Having twice got himself out during England's 75-run defeat by Pakistan in the first of a four-Test series at Lord's last week, a determined Root did not give a chance in just over six hours at the crease on Friday.
Earlier, England captain Alastair Cook, who won the toss on a good pitch and in sunny conditions ideal for batting, made 105 and shared a second-wicket stand of 185 with Root.
Pakistan's left-arm pace trio of Mohammad Amir, Rahat Ali and Wahab Riaz became increasingly threatening as the skies clouded over late on. Amir took two for 63 and Rahat two for 69, with both men bowling 20 overs on Friday.
But star leg-spinner Yasir Shah, who took a man-of-the-match winning 10 wickets at Lord's, returned figures of none for 111 in 31 overs.
Amir had opener Alex Hales dropped in the gully on six by Asad Shafiq. But three balls later, having repeatedly moved the ball away from Hales, Amir skilfully swung the ball back into the right-handed opener and knocked over his middle and off stumps.
England were now 25 for one after seven overs. Cook cashed in on a rare Shah short ball with a pull to the mid-wicket rope.
Root, in the manner of Yorkshire and England great Geoffrey Boycott, forced both Riaz and Amir through cover-point off the back foot for fours.
At lunch, England were 95 for one with Cook 40 not out and Root 41 not out. Root just beat Cook to the fifty milestone, getting there in 86 balls to the captain's 90.
Cook reached the landmark in style with a straight-driven four -- a clear sign he is in top form -- off Shah. A similar stroke saw him into the 90s and raised the 150 stand. Cook's two to mid-wicket off Amir saw him to a hundred in 157 balls including 14 fours and the normally restrained Essex batsman celebrated his century with several fist pumps.
Left-handed opener Cook's 29th Test century saw him extend his own England record and drew the Essex batsman level with Australia great Don Bradman's tally. Bradman, however, played just 52 Tests while this was Cook's 131st match in the format.