Root’s 93 was the most substantial of three half-centuries – James Vince hit the other, in his first one-day international innings – as England set a target of 324 for seven.
Then the home attack made short work of the tourists’ response, David Willey bagging a career-best four for 34 and Liam Plunkett taking three for 44.
Sri Lanka were bowled out for 202 with 7.2 overs unused, despite the best efforts of Dinesh Chandimal (53) – and England went above them into fifth in the International Cricket Council rankings.
Vince (51) and Jason Roy began well on a showery morning, after Sri Lanka won the toss.
But as Danushka Gunathilaka recorded career-best figures of three for 48, it was not until Buttler (70) brought the impetus in a stand of 109 with Root that England were able to profit properly.
On a pitch which gave Sri Lanka’s two spinners enough help to get through their 20 overs for 100 runs, Root’s was something of a throwback contribution.
Coming the day after he had talked up the chances of a first England ODI double-century in the offing for one of the current top six, Root had to battle for his six fours from 106 balls.
Buttler stayed on the modern message, though, with seven fours and a six from 45 until he was yorked by Chaminda Bandara for the left-armer’s first wicket on debut.
Root could not take advantage of a late slice of luck, dropped in the deep on 90, and fell short of his century in the final over when he was bowled having a mow at Nuwan Pradeep.
