Then Starc (2-77) removed England skipper Ben Stokes (0) for the fifth time this series with a near unplayable delivery that shaped away from the left-hander and the TV umpire review found it had caught a thin edge on its way through to Alex Carey.
After some nervy moments in the nineties as the pitch began showing signs of variable bounce, Root's straight drive off Michael Neser allowed the 163-test veteran to scamper through for two runs and complete his latest milestone off 146 balls, with 11 boundaries.
It was Root's 41st test century, but just his second in an Ashes test Down Under. He's level with former Australia captain Ricky Ponting on the all-time list of test centurymakers. Only Sachin Tendulker (51) and Jacques Kallis (45) have more.
Root made 138 not out in the second test at Brisbane which was his first century in Australia in his four Ashes tours.
Jamie Smith made an enterprising, and at times fortunate, 46 before he holed-out to the part-time medium pace of Marnus Labuschagne just before the break.