The Loudoun County Board of Supervisors this week sent an urgent message to the state legislature requesting that funding for a Rte. 7 bypass and the widening of Rte. 28 be considered "a vital priority" that should be moved ahead with "all deliberate speed" during the current session.

The unanimously approved policy statement was "the most important thing the supervisors did this week," said Chairman Frank Raflo.

"With the expected changes in the road-funding formula, if we had been divided on this issue, it would have been a perfect excuse for the legislature to ignore Loudoun County altogether when it came to road funding," he said. "It was important that the legislature know we won't let ourselves be batted about by VDH&T the Virginia Department of Highways and Transportation ."

The Rte. 7 bypass, at the western end of the county, has been waiting 20 years for completion. Rte. 28, in the county's eastern end, must be widened to four lanes to accommodate Loudoun's industrial corridor, the statement said. The statement said that 350 miles of secondary roads must be paved.

In other business, the board said it will consider the possibility of building a regional youth detention facility, after being told that the county faces a Dec. 5 deadline, after which, according to a federal law enacted in 1976, no juvenile may be housed overnight in a county jail.

Juvenile Court Judge John Ryan and Sheriff John Isom told the board that youthful offenders are driven to shelter in other counties, sometimes as far away as 450 miles, to avoid putting them in an adult jail.

Said Ryan, "Not counting mileage, wear and tear on the cars, and pay for two deputies to accompany the prisoner, it costs $100 a night for Fairfax or some other jurisdiction to keep him [the young prisoner] until the next morning. Then the two deputies have to drive back, pick the prisoner up and bring him to Loudoun to appear in juvenile court by 10 a.m.," he said. "It just doesn't make sense to keep doing it that way."

According to Ryan, the county spent $23,000 in 1984 to house 45 offenders under 18 in the youth shelters of other jurisdictions. Although it cost Fairfax County $3.5 million to build its youth shelter, Ryan said, it may be possible to build a regional shelter in Loudoun designed to serve Fauquier, Clark and Warren counties for much less. The board will postpone action until Ryan and Isom supply more definite cost figures, Raflo said.

In another matter, a recent application by a Leesburg development firm to subdivide 70 acres on the east side of the Blue Ridge Mountains was the catalyst for a motion by Blue Ridge Supervisor James F. Brownell. He requested that the county's zoning ordinance be amended to specify not only how mountain ridges are to be developed, but also to define a mountain ridge for planning purposes. "That's going to be tough," said county planner Frederick Carr. "We don't have a working definition of a mountain ridge."

The Sunny Ridge Development Corp. has told the Planning Commission that it wants to subdivide a tract along Rte. 761 into 15 residential lots of three to five acres each.

Before the motion was approved, Brownell cited soil erosion, surface and water pollution from construction runoff, ground water depletion and/or contamination and increased transportation safety hazards on the narrow unpaved mountain roads as the potential dangers in mountain ridge development. Although the motion could result in an amendment to the county zoning ordinance limiting or even prohibiting such development, the amendment will not affect the Sunny Ridge proposal now in public hearings before the planning commission, Carr said.

In other action, the board:

* Approved a $224,000 School Board budget appropriation in unexpended funds from the 1983-84 school budget. The appropriation includes $70,000 for nine storage buildings, $20,000 for printing stock paper, and $10,000 for purchase order forms and diplomas.

* Heard county administrator Philip Bolen report that a recent meeting with New York bond rating firms may result in a higher bond rating for Loudoun. Currently the county is rated A by Moody's and A-1 by Standard & Poor's. Bolen said he based his expectations on the county's extensive water and sewer development, low unemployment and the new jobs brought into Loudoun by development near Dulles Airport.