Shane Long continued his hot scoring streak as he fired the Royals into a first-half lead with a cool finish to net his eighth goal in the last seven games.
Jimmy Kebe grabbed the second when he lobbed the poorly-positioned Neil Sullivan after the break and Jobi McAnuff made sure the points were heading south when he blocked a clearance that flew into the bottom corner.
The win sees McDermott's men keep fast on the tail of the top six and on this showing a promotion charge in the second half of the season is well within their capabilities. But the boss is looking only at his side's next fixture.
"I just think of Hull next Saturday. If we win against Hull we are in a better position. We can't look too far ahead," he said. "We have played well this season and have had some really good performances and you have to play well to win here.
"You have to do the right things first and that's what you saw from our players. To come here and get a keep a clean sheet and win is fantastic."