You can make the roof of your hall with rectangular roof geometry from the form type section in the hall settings dialog.
Location of the Rectangular Hall Type
Select the hall and click the right mouse button. Click the Properties line from the right click menu that opens.
Enter the hall settings.
Select the hall you want to change the roof geometry for from the hal list.
Choose rectangular hall the form type section
The hall you choose will consist of a rectangular hall type.
Hall view when hall 1, 2 triangular hall type and hall 3 rectangular hall type is selected