....
<configuration>
  <appSettings />
  
  <connectionStrings>
    <add name="dbConnectionString" connectionString="Data Source=.\sqlexpress;Initial Catalog=......
  </connectionStrings>

  <system.web>
  
    <roleManager enabled="true" defaultProvider="myMemb">
      <providers>
        <add name="myMemb" connectionStringName="dbConnectionString" type="System.Web.Security.SqlRoleProvider"/>
      </providers>
    </roleManager>
    <membership defaultProvider="myMemb">
      <providers> 
        <add name="myMemb" connectionStringName="dbConnectionString" type="System.Web.Security.SqlMembershipProvider"/>
      </providers>
    </membership>
....