I'm looking to take advantage of some empty space under my roof that is currently inaccessible due to a dormer wall. I'd like to build in some fitted shelving or closets under the roof.

I'm wondering if I can just add headers on the angle of the roof so I can widen the area between studs? Essentially a door frame with a slanted header.

I'm based in Toronto, Canada if that helps with anything. Thanks for your help.