The wide margin was a little harsh on the Royals as Birmingham, who lost for the first time under manager Gary Rowett at Blackpool last week, led 4-1 at half-time through Paul Caddis’ opener and a slick Demarai Gray hat-trick.
Glenn Murray had pulled it back to 2-1 earlier, but Rowett’s side refused to let up and Andrew Shinnie and David Cotterill rounded off a clinical afternoon’s work.
Blues’ cult hero Nikola Zigic was named on the bench after re-signing for the club until the end of the season on a deal worth less than a tenth of his previous £65,000-a-week salary, but that was where he stayed until the game was well beyond Readin.
Birmingham went ahead in the fourth minute through a clever free-kick as Jonathan Grounds knocked it to fellow full-back Caddis, who smashed home right-footed past Adam Federici.
Simon Cox and then Murray wasted decent chances to level as Nigel Adkins’ side battled back.
But City made it 2-0 against the run of play as Reading lapsed at the back. They allowed Gray to pick the ball up from deep and burst forward, and he fired home from the edge of the area as the visitors backed off.
Reading pulled one back in the 19th minute when Jordan Obita scuffed a volley into the box but succeeded in teeing it up nicely for Murray, who looped a header over Darren Randolph and in.
The brilliant Gray restored the two-goal cushion in the 24th with more delightful solo play – Jake Cooper gave the ball away cheaply and the youngster showed quick feet before guiding the ball into the far corner.
If anything, it was Birmingham who looked shaky at the back, as Reading kept their heads up and pressed.
Yet Gray had his hat-trick in added time, again on the counter-attack. David Davis picked the ball up and with Gray to his left and Cotterill to his right against two defenders, slid it to the former who took it well before coolly slotting it home.
Thirty seconds into the second half, Shinnie made it 5-1 – Birmingham’s most-improved player under Rowett forced his way in from the right, jinked into the box and smashed a low shot past the despairing Federici.
In the 59th minute the Royals keeper’s misery was compounded as Cotterill’s free-kick from the left ghosted straight into the far corner of the net.
Murray saw an effort hit the bar as the clock ticked down, while Zigic received a hero’s welcome as he came off the bench for the final seven minutes.