Two District of Columbia roofing inspectors described as doing almost no work on their jobs were fired yesterday as Mayor Marion Barry ordered a full investigation into the operations of the agency that employed them.

Carroll B. Harvey, acting director of the D.C. Department of General Services, the city's housekeeping agency, disclosed that he had sent letters to the two employes telling them they would be terminaed within 30 days.

The Washington Post reported yesterday that the two employes, Lawrence Hollinshead and John Ross, were followed for seven days selected at random last month and were found to spend most of their time on long coffee breaks, personal errands and other forms of dawdling.

After the mayor read The Post's story, Barry's communications office issued a press release saying the mayor had ordered Harvey to "immediately and fully investigate" the charges it contained.

The investigation will extend to the work practices of the entire Bureau of Repairs and Improvements, which employs the two inspectors and about 250 other craftsmen, the press release reported.

Barry also took the unusual action of announcing that he verbally had reprimanded Harvey, the department head, for not ensuring against such practices. Harvey could not be reached for comment.

"It is not now, nor will it ever be a policy of mine to allow any District government employe to be paid for work he or she does not do," Barry wrote in his letter to Harvey. "Whenever such matters are brought to my attention. I assure you they will be fully investigated and the appropriate action taken."

Harvey, who has been nominated by Barry for permanent appointment to his departmental directorship, is the second city department head reprimanded publicly since the mayor took office in January 1979.

The first was Albert P. Russo, the since-retired head of the Department of Human Resources, who was criticized last year for failing to enforce a city law regulating halfway houses and nursing homes. The reprimand followed a fire in which 10 St. Elizabeths Hospital outpatients died.

Barry's press release said the two roofing inspectors, Hollinshead and Ross, were reassigned to other duties within the department last Friday, one day after Post reporter Art Harris contacted bureau chief Harold T. Henson to discuss the findings that were reported in yesterday's story.

The press release quoted Henson as saying that Hollinshead and Ross went on sick leave and repeated attempts to reach them by telephone failed. The official letters informing them of their terminations in 30 days were then sent to their home addresses, the release reported.