The defending premiers fought back from a 12-0 half time deficit, but the handling skills of both sides were hampered as mother nature unloaded on Sydney.
After a scrappy start - marred by torrential rain in the opening five minutes - the first-placed Eels posted first points in the 21st minute in fortunate circumstances.
When hooker Drew Dalton - who earlier pulled off a memorable 40/20 kick - put through a sloppy grubber, fullback Michael Basan showed the presence of mind to toe the ball ahead and score. John Williams converted to give the visitors a 6-0 lead.
Williams extended the Eels' advantage three minutes later, accepting a clever flick-pass from Henry Perenara to cross untouched on the left touchline. Williams piloted the conversion to extend the ladder-leaders' advantage to 12-0.
Despite a string of Parramatta raids as the first half wound down, the Roosters were able to scramble their way out of danger to keep the score to 12-0 at the break.
As conditions began to deteriorate, the Roosters hit back in the 49th minute when Chris Walker showed great handling skills to latch on to a Scott Porter (promoted from Flegg) grubber to score. With torrential rain making it hard to see, Porter showed great poise to convert and bring the Roosters within six.
But with points at a premium, Parramatta appeared to wrap the contest up with 16 minutes remaining when Luke O'Dwyer brushed off a feeble attempt at tackle by Walker to score. Brett Delaney's conversion extended Parramatta's lead to 18-6.
Dynamic speedster Michael Lett gave the home side a sniff with a determined try in the 72nd minute, setting up a frantic close to the top of the table clash.
But neither side was able to build up any attacking pressure as the rain only worsened over the closing stages.
Despite the side's second consecutive loss, coach Shane Flanagan was upbeat after the clash.
"The conditions made it hard for both sides, but I was pleased with our attitude in coming back," he said.
"All is not lost, it's disappointing, but I know it will turn around if we keep working hard."
PARRAMATTA EELS 18 (Michael Basan, John Williams, Luke O'Dwyer tries; John Williams 2/2, Brett Delaney 1/1 goals) defeated SYDNEY ROOSTERS 12 (Chris Walker, Michael Lett tries; Scott Porter 1/1, Chris Walker 1/1 goals) at Aussie Stadium; Friday 24th June 2005; Referee: Ben Cummins.