Caversham was reduced to 10 men in the 25th minute, when Tom Jackson produced a two-handed push directly in front of referee Allan Martin, and since the tall striker had been yellow carded minutes before, he inevitably headed for an early shower.
Roslyn happily seized the advantage, and went on to dominate possession, using its passing ability to pen Caversham back in its own half, and racked up a series of shots, corners, and free kicks.
Caversham keeper Calum Flaws denied the white attack with some fine goalkeeping, and Tom Schwarz tackled and headed danger away from the heart of his defence.
But while Caversham was reduced to playing long, to hit the dangerous forward trio Patrick Fleming, Darren Overton and Harley Rodeka, Roslyn used its extra man well, and set up chances that that could have tripled the score.
Once again Mike Cunningham was the orchestrator of Roslyn's best moves, with a near flawless display of receiving, shielding, and sliding tailored passes through Caversham's ranks.
Cunningham's quality is such, that even when Roslyn was in trouble, he was always available as a let-out man, and although tightly marked, was able to shimmy past tackles and create order from chaos.
James Govan also did sterling work as a target man in attack, holding the ball up waiting for support, and showing increasing confidence in turning his marker and lacing shots, the best of which struck Caversham's left upright with keeper Flaws a spectator.
Right-sided Brad Johnstone showed courage to play on despite a shoulder injury, and along with Tom Connor, Mike Still, and Fraser Cameron made wide runs in an effort to create space in Caversham's central defence.
Later runs through the middle had Aajay Cunningham and Sam Mepham carving forward, and only an extremely marginal offside call deprived Mepham of a headed goal.
Big mileage games by Darren Overton, Hamish Chang and Rodeka tugged at Roslyn's defence, but man of the match Watson and his central partner Tim Mather blotted out most threats.
Yet in the last quarter, with nothing to defend, Caversham applied strong pressure to earn free kicks and corners which had keeper Peter Evans fully utilising his height and reach.
The best chance for an equaliser sat up nicely for Caversham's Schwarz only a few metres out from Roslyn's goal line, but the big defender looked exactly that when he skied the ball into the adjoining paddock.
Caversham coach Richard Murray was disappointed to be handicapped by Jackson's sending-off, but praised his side's attitude and work rate.
Roslyn's Colin Thom said Caversham was "a very good side and we had to compete physically, as well as mentally. Keeping our discipline and team shape was imperative.
"It was another masterclass from Mike Cunningham in midfield, but Tim Mather was impeccable in defence. He and James Watson have developed into a sound partnership, and Watson's goal was an absolute cracker."
At Memorial Park, an Aaron Burgess penalty was enough for Dunedin Technical to beat Mosgiel 1-0, and stay seven points clear at the top of the FPL.
But a logjam is developing below Technical with Caversham and Roslyn equal on 26 points, followed by Mosgiel (25) and University on 18.
Dom Higgins got Varsity's goal in the 1-0 win over Spirit in Invercargill, Grants Braes drew 2-2 with Northern, and the week's upset was Green Island beating Queenstown 2-1 at Sunnyvale with goals from Taylor McCormack and Jack Kirkwood.
