The roof was poured yesterday on a 10-story intown apartment building - only 26 working days after the stacking of precast wall and floor sections began on an already completed foundation.

Using mostly precast panels for walls, balconies and floors and preassembled kitchen and bathroom modules.Forest City Dillon of Cleveland now has the $6.1 million, 171-unit Campbell Heights apartment structure for the elderly tenants ready for interior finishing. The building at the northeast corner of 15th and U Streets NW is expected to be ready for tenants in September.

Philip Breiding, FCD superintendant on the project, said that a crew of 32 - including cement finishers, crane operators, laborers and a stone mason - starting the stacking operation on April 19 and finished the sectional work yesterday. He estimated that use of the precast panel sections and modular kitchens and bathrooms would trim three to six months from the time required for conventionally pour-in-place construction.

Breiding said steel pilings were placed 30 feet into the ground on the site of the former Dunbar Hotel in February and then a poured foundation was placed on the pilings. When panels and modules were on the site in April, the stacking-by-crane began. Floor and wall connections were made during the stacking by using steel rods and mesh along with some poured concrete for finishing.

The wall sections are 8 feet high, 28 feet long and 8 inches thick. The floor sections are 8 inches thick.

The Campbell Heights apartment project, which will have HUD Section 8 housing assistant payments as rent subsidies for the elderly, was designed by Sulton-Campbell & Associates of D.C. The building was named for the late architect, Leroy Campbell, who died during the design stage of the building sponsored by the National Housing Partnership.

NHP, a Washington-based organization that is privately owned and supported by private industry, was created by Congress 10 years ago to stimulate the creation of rental housing for low and moderate income families. NHP also has plans for a late summer start on a 10-story, 176-unit private market rental apartment on an urban renewal area site at the northeast corner of 15th Street and Rhode Island Avenue NW.

Forest City Dillon also used the precast concrete, stacked system of modular building on a completed high-rise for the elderly at Fort Lincoln in Northeast and has another smiliar building being started there.