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.