Align sandbox grid to existing face

This is the first time I have created a sandbox grid to make a textured face. Once I create it I am having trouble aligning it to an existing face (which is on an angle).

I can’t seem to select corners of the grid and map them to corners of the face, the main problem for me I think is I am not sure of the correct terms for this, so when I search in tutorials I’m kind searching blind!

Image below of what I mean.

Thanks in advance.

Are your roof plane corners 90 degrees?
Could you provide another screen shot but in ‘Perspective’ mode? This one is confusing.

Hi Hugh, hi folks.

Click in sequence on the Scenes Tabs of this SU file for ideas.

Alignig Sandbox grid.skp (356.2 KB)