A powerful 50km/h wind greeted players for Sunday's match at Fremantle Oval, and West Coast bounced back from a goalless first half to take a one-point lead into the final quarter.
But with Fremantle kicking with the aid of the breeze and the rain tumbling down, former Dockers skipper Hayley Miller registered the only goal of the final quarter to seal the 3.5 (23) to 2.4 (16) victory.
Fremantle (2-3) entered the match off the back of losses to Brisbane (70 points), North Melbourne (100 points) and Sydney (25 points), but their win over West Coast keeps their finals hopes alive.
West Coast are now 3-2, with Daisy Pearce's team missing a huge chance to go two games clear inside the top eight.
Eagles star Roberts (24 disposals, five clearances, one goal, 448m gained) almost lifted her side to victory with a huge second half.
Fremantle's Irish star Orlagh Lally won the western derby medal as best afield after tallying 24 disposals, three clearances, 456m gained and one goal.
"I think I'm used to this weather, it felt a bit more like home, so it was nice to get out there in the rain," Lally said.
"It's always sweet to win against the local rivals."
Kiara Bowers (21 disposals, 13 tackles, one goal), Aisling McCarthy (22 disposals) and Gabby Newton (21 disposals, nine tackles) were also huge for the Dockers.
The Eagles won the inside-50m count 11-4 with the aid of the strong breeze in the opening term, but Lally kicked the only major after reaching up high to pull in a good mark 25m out from goal.
West Coast suffered a blow late in the first term when Liz McGrath had to be carried off after rolling her left ankle, with the ruck later seen on crutches.
Bowers was having a field day with 10 first-half tackles, and kicked the only goal of the second quarter when she floated behind the pack to take a mark before playing on to kick from point-blank range.
West Coast's Zoe Wakfer pushed Bowers to the ground as the goal was kicked, sparking a mini melee between the sides.
The third quarter was the Roberts show as the Eagles star tallied seven disposals, three strong marks, one goal and a score assist to thrust her side into the lead heading into the final change.
The downpour at the start of the fourth quarter was so heavy that players had to put a hand over their face at times to protect their eyes from the rain.
The pivotal moment came with 12 minutes remaining when Fremantle ruck Mim Strom took a big mark and quickly launched the ball deep into attack for Miller to run onto it and kick what turned out to be the match-winning goal.
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