On The Roof Gang - A little history

In July 1928, the CNO announced the establishment of a school to instruct radio operators in intercept operations, particularly for Japanese kana. The first class would begin on October 1, 1928, and instructors were to be two of the self-taught radiomen from the Asiatic fleet.
The first class was considered a success, so five more were held in 1929. The instructor for the first three was Chief Radioman Harry Kidder (A building is named in his honor at the Center for Information Dominance at Corry Station in Pensacola, Florida). The last two were taught by Chief Radioman Dorman Chauncey. Both were veterans of intercept operations in the Asiatic Fleet. Chauncey had conducted intercept at the U.S. Navy sites in Hawaii and Peking.
The first classes of trainees were composed largely of experienced radiomen of senior enlisted rank. To ensure that there would be continuity of service, the second and third groups of trainees were relatively junior.
Marine Corps personnel participated in the training from the third class, which began in November 1929. The class that trained from December 1930 to April 1931 was composed entirely of Marine enlisted men. Marines engaged in intercept activities for most of the 1930s, but the number dwindled late in the decade, since intercept operations were not a Marine rating and promotion possibilities were less for intercept operators than general service radiomen.
With a larger pool of intercept operators to deploy, additional collection sites were opened: Guam, Olongapo, Philippine Islands, and Astoria, Oregon. Some intercept was conducted aboard ships, principally the USS Agusta and the USS Gold Star.
Since these classes were held in a wood structure set atop the Navy Headquarters Building in Washington, and since the radiomen could not explain their classwork to others, they eventually acquired the nickname, "The On-the-Roof Gang."
Some of these fine men survive today.

NOTE: The Navy Building in the District of Columbia, by the way, was demolished after World War II. Part of the area where it once stood is now occupied by the Vietnam Memorial.
Taken from the NSA Website.