Petry earned a helper on the play, making it the third straight game in which he's earned a point. Dale Weise got the secondary assist.
That's where things would stand after 20 minutes, with the Canadiens also ahead 17-7 in shots.
The Canadiens had two more power play opportunities in the second, and despite generating four shots on net with the man advantage, they couldn't capitalize.
In fact, neither team would score in the middle frame. The Habs once again outshot the Flames in the second, this time by a 15-9 margin, and held onto their 1-0 lead heading into the final stanza.
Montreal added to its lead at 9:45 of the third. Ryan Poehling recovered the puck off the faceoff to the left of David Rittich and, in front of the net, exercised supreme patience in waiting for the perfect moment to lift the puck over the Flames starter to make it 2-0.
It was Poehling's first goal - and first point - of the season.