Saskatchewan will play at Winnipeg next Sunday in the West final.
After both teams kicked field goals on their initial overtime possessions, Calgary had the ball first in the second overtime and Rene Paredes missed a 44-yard try. After two runs by William Powell, Lauther kicked the winner.
Paredes forced overtime with a 47-yard field goal with 59 seconds left in regulation.
The teams combined for eight turnovers, six in the first half. Riders quarterback Cody Fajardo threw four interceptions, and Calgary’s Bo Levi Mitchell had two picked off.