What drives pre-engineered truss heel heights? 1

[StrEng007]

What method does a truss designer use to determine the base cut at the heel of a truss?
I know the equation used to determine the overall heel height; I'm not sure what rule the truss designer applies for the base cut.

Two examples from truss shop drawings (2x4 top chords in both examples):

Truss Heel, 6 1/16" = 3.5" / (cos 18.43°) + Base Cut
Base cut = 2 3/8"

Truss Heel, 6 1/2" = 3.5" / (cos 26.56°) + Base Cut
Base cut = 2 9/16"

 

[StrEng007]

can enter by soffit vents

I definitely understand the need and have done some reading on different different thicknesses of insulation and how they pertain to R-values.

The thing I'm still trying to wrap my head around is the lack of continuity with the diaphragm sheathing when there is a gap present for that air flow. I always specify continuous blocking at the diaphragm boundaries (have done so for a long time). Interestingly enough, my work is located in the strongest wind zone in the US, but also requires the lowest level of insulation for attic framing.