This can be achieved by creating a simply IFC Property mapping rule to the Pitch parameter of the roof element. After the IFC Property has been defined you can then link this to the Categories and Properties label to show the IFC property on floor plan.
To achieve this result, follow these steps:
1) Navigate to the File Menu>Interoperability>IFC>IFC Scheme Setup. Highlight IfcSlab and click New Property/Classification. Defined a Property Set name followed by a Property name and click OK.
2) Highlight the Property within IFC Scheme Setup and click New Rule. Click the Add Content button and open up the Roof subheading and add Pitch from the list of available parameters. Close IFC Scheme Setup.
3) Open up the Label tool and choose the Categories and Properties label. Click OK and apply it to the Roof element. Select the label on plan and open up the Label Selection Settings, under the subheading Custom Settings>Content. Set the option Select Data to Create Sorting. Click on 1. ROW followed by the tiny black arrow which points to the right to assign the IFC Property we created in the previous steps. Click OK and you will see the pitch angle shown on floor plan.
Please note this was written for AC20 but can be achieved in AC19/AC18 in exactly the same way as above. However there will be differences in naming conventions.
File Menu>Interopability is File Menu>File Special within AC18/AC19.
Add Content button is Add Parameters within AC18/AC19.
Categories and Properties label is Generic Label within AC18.
Hope this helps.