Roy’s 120 at the top of the order in a total of 342-8 gave the hosts the breathing space they needed to go 2-0 up with three to play in their one-day international series against Australia.
Jos Buttler, emergency captain after Eoin Morgan was a last-minute absentee with a back spasm, piled in with an unbeaten 91 - and despite a brilliant 131 from Australia’s Shaun Marsh in reply, England bowled the tourists out for 304 in 47.1 overs.
Roy, who also gave glowing mention to his opening partner Jonny Bairstow for setting the tone with a rush of early boundaries after England were put in on a cloudy morning, had his own national-record 180 in his sights until he was caught behind in the 36th over.
Nonetheless, as he picked over his innings with England batting coach Graham Thorpe, he concluded it was the best of his five ODI hundreds.
“I had a long discussion with Thorpey just after I got out and said it was probably my favourite innings - as far as the tempo of it, the way I knuckled down early,” said Roy.
“I went through some difficult patches, but then came out the other side.
“When I wanted to start expanding my game, unfortunately I got out - but I was still happy.”
Roy’s brilliant batting can sometimes have a seat-of-the-pants look to it, but this time he appeared more measured than ever - all the more impressive after his second-ball duck in the first ODI at The Oval three days ago.
“I was able to knuckle down and get myself in,” he added. “(Jonny) is great to watch isn’t he at the moment? He’s in some incredible form.
“It was a nice feeling to be able to just relax and take a watch.”
It was heartening too that Roy hit such heights in his first ODI back at this venue, where he was left out of England’s Champions Trophy semi-final team last summer.
He said: “It’s disappointing to be dropped at any ground. Playing for England is a huge honour.
“I wouldn’t say being dropped in that Champions Trophy has anything to do with making any hundred feel any better than the next.
“It was a big aim of mine to come back after a long winter and start well.
“Obviously, we had that hiccup against Scotland, and then the first game (of this series) I was a bit panicky, a bit wary of where my game was at.
“But I’ve put in a lot of hard work over the last two days, and got a few rewards today.”
Marsh made England sweat a little nonetheless.
Roy added: “Marsh batted out of his skin, and then (Ashton) Agar came in and started smacking it as well.
“Those were two very big wickets, but at the half-time break we were confident.”
Australia’s number three was excelling at a ground he will be calling home for the next two years after signing for Glamorgan.
Marsh said: “It’s bittersweet. It was nice to get a few runs but we couldn’t quite finish it off there at the end.
“Obviously, it’s a disappointing result for us. We felt like we were in the game right till the end, but unfortunately (we) just weren’t good enough today.”
Australia captain Tim Paine has had two stitches on his upper lip after taking a nasty blow while keeping wicket in Cardiff - but still has all his teeth intact.