A couple of rash challenges from Matthew Upson and Tony Rougier resulted in cautions with the only other incident of note before Forster scored after 40 minutes.
He collected a header from Andy Hughes, outpaced Kit Symons on the right before firing past Alex Kolinko in the Palace goal from 15 yards.
Palace tried to up the pace after the break but really didn't have enough midfield invention to cause Reading many problems, Palace's best chance went to Tommy Black.
The result lifted the visitors to fourth and this was their fifth clean sheet in a row in a run of eight wins in ten games.
Their manager, former Palace player, Alan Pardew said: “I love it here, it holds a special place in my heart. I was proud to bring my team here and win."
Although he was disappointed with his team’s performance when in possession, Francis felt his players: “Had enough chances to claim a point but deserved no more, we were not able to impose ourselves."
“We were not creative enough in the middle and thus did not create enough chances.”