You said you created a custom membership provider, right? Aah, you didn't create a Custom Role Provider did you?

Right, you'll need to write another class. Your own Custom Role Provider. Implement all the methods that you need. For your specific purposes I believe you will need to implement the "IsUserInRole" and "GetRolesForUser" (something similarly named) methods.

Then obviously, set your role provider as the default role provider to be used by the application.