Use the Element Classification field in the Tags and Categories panel of element settings dialogs.
Set this field to "Roof".
You can list this field in Interactive Schedules.
Note that in such a case you will have to set this field to all elements you want to appear in the schedule for correct result.
Check the following video for an example:
http://www.youtube.com/user/Archicad#p/c/1CA38C58B1DCFE58/33/DTc3efqxVYg
Note that in this video, the "Element Classification" field was called "Export to IFC as" (this was during beta).
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