Gloucestershire 134 for 1 (Dent 61*, O. Price 49*) trail Glamorgan 450 for 6 dec (Root 98, Cooke 86*, Lloyd 81, Carlson 76) by 316 runs
Paul van Meekeren was the pick of a Gloucestershire attack, who found it hard to extract any help from the true batting surface. The Dutch seamer's figures of 3 for 92 from 21 overs were reward for sustaining impressive pace on his first Championship appearance for the hosts.
By the close, Gloucestershire had moved to 134 for 1 in their second innings, Chris Dent leading the way with 61 not out. With bad weather forecast tomorrow, the match already seems to be heading for a high-scoring draw.
The day began well for the home side. With only a run added to the Glamorgan total, Colin Ingram edged the fourth ball of van Meekeren's opening over to first slip where Ben Charlesworth took a waist-high catch.
The lively van Meekeren struck again in the seventh over of the morning, squeezing a delivery through Sam Northeast's defence to bowl him for 28. At 183 for 4, Glamorgan were in danger of undoing their first-day efforts with the bat.
But Root had other ideas, getting off the mark with a sumptuous square driven four off van Meekeren. Soon Carlson was square cutting the same bowler for four before bringing the 200 up with a boundary through extra cover off Dom Goodman.
It took the pair just 17 overs to complete a century stand, Carlson reaching a run-a-ball fifty with a lofted shot that almost saw him caught at mid-on off Josh Shaw. The partnership was worth 113 when he pulled van Meekeren to square leg where Dent took a smart low catch.
Root also reached an entertaining half-century, off 63 balls, before lunch, which was taken at 311 for 5. Glamorgan had added 159 runs a session prolonged by ten minutes to help make up for overs lost to rain on day one.
Cooke had also begun to enjoy himself on a pitch offering consistent bounce and a quick outfield providing full value for shots. He and Root put together a 50-stand in 69 balls before the second new ball was taken at 361 for five.
On 29, Cooke appeared to survive a chance to Ollie Price at second slip off Tom Price, but it was a rare moment of alarm as he and Root progressed their partnership into three figures with a succession of well-timed strokes.
Root had struck 14 fours and a six during a fluent innings when, on the verge of what would have been his eighth first-class ton, he edged left-arm spinner Zafar Gohar to Ollie Price at slip.
Cooke had moved imperiously to fifty off 73 balls and was still there when a fifth batting point was secured, having taken his boundary count to 15 fours and a six. The declaration came immediately and by tea Gloucestershire had replied with 15 for no wicket.
The final session saw runs continue to flow, this time for the home side. Dent and Charlesworth had put together an opening stand of 39 when the latter was bowled off an inside edge by leg-spinner Mitchell Swepson for 15.
It was a first and last success of the day for the Glamorgan bowlers as Dent moved to a comfortable half-century off 113 balls, with seven fours, finding an equally assured partner in Ollie Price, who was unbeaten on 49 at stumps, having hit eight elegant boundaries.
A decent sized Festival crowd, including a host of past players from around the counties attending a Professional Cricketers' Association lunch, had witnessed 432 runs and only five wickets in the extended day's play.