If you have chosen degree's as your project units, this makes sense when creating roofs but not for calculating the slope of a floor, as this is measured as a ratio, eg 1:100 fall. This makes creating the slope of a floor something that you have to calculate manually.
So it would be possible to have two slope settings available in the project units and have both units available in the properties browser.
Slope: 20 deg
Alternative Slope: 1:100
PS I am not talking about reporting values tags, you can't change the slope of an object from the tag