First it was Hurricane Gloria. Just days after Prince William employes moved into the brand-new county government center in September, workers noticed the water seeping through the walls and dripping from the ceiling during the storm.

Then, during this week's driving rains, a repeat performance forced county employes to jury-rig elaborate plastic chutes to channel the worst leaks away from desks and expensive equipment and into pails.

Computers, police dispatch equipment and desk tops had to be covered at night.

The $5.1 million James J. McCoart Administration Building, it seems, is not watertight.

"I only come to work on the sunny days," said Steve MacIsaac, an assistant county attorney, whose desk is situated beneath a gaping hole where ceiling panels have been removed for workers to examine the source of the seepage.

"Don't believe anything you hear," said County Attorney John Foote. "Steve just likes having holes in his roof."

"That's right," parried MacIsaacs, "it's for a skylight."

"Some things are not funny, and this is one of them," said County Executive Robert C. Noe, grinning broadly.

Tomorrow the county Board of Supervisors is throwing a party to dedicate the McCoart building (named for the former county supervisor who died April 8), which became the official residence of Prince William County's swelling bureaucracy Sept. 30.

The occasion will be festive, county officials say. But privately, they are concerned about who will pay to fix the leaks in the building about five miles southeast of Manassas. The bill could run as high as $50,000.

The county says it doesn't expect to pay for a problem it contends is the fault of the architect or the contractor, Centex Construction of Fairfax County.

The building, as is the case with most large office complexes, has one-inch-wide metal gutters set just behind the bricks. These gutters, called "flashing" in construction parlance, are designed to catch the water that seeps through the brick walls, then funnel it back outside the building through tiny plastic tubes called weep holes.

Here's the catch: The gutters, which should be clean and unimpeded, are clogged with rock-hard mortar. What should look like a trench instead looks like a small mound. Rather than being trapped in the gutters, the water simply cascades over mortar and down the inside of the brick walls and into offices.

To plug the leaks for good, county officials say, workers will have to break through the inside dry wall, then chip away the mortar to open the flow in the gutters. Then they'll have to restore the dry wall, a time-consuming and costly process.

So far, said Foote, the only damage has been "$2,000 in ceiling tiles, plus emotional distress."

But county data processors and police and fire dispatchers worry that another heavy rain could damage computers and transmitters that cost hundreds of thousands of dollars.

"It's certainly not good for the equipment back there to get wet," said Jim Naylor, the county's director of project management. "I think it's very serious."