With 74 minutes gone the game looked to be heading to the seventh stalemate in the last eight meetings between these two sides.
Then Barnet manager Paul Fairclough introduced Roach three minutes later and Barnet were in the driving seat.
Roach had previously netted after coming on as a sub in the games against Dagenham and Redbridge and Leigh RMI.
And following a teasing cross from the left by defender Simon King he did it again, poking past Leigh Walker in the Scarborough goal.
It was no more than Barnet deserved and with better finishing the Bees could have been out of sight going into the break, though credit must go to visiting 'keeper Walker.
He produced several spectacular saves to twice deny Nick Bailey and Giuliano Grazioli.
Yet the former Sheffield United trainee was powerless to stop Dwane Lee's turn and shot from outside the area midway through the first half but unfortunately for Lee, the ball hit the post and ricocheted to safety.
Both sides had their chances in an exhilarating second half but it appeared neither had the midas touch.
Damien Batt was denied what would have been a stunning goal when Walker palmed away his rasping effoprt.
Immediately after, Barnet keeper Scott Tynan produced a similar save from Chris Senior's 20-yard piledriver, before the impressive Ian Hendon cleared Tony Hackworth's shot off the line.
Enter Roach, exit Scarborough's hopes of victory.
At the death, Dean Sinclair had a goal unfairly ruled out for offside and Grazioli should have doen better when one on one a minute later.
The home side could not add to their lead but go into the New Year sitting proudly at the top of the Conference, having already equaled last season's goal tally.