About four years ago, Paul LeClerc, president of the New York Public Library, began noticing the streaks blackening the marble facade of the Beaux-Arts main building when he walked into work each day.

Mr. LeClerc consulted experts, who told him that pollution and moisture were corroding the statuary ornament and wearing away the marble surface. Precious detail on the neo-Classical Carrère & Hastings building was disappearing, like the edges of elegant cornices and the features of carved faces.

Today, the library is to announce a plan of action: a three-year restoration of the facade, stairs and plaza that is to be completed in time for the building’s centennial in 2011. Already some of the building is shrouded with netting; the first scaffolding is to appear in February on the Bryant Park side of the building.