I am going to put up a 48' wide by 24' deep pole barn, open faced for my tractor and attachments. How do I determine the length opf 6x6 posts for the 4 outside corners so that I have 8 " at the rear of the building? I am putting them in concrete 3' deep and at the back 24' away I want the lowest point to be ballpark 8' high to also maintain my 6/12 pitch?

Am thinking somewhere around 20' for the front poles and maybe 12' for the rear??

To maintain a 6 /12 pitch on the size building you plan on the front posts must be 12 feet longer then the rear. To have a rear height of 8 feet rear the post should be 8feet less the height of the girder plus 2 inches.

This will give you approximately 8 feet at the rear .

Chuck