ASP.NET 5 Custom roles for Windows Authentication

I'm building an intranet type site with ASP.NET 5 that uses Windows Authentication. I have the authentication working, but I don't want everyone on the domain to have access to the intranet site. I can't use domain roles so I've set up my own custom roles in my SQL Server. I have a table that maps the domain username to roles. I want to restrict access to the intranet site to only users that have a role defined in my SQL Server role table. How would I set up custom roles for Windows Authentication in ASP.NET 5? Thanks!