McDermott's men could have gone level on points with leaders Southampton but were left fighting to avoid defeat after Kyle Bennett gave the hosts a 27th minute lead at the Keepmoat Stadium.
Alex Pearce headed home Ian Harte's free-kick in the 51st minute to claim a point for Reading, who are now two points adrift of Saints while Rovers remain rooted in the relegation zone.
Kevin McNaughton's sixth minute own goal helped the Tigers on their way and James Chester and Aaron Mclean made sure with further efforts in the 47th and 55th minutes respectively.
Jake Buxton headed home a Ben Davies free-kick deep into injury time as
Forest had Marcus Tudgay sent off in the last minute but the game looked destined to end goalless until Buxton's strike. The defeat leaves Forest four points clear of the relegation zone.