The Reston Center's roof collapsed yesterday morning under the weight of a night of heavy rain, only hours before a group of young riding students were to take lessons there.

"If it'd happened later," said the center's chief instructor Susan Arnold, "I would have had some dead students. We would have been trapped."

Three workers inside the center and 27 horses escaped injury at about 8:45 a.m. when the roof, supported by a network of exposed beams, crashed into the stable's center ring, where the youngsters were scheduled to ride. "There was a loud crack, like a horse kicking in the ring, and then the roof went just like that," said stable manager Ron Sakell, one of the three workers.

Later, on recommendation of county officials, the remainder of the building was leveled as a safety measure by its owners, the Reston Home Owners Association.

The accident appeared to have been caused by an accumulation of water on the roof, "with some deterioration of the roof trusses," according to Richard E. Lawson, deputy director of inspections for Fairfax County's building department. A final determination of the cause, he said, probably will be made today.

The 15-year-old stable, with its elaborate cedar shake mansard roof, was a familiar architectural landmark in the Hunters Woods section of Reston. What passersby saw yesterday was, for the most part, a crumpled heap of wood. The paddock areas on either side of the ring survived the crash but their outside walls were listing badly.

Stable manager Sakell and workers Carolyn Dahlberg and Clarence Allen were the only three persons in the stable when the roof collapsed.

Among the 27 horses led to safety was Sakell's $20,000 show stallion, Sunday, which was prancing alone in the ring when the roof came down.

There was just one spot where nothing crashed into the ring," Sakell said. "And somehow Sunday found it. I was in the ring with him one minute before the roof collapsed."