"We've taken a good step as the first 90 minutes is done. You've seen all the drama and comebacks this week so we can't relax," Roofe told Sky Sports.
It was the 15th goal of the season for Roofe, who could have doubled his team's advantage, only to be denied by Roos as Leeds came alive following a sedate opening period.
Derby were awarded a penalty after Harrison appeared to foul Jayden Bogle in the 77th minute, only for referee Craig Pawson to overturn his decision and award a free kick to the visitors after consulting the linesman.
"I knew that Jayden Bogle had come in behind me but I was surprised it was called a penalty. It's all about perspective but I was going for the ball, I had no intention to take him down," Harrison said.
Leeds held on as Derby struggled to create clear chances at home. The teams meet at Elland Road in Wednesday's second leg.
Aston Villa defeated West Bromwich Albion 2-1 in the first leg of the other semi-final earlier on Saturday. The return leg is at The Hawthorns on Tuesday.
The finalists will meet at Wembley on May 27 to decide who will join Norwich City and Sheffield United in the Premier League next season.
(Reporting by Shrivathsa Sridhar in Bengaluru, editing by Ed Osmond and Ian Chadband)
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