You don't eh? He's got a roof made of 2X4's on 24 inch centers.
That's sufficient for a light roof in a place where it doesn't snow
much and with a span of maybe 7 ft max. And that's assuming whoever
built it used only quality construction grade 2X4's, not some random
studs. Anyone taking that care likely would have used 2X6's to begin
with.

The previous advice to build out the rafters and correctly reinforce
them is the right advice. If done correctly, it will make it strong
and fix the insulation issue at the same time.