Dwayne Stafford, 26, charged down the stairs from his cell at the Cannon Detention Center as Roof went for a shower at around 7.45am.
He struck him causing bruises to his face and back in what has since been described by cops as an 'unprovoked' attack.
According to Charleston County Sheriff's Major Eric Watson, Roof, 22, was returned to his cell after a medical examination.
Although authorities want Stafford charged with third-degree assault, Roof has decided not to press charges.
Stafford himself is in prison awaiting trial on charges relating to a first-degree assault and armed robbery.
Officers at the prison are currently investigating how Stafford managed to get through a cell door to attack Roof.
Roof himself has been jailed since June last year, awaiting a trial which could lead to a death penalty.
His defence lawyers are hoping if he pleads guilty, prosecutor won't push for the death penalty and take life imprisonment instead.