Matt Phillips also smashed a fierce 30-yard strike against the crossbar - one of West Brom's 16 first-half shots.
Relive West Brom's victory over Brentford
The game evened out after half-time and the Bees went close to grabbing an opener themselves, Marcondes' first-time shot tipped wide by Sam Johnstone.
Taken from BBC website just after game finished 1-1.