I think this may be how revit is calculating the slope with respect the host surface but i can t wrap my brain around it.
Revit roof slope arrow annotation.
This create an uni directional slope parallel to the arrow.
If you press tab when dimension spot slope in elevation view and the roof is a hip roof with 15 degrees on all 4 roofsides the angel will show alternate 15 and 10 7 degrees.
Revit snaps to the cornerpitch of the roof.
Optional change the following on the options bar.
How to set up the dimension for roof slope in degree products and versions covered revit 2015 revit 2016 revit 2017 revit architecture 2015 revit architecture 2016 revit lt 2015 revit lt 2016 revit lt 2017 revit mep 2015 revit mep 2016 revit structure 2015 revit structure 2016.
The tail of the slope arrow must be on one of the sketched lines that defines the boundary.
Click annotate tab dimension panel spot slope.
In the type selector select the type of spot slope to place.
Enter a value for offset from reference.
To create a slope for an odd shaped roof like the one below use slope arrow.
The tail is the low point and the head is the high point.
For slope representation enabled in an elevation or section view select arrow or triangle.
Ceilings for example to create a flat sloping roof draw a slope arrow from the lower edge to the upper edge of the roof boundary.
Draw the arrow then click it and change the properties to select the right levels and offsets.