The Roof Wizard in ArchiCAD 8.1 will use the RoofMaker Library 8.1. The Wizard does NOT use the Object Library_81/06 Wood-Plastic 81/ Roof Construction_81 library.
The name of the Library part Roof Wizard will place is Rafter2_1.
Please check if this object is present in the RoofMaker Library.
Maybe you should also check in Add-On manager whether the correct Add-On version is loaded.
Gergely Kmethy
VP, Customer Success, Graphisoft