PURLIN MITER and BEVEL ANGLE FORMULAS:
GIVEN: Slope at the Framing Point and Deck Angle
Pitch Angle in the following formulas means arctan(Slope at the Framing Point)
Plane of the Roof means the plane tangent to the curve at the framing point.
The compound angle is cut on the face of the Purlin set perpendicular to the plane of the roof.
Saw Miter = arctan(sin Pitch Angle ÷ tan Deck Angle)
Angle on the Stick = arctan(tan Deck Angle ÷ sin Pitch Angle)
or Angle on the Stick = 90°  Saw Miter
Saw Blade Bevel = arcsin(cos Pitch Angle × cos Deck Angle)
Miter Line on the Valley: In the context of an ellipse, the following angle
is measured with respect to a right angle to the tangent (meaning Slope or Pitch) at a given point.
arctan(sin Pitch Angle × cos Pitch Angle × cos Deck Angle ÷ tan Deck Angle)
Example:
At (x, y) = (9.33333, 1.79506) on the MAIN SPAN,
the Slope = 1.29987
(Invoking symmetry, we may use a positive value of the slope)
Pitch Angle = arctan(1.29987) = 52.42864°
Saw Miter = arctan(sin 52.42864° ÷ tan 53.13010°) = 30.72919°
Angle on the Stick = arctan(tan 53.13010° ÷ sin 52.42864°) = 59.27081°
or Angle on the Stick = 90°  30.72919° = 59.27081°
Saw Blade Bevel = arcsin(cos 52.42864° × cos 53.13010°) = 21.45987°
Corresponding point (x, y) = (9.33333, 1.79506) on the VALLEY
, Slope = 1.03989
With respect to a right angle drawn to this line the Purlin meets the Valley at
arctan(sin 52.42864° × cos 52.42864° × cos 53.13010° ÷ tan 53.13010°)
= 12.26950°
The compound angle is cut on the face of the Purlin set in the plane of the roof
Saw Miter = Jack Rafter Side Cut Angle = arctan(cos Pitch Angle ÷ tan Deck Angle)
Angle on the Stick = Sheathing Angle = arctan(tan Deck Angle ÷ cos Pitch Angle)
or Angle on the Stick = 90°  Saw Miter = 90°  Jack Rafter Side Cut Angle
Saw Blade Bevel = Backing Angle = arcsin(sin Pitch Angle × cos Deck Angle)
Example:
Saw Miter = arctan(cos 52.42864° ÷ tan 53.13010°) = 24.57518°
Angle on the Stick = arctan(tan 53.13010° ÷ cos 52.42864°) = 65.42482°
or Angle on the Stick = 90°  24.57518° = 65.42482°
Saw Blade Bevel = arcsin(sin 52.42864° × cos 53.13010°) = 28.39561°
SPECIAL CASES: The shapes in section may be circular, elliptic, parabolic, hyperbolic, etc.,
or sections thereof.
"Eaves" or "Ridges" may intercept at angles other than 90°.
Equal shapes intercepting at right angles create equal 45° Deck Angles.
The simplest case is two circular sections intercepting at 90°.
Equation: x ² + y ² = Radius ²
Slope at Framing Point =  x ÷ y
Pitch Angle at Framing Point = arctan(x ÷ y)
sin Pitch Angle = x ÷ Radius
cos Pitch Angle = y ÷ Radius
Valley Ellipse Major Axis = Diameter ÷ sin 45°
= Ö2 × Diameter
Example:
Circle: Radius = 5
Framing Point on Circle: at x = 4 y = 3
Slope at Framing Point = tan Pitch Angle =  4 ÷ 3
Pitch Angle = arctan (4 ÷ 3) = 53.13010°
sin Pitch Angle = 4 ÷ 5 cos Pitch Angle = 3 ÷ 5
Valley Ellipse Major Axis
= 5 × Ö2
= 7.07107
Corresponding Framing Point on Valley Ellipse:
x = 4 × Ö2 = 5.65685 y = 3
Since tan 45° = 1 and sin 45° = cos 45° = 1 ÷
Ö2
some Deck Angle terms in the Purlin formulas are eliminated and the equations reduce to simpler forms.
Angles on the Purlin face perpendicular to the plane of the roof:
Saw Miter = arctan(sin Pitch Angle)
Angle on the Stick = arctan(1 ÷ sin Pitch Angle)
= arctan(csc Pitch Angle)
Saw Blade Bevel = arcsin(cos Pitch Angle ÷
Ö2)
@
arcsin(.70711 × cos Pitch Angle)
Example:
Saw Miter = arctan(4 ÷ 5) = 38.65981°
Angle on the Stick = arctan(5 ÷ 4) = 51.34019°
Saw Blade Bevel = arcsin(3 ÷ (5 ×
Ö2))
= 25.10409°
Angles on the Purlin face set in the plane of the roof:
Saw Miter = arctan(cos Pitch Angle)
Angle on the Stick = arctan(1 ÷ cos Pitch Angle)
= arctan(sec Pitch Angle)
Saw Blade Bevel = arcsin(sin Pitch Angle ÷
Ö2)
@
arcsin(.70711 × sin Pitch Angle)
Example:
Saw Miter = arctan(3 ÷ 5) = 30.96376°
Angle on the Stick = arctan(5 ÷ 3) = 59.03624°
Saw Blade Bevel = arcsin(4 ÷ (5 ×
Ö2))
= 34.44990°
Miter Line on the Valley =
arctan(sin Pitch Angle × cos Pitch Angle ÷
Ö2)
@
arctan(.70711 × sin Pitch Angle × cos Pitch Angle)
Example:
Miter Line on the Valley =
arctan(4 × 3 ÷ (5 × 5 ×
Ö2))
= 18.74779°
