The young Cumbrian raced 85 yards to score the try that confirmed Warrington's superiority and ensured that they would be progressing to the quarter- finals.
"I just looked up and saw the space ahead of me," Roper said. "Halifax were arguing with the referee and I took advantage."
His coach, Darryl van de Velde, called it: "A try out of nothing and the turning point of the match."
Once it had been scored, it was a question of how many Warrington would win by.
The only surprise was that it had taken Warrington so long to secure a decisive lead. Ahead after Steve McCurrie charged down Gavin Clinch's kick in the fifth minute and Mark Forster took Lee Briers' pass for the first try, they gave Halifax absolutely nothing in defence and only failed to score further tries through their own impatience.
Three penalties from Briers, balanced by two from Graham Holroyd, gave them a modest six-point lead until Roper struck. Two more goals from Briers stretched the lead before a comedy of errors took them out of Halifax's sight.
Scott Wilson kicked ahead and Jamie Bloem foolishly tried to trap the ball rather than catch it. Warrington's big close season signing, Alan Hunte, could have gone straight into score if he had not lost his footing, but he accepted his second chance when Damien Gibson became the second Halifax player to make a mess of the loose ball.
Halifax had not helped themselves with their indiscipline - Gary Mercer had only just returned from the sin-bin when Roper scored his pivotal try - and they were given a real runaround in the last seven minutes.
Toa Kohe-Love, another who missed much of last season, scored one try when the ball was moved direct from the scrum and Lee Penny rounded it off after some more slick handling from a side which has grown in confidence during the winter.
Roper's explanation is that, with their long-term financial problems resolved, Warrington are now able to concentrate on rugby. If they can continue to do so, this performance illustrated that they will be a force to be reckoned with.
"I was very pleased with our defence," Van de Velde said. "That was where the football game was won. Clinch pushes them around the field and we knew we had to get tight on him."
Warrington did that so effectively that Clinch has rarely had a more anonymous game since coming to England. Now Warrington have to do it all over again; their first Super League fixture of a season that suddenly promises so much more than last is at Halifax next week.
Warrington: Penny; Roach, Kohe-Love, Hunte, Forster; Wilson, Briers; Hilton, Farrar, Nutley, McCurrie, Gillies, Roper. Substitutes used: Hanger, Chambers, Wainwright, Knott.
Halifax: Holroyd; Bloem, Gibson, Craig, Bouveng; Chester, Clinch; Broadbent, Rowley, Skerrett, Marshall, Gillespie, Mercer. Substitutes used: Cardiss, Clark, Moana.
Referee: R Smith (Castleford).
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