The Nationals held a 1-0 lead after four innings when the contest was halted for 1 hour, 17 minutes by rain. When play resumed, the Giants tallied a score-tying run when shortstop Ian Desmond's error allowed Joaquin Arias to score on a Buster Posey grounder.
But the Nats responded in the sixth when Desmond (3-for-4, two runs scored) led off with a single and LaRoche followed by golfing a pitch from Guillermo Moscoso (1-1) deep over the right-field fence for a two-run homer.
Tanner Roark (2-0) notched the decision and Rafael Soriano picked up his 30th save for Washington, which took the opener of a three-game series.
Arias finished 4-for-5 with two runs scored and Hunter Pence supplied an RBI for San Francisco.
Justin Bieber crashes Drake Bell's album release party
Putin thinks Obama would save him if he were drowning