Tudors keeper Laurie Walker was the busiest of the two stoppers, but it was Jonathan North picking the ball out of the net from his ex-Watford team-mate Parkes in the 61st minute.
Wealdstone had a corner cleared and from that Hemel led, Karl Oliyide latched onto the cross at the near post, North and Eddie Oshodi did well to prevent it going in but Parkes fired home the rebound as home fans around the ground breathed a sigh of relief.
Parkes almost doubled his tally twice, first with a long-range effort going just wide and second heading wide of the goal when he should have done better.
But Hemel held out for a vital win as they look to cement their play-off position.
Chris Woods