After neat interplay on 31 minutes, involving Alex MacDonald and Danny Hylton, Roofe cut in from the left and hammered a ferocious right-footed shot past Cambridge goalkeeper Will Norris into the far corner of the net.
It was on-loan West Brom forward Roofe’s fifth goal in six games, though his first at home.
Oxford added a second in first-half stoppage time when Player of the Year Hylton headed in a corner.
Yet the home team’s joy was tempered by the fact that centre-half Johnny Mullins had to be stretchered off with a back injury at the end of the first half after he fell badly.
Cambridge had gone close with Sullay Kaikai’s shot deflecting just past the far post.
But they were outplayed and Oxford were unfortunate not to score more, with Chey Dunkley seeing his header cleared off the line, MacDonald rifling a right-footed drive against the post, and sub Kyle Vassell failing to score when through on goal.