Roark (5-2) pitched seven crisp innings, allowing six hits, walked one and struck out six to win his second straight decision. The right-hander only allowed two runners past first base and got help from catcher Matt Wieters, who threw out a pair of baserunners.
Giants starter Matt Moore (2-6) took the loss despite allowing only two runs over seven innings. The left-hander gave up six hits, struck out five and didn't walk a batter but got no run support.
The game was halted briefly in the top of the eighth after Washington slugger Bryce Harper was hit by a pitch from San Francisco reliever Hunter Strickland.