A 21-year-old Long Island roofer was accused yesterday in Federal Court, Brooklyn, of stealing $2,800 worth of National Guard weapons to sell to Fidel Castro, the Cuban rebel leader.

Louis P. Almeida Jr. of Huntington Station, L. I., waived examination when arraigned before United States Commissioner Martin C. Epstein.  He was held in $1,000 bail.

According to Assistant United States Attorney John Wydler, Almeida and an unidentified accomplice broke into South Huntington High School and stole two rocket launchers, three .30 caliber machineguns, thirteen carbines, and four pairs of binoculars.  Mr. Wydler said Almeida had said that he was unsuccessful in making a contact with Fidel Castro's representative in Florida and had thrown away the weapons.