Leeds will take a one-goal advantage into Wednesday night's second leg at Elland Road after Roofe bagged his 15th goal of the season in the 55th minute.
The Whites striker found the corner of the net following a fine cross from Manchester City loanee Harrison who was played in down the left flank by Stuart Dallas.
"I've watched it back and it was great play just near the half way line between Stuey and Jack and they worked it into the position," said Roofe.
"For me, I know if I get into that area the ball is coming and what a ball it was. To be honest I think the ball made it."
Roofe also admitted he was left thankful to referee's assistant Eddie Smart who called over referee Craig Pawson to overturn a Derby penalty.
Pawson initially pointed to the spot in the 78th minute after Harrison and Jayden Bogle collided in United’s box as Harrison attempted to clear a through-ball from Harry Wilson.
"While I was playing I didn't think it was a penalty," said Roofe.
"That was my view on the pitch but obviously you analyse it with the camera and the replays.
"I'm not sure but I am thankful that the linesman didn't give it so thankful for that and we have won the game."