ASP.NET Security over WCF instead of directly to SQL Server

 We have a client who would like us to add an extra layer of security thus making it impossible to access our SQL Server database directly from our ASP.NET web server and have a middle layer thus creating a 3-zone or 3-tier solution.  Right now our ASP.NET application uses the the built in ASP.NET Security/Membership/RoleManager providers and connects directly to our SQL Server database. 
Instead we would like to change this so that it uses WCF to get the at the information and does not directly go against ASNET_USERS, ASPNET_MEMBERSHIP, etc. in our production SQL Server database.  Is there any examples on how one may accomplish changing the providers to use objects instead of direct connetions to SQL Server databases.

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