His latest effort keyed a rare Marlins win at Washington.
Stanton hit his NL-leading 15th home run and also doubled and singled in another power performance in the nation's capital, leading Miami over Washington 3-2 Monday.
Stanton doubled and scored in the first inning, then launched a long two-run homer in third.
"I don't know," Stanton said. "There's certain things you can't explain like that. It's just good it worked out today. But I understand that I do a little bit better here, but you can't really attest it to anything."
Stanton is batting .333 (43 for 129) with 14 home runs and 27 RBIs in Washington since his career began in 2010. He has homered at Nationals Park more than any other visiting player during that time.
"We need to come out with some momentum and we were able to do that today," manager Mike Redmond said. "(Stanton) crushed that (homer) and had some great at-bats, and that's what it's going to take for us to win games on the road."
The Marlins improved to 7-17 on the road; they're 20-8 at home. They won for just the second time their last 14 games at Washington.
"They've had our number for a while now," Stanton said. "So it was great to start the series off (with a win)."
Washington trailed 3-0 and had only one hit before Jayson Werth singled with one out in the sixth. LaRoche, in his second game back from the disabled list, homered into the second deck in right to cut the deficit to 3-2.
"Great to come back. Frustrating game today," LaRoche said. "I feel we just, we didn't give ourselves a ton of opportunities. Not sloppy, we didn't beat ourselves in any way, just weren't on base enough."
"He gave us exactly what we needed," Redmond said of Eovaldi. "We needed him to get as deep in that game as he could."
The Marlins got to Roark for a two-out run in the first when Stanton doubled to right just beyond the reach of Werth and scored on Casey McGehee's single. McGehee is hitting .434 (23 for 53) with runners in scoring position.
Stanton and the Marlins struck again with two outs in the third. After Derek Dietrich was hit by a pitch, Stanton launched the first offering he saw to straightaway center, with the ball bouncing high off a wall beyond the wall.
"If it's a hanging breaking ball, he can hit it over the fence, and that's what it was," Nationals manager Matt Williams said. "If you hang that breaking ball over the plate, sometimes it gets whacked. And the guy whacked it today."
Miami had a chance for more in the sixth when McGehee doubled with one out, but he was picked off second by Roark.
Washington 3B Ryan Zimmerman, on the DL since April 13 with a broken right thumb, had an X-ray on Monday and has been cleared to begin hitting. ... Redmond said 37-year-old Randy Wolf, who allowed six runs (four earned) on eight hits Sunday in his first start since 2012, will remain in the rotation and pitch Saturday against Atlanta. ... Miami's Henderson Alvarez (2-3, 3.21) opposes Blake Treinen (0-2, 1.56) on Tuesday.
33,677 (81.1% full) - % is based on regular season capacity
82 degrees, sunny
Home Plate - Fieldin Culbreth, First Base - Seth Buckminster, Second Base - Jim Reynolds, Third Base - Brian Knight
Giancarlo Stanton hit a 447-foot home run in the third inning Monday, his MLB-leading sixth homer of at least 440 feet this season. His average home run in 2014 has been 431 feet, 34 feet longer than the MLB average.