The maximum vertical rise would be 10.4 ft.
A 13% grade is the slope, or rise/run.
13% = 13/100
Setting up a proportion with the 80 ft horizontal run and an unknown rise, we have:
13/100 = x/80
Cross multiplying we have
100*x = 13*80
100x = 1040
Divide both sides by 100:
100x/100 = 1040/100
x = 10.4