Roark set down the first 16 batters before San Diego catcher Rene Rivera, an .074 batter entering the game, hit a solid single to center field with one out in the sixth inning.
Roark (2-0) walked one and struck out eight, including Jedd Gyorko to end the game on his 105th pitch. The 27-year-old right-hander extended his run of consecutive scoreless innings to 18, improved to 4-0 in his career at Nationals Park. In 35 innings there he has given up one earned run.
Ian Desmond had three hits for Washington, including a run-scoring double — the game's only extra-base hit — which highlighted a three-run first inning off losing pitcher Andrew Cashner (2-3).
Washington got all the offense it needed in the first inning as singles by Anthony Rendon, Jayson Werth, and Adam LaRoche produced one run. Desmond's double to the right-field corner brought in another. Danny Espinosa's sacrifice fly gave the Nationals a 3-0 lead.
Washington increased its advantage to 4-0 in the sixth when Kevin Frandsen opened with a single, moved to second base on a sacrifice bunt by Roark, and scored on a single to left by Denard Span. Frandsen was starting in left field for Bryce Harper, who was out of the lineup with a jammed thumb.
Notes: X-rays on the jammed thumb of Harper were negative, according to manager Matt Williams, who said that Harper would see a hand specialist on Saturday. Williams said that Harper was available to pinch-hit. ... The Nationals had their second successful replay challenge in three games as Werth was ruled safe at second base, taking two bases on a booted ground ball. ... Padres outfielder Seth Smith was out of the lineup for the second straight day and will remain day-to-day after suffering a strained groin Thursday night against the Nationals. He was scheduled to take cuts in the batting cage on Saturday and for light jogging on Sunday. ... Washington right-hander Taylor Jordan (0-3, 6.23 ERA) will face San Diego right-hander Ian Kennedy (1-3, 3.60) on Sunday.