The host nation won the toss and made the most of favourable bowling conditions on a day when the rain finally relented, bowling the West Indies out for 212.
"I get them two or three times a year and unfortunately it's come now".
The last thing England need is to lose the destructive Roy now, particularly as the ideal stand-in in Alex Hales is indisposed, and England will keep him in the squad in the hope the "tightness" in his left hamstring heals before the business end of this tournament.
Root, who made 107 in England's defeat against Pakistan, made batting look easy, scoring a run-a-ball 50 without having to take unnecessary risks as he and Bairstow tamed the West Indies quicks.
Root put on 95 with opening partner Jonny Bairstow (45) and 104 with Chris Woakes (40), who was promoted from No 7 to No 3, before guiding his side home alongside Ben Stokes (10no off 6) as England moved second in the table, above Australia on net run rate, with West Indies down in sixth sport with one victory from four fixtures.
Root reached three figures off 93 balls with 11 fours and is now the tournament's top-scorer - with a total of 279 runs in four innings.
England were not ideal and coach Trevor Bayliss will not be happy with the two dropped catches by Mark Wood and Chris Woakes that could have proved costly if Chris Gayle and Andre Russell had been at their ebullient best.
Two balls after his dismissal West Indies were in deep trouble at 55/3 when Shai Hope was trapped in front of the wicket by Wood.
Root was a force with the ball, too, catching Shimron Hetmyer and Jason Holder off his own bowling as the two-time winners - who have now lost six World Cup matches in a row against England - tumbled from 144-3.
Mark Wood (3-18) finished off the innings when he bowled Shannon Gabriel with a yorker as the last four West Indies wickets fell for 10 runs.
Joe Root picked up his first two wickets in one-day worldwide cricket since January 2018, both caught-and-bowled, and Barbados-born paceman Jofra Archer (3-30) picked up his first wickets against West Indies.
Make no mistake, this was a pivotal match for an England team who could not have afforded a second defeat in their first four matches and still lived up to expectations as firm World Cup favourites.