The Nickelback singer worked with the 'What The Hell' singer on tracks for her fifth album - a relationship that proved so fruitful it has led to them becoming engaged - and was blown away by her vocal ability from the start.
He exclusively told BANG Showbiz: ''It's easier to write for somebody who has got a vocal range like she's got. It's ridiculous how high she can go. She just opens up her throat and it goes higher and higher - and it's amazing, it's like taking a bazooka to a knife fight!''
Chad, 37, also said he had to approach writing with Avril, 28, in a different way to how he would with his rock group, but in the studio he mostly contributed lyrics.
He added: ''Obviously with her I wasn't going to tune the guitars down and put on as much distortion as possible.
''You start acoustically, with a piano, and you start moving that way. When I'm with her, I'm mostly the lyric guy, she does a lot of the melody and I'll help out with lyrics and chord progressions wherever I can.''