Abstract: This invention relates to an improved manhole extension or shield of the type constituting the subject matter of my U.S. Pat. No. 3,294,000 which improvement comprises providing at least one, and preferably both of the outwardly-facing channels with a circular fence projecting down into the casting bordering the mouth of the manhole so as to bridge any gap left between the latter and the annular flange from which it depends thus cooperating with said casting to retain the inflatable gasket.