Roark (7-0) yielded one walk and struck out six in the stellar performance, becoming the first Nationals pitcher to reach 7-0 since Tyler Clippard started the 2010 season 6-0 out of the bullpen.
He retired 13 batters in a row to finish his start as the Nats followed up a 6-5 win in the day's first game, which was rescheduled from Monday in the wake of a shooting rampage at the nearby Washington Navy Yard in which 13 people died.
Denard Span extended his MLB-best hitting streak to 28 games for the Nationals.
Atlanta's Freddy Garcia (1-2) was hung with the loss despite surrendering just one run on seven hits over seven innings
The National League East-leading Braves' magic number to clinch the division crown remained at four.