But the all-rounder's run-a-ball 19 came to an end when he lobbed a gentle catch off Eranga to wicketkeeper Prasanna Jayawardene.
Broad, primarily a seamer, kept the runs coming, however, with a flurry of boundaries, including two dismissive square-cuts off Pradeep.
However, he pulled Pradeep straight to deep mid-wicket to end a run-a-minute innings.
Liam Plunkett was then dropped on two when Eranga at mid-wicket put down a routine chance off Pradeep.
Root then rounded off an excellent session for England with a three off Herath that saw him to 150 in 246 balls including 12 fours.
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It was the first time in 14 Tests that England had reached the benchmark innings score of 400.
Root, England's overnight century-maker, was 152 not out after dashing left-handed batsman Broad dominated a seventh-wicket stand worth 64.
England, who had been struggling at 22 for two and 74 for three, resumed Friday well-placed on 344 for five.
Root, recalled after being dropped for the final Test