Gers had to endure a second-half downpour but Roofe’s stoppage-time strike from the halfway line sank the Belgian outfit, with the former Leeds forward marking his first appearance in over a month due to an calf injury in style.
The sunny disposition Gers have been displaying since Saturday’s Old Firm triumph grew brighter when James Tavernier slotted home his latest penalty midway through the first half.
Standard striker Jackson Muleka had two efforts crash against the woodwork before the break but the biggest threat to Steven Gerrard’s team after the interval came as the heavens opened.
There were just under 4,000 fans at the Stade Maurice Dufrasne but there were real concerns they would not get to see the game finish as the ball began sticking in the drenched pitch.
Thankfully for the Scottish Premiership leaders, Danish referee Jakob Kehlet allowed the match to go the distance as Roofe’s spectacular effort confirmed the Light Blues’ first away win in a group stage since Walter Smith’s team beat Lyon back in 2007.
The confidence surging through the Gers ranks following their derby triumph was evident in the early stages as they popped passes round the Belgian’s high press.
Tavernier was left a little red faced when he swung at a Borna Barisic cross but connected only with fresh air as he allowed his eye to be distracted by Alfredo Morelos, who was desperately trying to get out of his captain’s way.
But the right-back made sure to get full force behind the ball as he drilled Rangers ahead from the penalty spot 18 minutes in.
The spot-kick award came as Connor Goldson got on the end of a Tavernier corner. His header was never likely to continue the Parkhead goal hero’s scoring hot streak but Nicolas Gavory was foolish enough to let it hit his arm and Kehlet immediately reached for his whistle.
Taverier stepped up to fire home his ninth goal in the last 10 appearances past Arnaud Bodart.
A Hagi strike dipped and swirled before flashing wide but the signs were starting to show that Standard had found their feet after a slow start.
Gers had the crossbar to thank on the half-hour mark when Muleka crashed a header off the bar from Collins Fai’s cross as Allan McGregor was left scrambling, while Goldson had to time his tackle to perfection as the Standard frontman threatened to burst through again.
Gerrard was left scratching his head moments later as Arfield’s lovely lay-off cut the Liege defence wide open, only for Morelos to inexplicably return the ball to his team-mate instead of taking a free shot.
The Ibrox boss’ concerns would have grown when Barisic was forced off with a thigh problem late in the half but Calvin Bassey did not let his manager down as he threw himself into the action with three vital blocks.
Muleka tried to pull a fast one as he punched a Maxime Lestienne cross against the bar just as the half-time whistle went. It was a blatant handball but it was missed by Kehlet, who instead booked McGregor for his protests.
There was another moment of hesitation from Morelos two minutes into the second period as the Colombian wasted a great chance to add a second, with his heavy touch allowing Bodart to rush and block his shot.
It took another decent stop from the keeper to deny Morelos in the 60th minute as Hagi exploited another huge hole in Standard’s back-line, while Arfield swept wide after a Bassey run.
The rain clouds were by this stage dumping an enormous amount of water on the pitch.
The Rangers dam at the back held firm, though, and Roofe wrapped up the win as he danced his way past three challenges before launching a lengthy strike over Bodart’s head from inside his own half.
There were ugly scenes at full-time as a touchline brawl broke out between the two teams after a member of the Standard back-room staff squared up to former Anderlecht striker Roofe on his way off the pitch.