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.
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.