can be used to adjust that to allow non-horizontal roofs to be treated as flat as well. For example if 10° is entered here then any roof surfaces of slope <= 10° in the block will treated as flat roof surfaces and the appropriate construction will be used in simulations. Also the appropriate thermal bridging settings will be used.
If you wish to allow pitched roofs to be defined for the current block then leave this option in its default checked state. Alternatively, you can force all non-horizontal surfaces to be walls by unchecking this option.
When the Allow pitched roof construction option is checked you can enter the angle which defines the surface slope 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 slope 40° is modelled as being a roof (using pitched roof constructions, roof glazing layout etc). And a surface having slope 50° is modelled as being a wall (using wall constructions, glazing layout etc).