St Edmund's tackled itself to a standstill in keeping Canberra Grammar School scoreless, running away with the grand final 40-0 at Viking Park on Sunday.

It was back-to-back first XV titles for St Edmund's after last year's victory against Marist College.

St Edmund's led 14-0 at half-time before finishing the game off strongly to the delight of first-year coach Pat Langtry.

"Our boys worked hard during the season and I think the way they defended today was the heart and soul of the victory," Langtry said.

"There was a period there late in the first half and early in the second half where Grammar threw everything at us and we stood our ground and kept knocking them over.

"I was really proud of them the way they did that."

St Edmund's narrowly defeated Grammar during their two previous encounters this season.

And despite the lop-sided scoreline, it again proved to be a tough contest.

"To say we were in complete control is probably not right," Langtry said.

"Grammar were very, very threatening.

"They had a lot of players out through injury and that probably didn't help their cause, but I just go back to the way we prepared.

"If you defend well in the big games, it's hard to get beat."

St Edmund's completed a clean sweep of the under-18 division, winning the second XV premiership with a 45-22 victory against Marist.