The England coach, who had his football counterpart Gareth Southgate at training yesterday, expressed a blase disregard for whether it stays open or not, joking that he will toss a coin to decide.
The former Wales great says he is playing with fire with his indifference as he believes it makes a significant difference.
“If the roof is closed it makes a big impact on the atmosphere. That will make it an even more hostile environment for the English players,” said Williams.
“When they came to Cardiff four years ago looking for the Grand Slam, I have never seen a side capitulate so badly. The Welsh crowd played a huge part in that record-equalling win as they roared Wales on to the title and a 30-3 victory – and they will do again on Saturday.
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Click through the gallery for the most bizarre moments in Six Nations history
England coach Eddie Jones has been warned about letting Wales play with the roof closed
“There are a few players who won’t have experienced playing in Cardiff and that could be crucial.”
Williams, who never lost to England in 11 internationals, predicts the visitors will be taken aback by the ferocity of the Welsh ‘welcome’.
“In my playing days it just came down to us wanting to win it more than them – the classic small nation versus big neighbour thing,” he said.
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JPR Williams is a former Wales international
“We get on pretty well with the people in the west country and up north, but not so well with those from the south-east.”
Wales’s players return to training today after two days off following Sunday’s win in Italy with a serious doubt over stand-off Dan Biggar (rib).
England, who rested battered back rows Tom Wood and Nathan Hughes, are considering promoting Exeter wing Jack Nowell to the starting line-up.