session expire

i want to detect when a seesion is expired and show a specific page and other when user is not admin and so on but i never get on session expired it generates a new session but it always get on the if is authenticated but generated a new session id see code:

if (Session.SessionID != null)
            {



                if (HttpContext.Current.User.Identity.IsAuthenticated)
                {
                    // auth OK


                    if (HttpContext.Current.User.IsInRole("Administradores"))
                    {
                        //admin
                    }

                    else
                    {
                        FormsAuthentication.SignOut();
                        Response.Redirect("~/SemFuncaoAdmin.aspx"); //not admin
                    }

                }
                else
                {
                    // not logged in
                    Response.Redirect("~/Proibido.aspx");

                }


            }

            else
            {

                //session expired
                FormsAuthentication.SignOut();
                Response.Redirect("~/SessaoExpirou.aspx");
            }

        }

after the 20 min i should when on the last else but it wents on if (HttpContext.Current.User.Identity.IsAuthenticated) and auto-generated a new session id instead of asking to login again what should i change?

i'm using membership