Simplest way to angle off an edge of box

[Anonymous]

I am modeling a floor plan and need to show a wood joist with its end angled off to fit a corner. I start by creating a box 1.5" x 12" x 8'. I want just the last 6 inches to be at an angle (45 degrees is current need but would like ability to adjust). So the box ends up being 8' on one side and 7'6" on the other. See attachment

[CAMedeck]

If your object has an Edit Poly modifier, you could use a Slice or Cut to sheer off the end and then Cap it under the Border sub-object. 

Or use a Slice Plane modifier then Cap Holes.