Martinez drove in four and leads the majors with 26 homers and 71 RBI. Boston has won 17 of its last 19 interleague games, including 11 of its last 12.
Washington has lost four straight and 16 of 21 to fall to 42-42, its first time at .500 since it was 17-17.
Red Sox starter Brian Johnson allowed two runs in 4 2/3 innings. Brandon Workman pitched a perfect seventh, and was awarded the win.
Roark allowed a career-high nine runs in seven innings and is 1-8 with a 5.19 ERA in 12 appearances since May 5.