President of Baseball Operations and General Manager Mike Rizzo made the announcement.
Roark is starting Friday night in place of Stephen Strasburg.
Strasburg left his start Sunday versus the Marlins after four innings with upper back tightness. He threw a bullpen session Thursday and is expected to return to the rotation next week against the Mets.
The 28-year-old Roark (4-4, one save, 4.54 ERA) has pitched in 34 games, starting six, this season for Washington.
Since his debut in August of 2013, Roark has started 42 games for the Nationals, going 21-12 with a 3.08 ERA in those games.