what is 2 x 10/2 x framing?

In researching the metal for the roof I find it needs to be at least a 3:12 slope, which means a 24 inch knee wall at the peak for my 8 ft wide building. I'm using 2x6 rafters and I'm sheathing the roof under the metal because I'm in a high wind area. I will also use either angle iron or strapping to attach the vertical legs of the knee wall to the container. I assume the knee wall should be out of 2x6, but maybe 2x4 would do? Also, I'm guessing it needs to be beefed up a bit more than the attached picture. But I don't know where the extra support needs to go.

In Australia we have;
The roof pitch required for a corrugated roof, is a minimum of 5°. With low ribs and small spacing, a higher pitch is required to ensure the water does not pond in the one area.
As a result this may cause the water to leak through the lapping roof sheets and into the building below.
Below is the standard in North America, I am surprised its so steep.
Do you get much rain where you are building?

From; roof slopes required

What is the minimum pitch for a metal roof?

The minimum slope for lapped, non-soldered seam metal roofs without applied lap sealant shall be three units vertical in 12 units horizontal (25-percent slope).
The minimum slope for lapped, non-soldered seam metal roofs with applied lap sealant shall be one-half unit vertical in 12 units horizontal (4-percent slope).1 Nov 2016

Note that the 3:12 pitch is only required if you don't use lap sealant between sheets. This is a roll of thick stretchy gummy "tape" that you apply to the top of one lapping edge before putting the matching edge over it. It is not expensive and only requires ordinary careful handling to do right. With that, you can have as little as 1/2:12 pitch, though I would go with at least 1:12 if possible just for safety. Even 1 1/2:12 is flat enough to walk on easily, so requires no special methods to install.

Glenn Herbert, no, that's not correct. The manufacturers would prefer you use a peel and stick on the whole roof deck and still want you to have a 3/12 slope.
This company has great information on roof installation! https://www.westernstatesmetalroofing.com/

I wonder if things are tighter because you have hurricanes down there>

The Western States site calls for their PBR panel (most common style) to have a 1:12 minimum pitch.
Recommended Minimum Roof Slope: 1/12

Glenn Herbert, thank you for pointing that out. You are correct about the PBR / R-panel only needing 1/12 slope. I was looking at the 7/8 corrugated which requires the 3/12 slope. I like the look of it better.