From my experience with revit it is one of those quirks that have no apparent explanation.
Revit mass roof.
Family parameters and materials can be applied to mass families and elements like roofs floors etc.
Create a mass that is specific to the context of the current project.
If that does not work i hack it and use a wall in place of a roof and see if that works.
You may have to adjust its xyz location before finishing the mass which can be accomplished in plan or elevation.
First you can create a mass and create roof by selecting the mass faces.
I will not discuss much about this for now.
Mass are quite easy to make but using this method might be require a lot of works.
5 once located properly use the roof by angle tool to generate a roof using any roof type.
In the type selector select a roof type.
When that happens to me i remove the family remove the massed roof reload it and then try and make it.
Open a view that displays the mass.
This mass cannot be reused in other projects.
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Amazing that a user in a revit related forum is advised to go to another application archicad 16 to do a simple gable mass to represent a sample house in a site.
Revit model space lets you work simultaneously with in place masses system families and mass families conceptual mass families.
Optional on the options bar specify the level of the roof.
Just load it into the project adjust dimensions materials etc.
Click massing site tab model by face panel roof by face.
Enter a name for the in place mass family and click ok.
For your information a parametric gable mass is already available in the revit libraries in the mass folder.
Click massing site tab conceptual mass panel in place mass.