England, 2-1 up in the five-match series, were 385 for seven at stumps on the second day - a first-innings lead of 237 runs after India had been skittled out for just 148 on Friday.
Root was 92 not out and Chris Jordan unbeaten on 19, with the pair having added an unbroken 67 in just 62 balls for the eighth wicket.
Joe Root celebrates after reaching his half-century during the second day of the fifth Test between England and India at The Oval in London. (Reuters Photo)
Saturday saw Root join two all-time great batsmen in becoming only the third England player to score a half-century in every Test of a five-match series after Wally Hammond against South Africa in 1938/39 and Peter May, also against South Africa, in 1955.
John Edrich scored fifties in all six-matches of England's Ashes-winning series against Australia in 1970/71.
"It's great that we have batted all day and given ourselves a good lead," England's Gary Ballance, who made 64, told Sky Sports.
"There's a bit of seam and if you bowl in good areas you can create chances. Hopefully it will still do that tomorrow (Sunday)."
India took four wickets in quick succession before tea with off-spinner Ravichandran Ashwin claiming two for three runs in eight balls, as England captain Alastair Cook (79), Ballance, Ian Bell (7) and Moeen Ali (14) were dismissed.
"We created some chances but we let go of some chances," said Ashwin, who ended the day with two for 55 in 20 overs.
"We tried to hit good areas and pick up as many wickets as possible in the morning because whatever was in the wicket was going to be there in the morning."
At tea, England were 246 for five, with Root 11 not out and Jos Buttler unbeaten on 12 on a day where overhead conditions largely favoured the batsmen and much of the early moisture in the pitch had evaporated.
The pair had put on 80 before wicketkeeper Buttler, bidding to score fifties in his first three Test innings, fell for 45 when he chipped fast bowler Ishant Sharma straight to Ashwin at short mid-wicket.
Root then hooked a short ball from swing specialist Bhuvneshwar Kumar for six before completing a 93-ball fifty.
That followed the 23-year-old Yorkshireman's scores of 154 in the drawn first Test at Trent Bridge, 66 in India's 95-run second Test win at Lord's, 56 in the hosts' 266-run victory at Southampton and 77 in their even more emphatic innings and 54-run success at Old Trafford.
Cook misses ton
England resumed Saturday's play on 62 without loss after India captain Mahendra Singh Dhoni, who made 82, had stood firm during the tourists latest collapse of the series.
Cook was 24 not out and Sam Robson 33 not out.
However, Robson had added just four runs to his score when, in the second over of the day, the Australia-born opener had his off stump knocked over by fast bowler Varun Aaron after playing down the wrong line.
But England's all left-handed second-wicket pair of Cook and Ballance were barely troubled as they took the hosts to 148 for one at lunch.
Ballance completed a 77-ball fifty when he deliberately uppercut Kumar over the slips for his 11th four of the innings.
That meant the Zimbabwe-born batsman had now scored three hundreds and three fifties in his first eight Tests.
Cook, so fluent before lunch, was dropped in the slips on 65 and 70 by Murali Vijay and Ajinkya Rahane respectively.
Varun Aaron (R) looks down after another chance to dismiss Alastair Cook (L) goes begging during the second day of the fifth Test between England and India at The Oval in London. (AP Photo)
But he was unable to score what would have been his first century in 31 innings since the last of his England record 26 Test hundreds, against New Zealand at Headingley in May last year.
Cook's luck ran out when he was drawn into a drive outside off stump by Aaron and edged low to Vijay at first slip.
It was the toughest of the three chances the opening batsman had offered, with England now 191 for two after a second-wicket stand of 125.
Cook's exit sparked a mini collapse, with Ballance prodding Ashwin tamely to Cheteshwar Pujara at silly point.
Sharma, the star of India's 95-run win in the second Test at Lord's but missing at Southampton and Manchester with a leg injury, then produced an excellent full-length delivery that Bell edged to Dhoni.
Ashwin struck again when Ali, trying to withdraw the bat, played on.
Commentary, England 1st Innings
England - Alastair Cook (captain), Sam Robson, Gary Ballance, Ian Bell, Joe Root, Moeen Ali, Jos Buttler (wicketkeeper), Chris Jordan, Chris Woakes, Stuart Broad, James Anderson.
India - Murali Vijay, Gautam Gambhir, Cheteshwar Pujara, Virat Kohli, Ajinkya Rahane, MS Dhoni (captain and wicketkeeper), Stuart Binny, Ravichandran Ashwin, Bhuvneshwar Kumar, Varun Aaron, Ishant Sharma.