documentclass{standalone}
usepackage{tikz}
usetikzlibrary{patterns}
usetikzlibrary{calc}
begin{document}
begin{tikzpicture}

tikzset{%
  miroir/.style = {%
    minimum width = 1cm,
    minimum height = .4cm,
    rectangle,
    transform shape,
    fill = gray!10,
    pattern = north east lines,
    append after command = {%
      pgfextra{%
        begin{pgfinterruptpath}[transform shape]
            draw[transform shape,
                  very thin, fill=gray!10,shorten >=0.2bp, shorten <=0.2bp
            ]
              (tikzlastnode.north west)
              -- ++(0,.2)
              -- ($(tikzlastnode.north east)+(0,.2)$)
              -- (tikzlastnode.north east)
              --cycle;
        end{pgfinterruptpath}
      }
    }
  }
}

node [miroir,transform shape,rotate=0] at (0,0) {};
end{tikzpicture}
end{document}