Feet & Inches Formula

I am making an excel spreadsheet that auto fills in a lot of items for a construction company. One of the most important ones is the Roof Pitch.

1/12 pitch is 1 inch rise every 12 inches.
2/12 pitch is 2 inch rise every 12 inches.

I would like it to auto calculate this and add it to the over all height of the building.

Example:

A house is 20 feet wide with a 2/12 pitch.
Since the Ridge is in the center we divide the Width in half.

So a 2/12 pitch over a span of 10 feet is 20 inches