Anthony Elding gave the visitors the lead after 11 minutes when Bees keeper Alex McCarthy could only parry Andrew Tutte’s long-range effort onto his head.
Dale felt they should have been two up two minutes later when Jason Kennedy volleyed home from a tight angle only to see his effort ruled out for offside.
The hosts clawed their way back into it just before the half-hour when Robbie Simpson started and finished a flowing move with a crisp finish from Myles Weston’s left wing cross.
But Keith Hill’s side deservedly retook the lead on the hour when Chris O’Grady fired home into the bottom corner from the edge of the box at the end of a five-man one-touch move.
Two minutes later it was three, Kennedy latching onto a right wing cross to fire home at the near post and celebrate with the travelling fans – an action which earned him a second yellow card and a sending off.
But even with 10 men it was always Rochdale who looked more likely to score again, although Brentford did manage some lacklustre efforts on goal.
Brentford’s Weston flashed a 30-yard drive past the upright before substitute Carl Cort twice went close with headed efforts when he should have done better.