The roof collapsed yesterday over a section of a General Services Administration warehouse in Fairfax County where $3.2 million in repairs is being carried out to reinforce the roof and stop leaks.

The collapse occurred about 10:30 a.m. when a wooden support truss gave way with a loud cracking sound that warned two men in the area to flee, a GSA spokesman said. "One truss failed and everything else came down around it."

The roof segment, part of the government's Franconia GSA Supply Distribution Facility, came down over an unloading dock and crushed three trucks but caused only a sprained ankle for one of the workers fleeing the scene, Walter Huber, chief of operations for construction management at GSA, said.

Roofing materials piled too high over the section of roof that fell probably caused the collapse, the GSA said yesterday. The agency said a 20,000 square foot segment was involved out of a total of a million square feet, and said damages were $100,000.

An investigation into the exact cause of the accident has begun, according to GSA.

Huber, in a tour of the damaged warehouse yesterday, said it was "odd" that the roof, which is supported by the wooden trusses, collapsed in a section where the trusses had recently been repaired.

Huber pointed to metal "splice plates" attached to the broken and splintered trusses, which he said were installed within the past two months to prevent the roof from falling.

Attempts to obtain comment from the Beiro AA Construction Company Inc. of Alexandria, which is doing the roof repair work at the warehouse, were unsuccessful yesterday.

The warehouse, which was built in 1953 by the Methodist Church and later purchased by GSA, is a depot for government office supplies in the Washington area.

Huber said yesterday "It is our belief that the design of reinforcements for the trusses was done properly and the repairs also were done properly." He said he is not sure why the roof fell in.

Repairs to the roof of the warehouse, which is located behind the Springfield Mall, were begun this spring because the roof leaked and because some of the wooden trusses, which hold up the roof, appeared to be weakened with age, Huber said.

Beiro Construction first repaired several of the damaged trusses, using the rectangular metal "splice plates," Huber said. Then, he said the company began placing a new tar surface on the roof.

Yesterday Beiro Construction employes were at work tarring sections of the roof away from the collapsed area.

The GSA warehouse services government buildings in a 200-mile radius and ships about 58 million dollars of office supplies a month, according to the GSA.

Huber said the section of the roof that collapsed yesterday was a "typical" part of the building and that the GSA will bring in a structural design engineer to examine the cause of the collapse and, if necessary, examine the wooden supports for the roof throughout the building. CAPTION: Picture 1, A $3.2 million GSA effort to strengthen the roof of a 25-year-old Franconia facility was set back yesterday as a repaired section collapsed. By Charles Del Vecchio - The Washington Post; Picture 2, Truck is crushed beneath collapsed section of roof at warehouse in Franconia. One minor injury was reported. By Charles Del Vecchio - The Washington Post