Watford started with purpose and went close inside a minute through Tom Ince, who found space on the edge of the area and fired wide. Birmingham responded when Tommy Doyle’s corner whistled past the far post moments later, evading Paik.
The frenetic start to the game continued when Marc Bola’s low cross flashed across the Blues goal before Christoph Klarer cleared while Maamma also spurned a couple of decent openings as Watford looked to impose themselves.
Hornets keeper Nathan Baxter kept the scores level at the other end when he denied Phil Neumann from point-blank range after a quick free-kick led to a Jay Stansfield cross. Baxter then held on to a Doyle shot from distance.
Birmingham took a hold of the match soon after, though.
First, Paik won the ball high before driving forward and firing into the bottom corner on the half-hour mark.
Then, Gray cut inside and unleashed a wonderful strike three minutes before half-time.
Watford full-back Bola forced James Beadle into a good save early in the second half after he was found at the left-hand corner of the box by Imran Louza.
However, Marvin Ducksch should have put Birmingham out of sight moments later, but somehow missed from six yards out.
Watford were rewarded for a strong response just past the hour when Maamma swept home Bola’s cross to halve the deficit.
They upped the pressure at the end but Birmingham held on to secure a fourth straight home win.
