Root was unbeaten on 45 and Jos Buttler not out 37 after he had a caught behind decision overturned when it was revealed he did not hit the ball shortly before the break.
Playing just his fourth Test innings, Denly capitalised after being dropped on 12 to deliver an otherwise composed 69 - until his dismissal midway through the second session.
Had Denly managed a ton in Englands penultimate Test before hosting Australia from August 1, he would have well and truly staked his claim for an Ashes berth. But his dismissal, caught behind off a bottom edge by Shane Dowrich while attempting to cut was welcome reward for a hard-working Shannon Gabriel and left Englands summer selections as uncertain as ever ahead of the remaining one-off, four-day match against Ireland in late July with four months worth of County Championship matches between now and then.
That said, Denly looked fluent in his more familiar position at No.3, having opened on debut in Antigua in place of Keaton Jennings, who failed to make the most of his recall in this Test.
Having resumed after lunch on 45, Denly brought up his 50 with a four, the 11th of his innings, before he was out three balls later.
Alzarri Joseph snared the wicket of under-pressure Jennings with a ball that was missing everything before it jagged the thigh pad and curled behind the batsman onto leg stump. Jennings grinned ruefully after the freak end to his innings. With scores of 17, 14, 8 and 23 in this series, having been dropped in favour of Denly for the second Test, he perhaps realised that his time as an England Test batsman could be over.
His was a much-needed wicket for West Indies, who had started brightly when Keemo Paul had Rory Burns (10) out to a regulation catch at square leg by Joseph on the first ball of the day to put England at 19 for 1 before it all began falling apart for the home sides pace attack, and none more so than Paul.
With menacing quick Gabriel apparently carrying a hamstring injury, he then suffered the insult Shimron Hetmyers spilled catch at third slip off Denly.
In the meantime, Paul was prostrate on the grass just outside the boundary rope, having pulled up in agony as he chased down a Denly cover drive two balls earlier. Paul - who had come into the side to replace suspended captain Jason Holder and claimed two wickets in Englands first innings, including that of Jennings with his first ball of the match - ended up having to be stretchered away with a right quadriceps strain.
Initial reports from the West Indies camp suggested correctly that Paul would not return to the field before tea, although the manner in which he went to ground and the fact he had to be wheeled away gave cause for serious concern. His injury meant the home side had to call upon a second substitute fielder, Jomel Warrican, with Darren Bravo already off receiving treatment for a finger injury picked up earlier in the match.
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