Drew Stafford had two third-period goals and Clarke MacArthur also scored in helping Buffalo snap a season-worst three-game home losing streak. Dainius Zubrus had two assists in his first game against his former team; the Sabres acquired the forward in a deal with the Capitals last month.
The Eastern Conference-leading Sabres (47-19-7) joined Nashville in becoming the second NHL team to reach the 100-point mark this season.
Brian Pothier and Donald Brashear scored for the Capitals (26-35-13), who continue displaying the telltale inconsistencies of a young team. Besides dropping to 1-8-2 in their past 11 road games, the Capitals failed to build off two home wins in which they outscored their opponents by a combined 12-2, including a 7-1 win over Tampa Bay on Sunday.
The Sabres, coming off a 2-0-2 four-game road swing, can thank their power-play unit for finally coming to life.
Buffalo converted two of four power-play chances to end a seven-game drought - the team's longest since 2003-04. The Sabres had squandered 25 consecutive power-play chances before Roy opened the scoring 40 seconds into the second period.
Buffalo never trailed and took control with a three-goal second period.
Roy's goal came courtesy of a fortunate bounce after Dmitri Kalinin's shot from inside the blue-line sailed wide and high. The puck bounced to the right of the net, where Roy slid it in the open side with goalie Olaf Kolzig looking the other way.
Briere sealed the win, putting the Sabres up 3-1 late in the frame when he scored on a wraparound after Zubrus forced a turnover behind the Capitals' net.
Briere's goal was his 30th of the season, giving the Sabres an NHL-best four players with 30 or more. It marks the first time Buffalo has had four 30-goal scorers since 1990-91.
Notes: The Sabres had a goal disallowed seven minutes in when officials correctly ruled Stafford directed the puck in with his glove. . . . Capitals LW Matt Pettinger, who missed his ninth game with a broken thumb, is expected to return within the next two games. . . . Sabres D Toni Lydman missed his third game with what the team is referring to as an upper body injury. . . . With nine games remaining, the Sabres are six wins short of breaking the the franchise record for victories set last year. . . . The Sabres' two power-play goals were their most in 13 games, dating to a 6-3 win against Philadelphia on Feb. 20.