I tried to make this as simple as possible. You tell the program how wide the building is, how tall the walls on each side of the ridge are and the pitches, push a button and it spits out the numbers you need to cut the rafters. These calculations assume the throat, the vertical distance above the plumb cut of the birds mouth, is the same around the entire perimeter of the building. The wall heights given do not have to be the actual wall heights but the exact difference in heights has to be calculated from them. For example, if the span goes from an 8' PL on one side of the ridge to a 10' PL on the other, 8' 0" and 10' 0" will be fine even though they both measure 1 1/8" taller. Where one side is downstairs and the other is upstairs, the wall heights must reflect the actual difference in height.

Entering Data
When entering measurements, feet are designated by a single quote. All numbers not immediately followed by a single quote will be considered inches. Fractions must have no spaces or they will not be interpreted correctly. 12' 8 1/2 is determined to mean 12 feet 8 1/2 inches. Decimal points are allowed. You could enter 1 1/2" as 1.5. You can do some addition and subtraction. A minus sign will cause the next number input to be subtracted from the total input number. Any number without a minus sign will be added to the total input number. All numbers to be subtracted must have its own minus sign. 12' 8 1/2 2 is equal to 12' 10 1/2". 12' 8 1/2 -2' -1/2 is equal to 10' 8". If I had left off the last minus sign it would have been 10' 9". Spaces and letters are all treated as spaces. They only make a difference if used immediately before a single quote or in the middle of a fraction.

Span Info

Span:
Width of the building including the ridge.
Wall 1 Height:
Height of a wall.
Wall 1 Pitch:
Pitch associated with Wall 1 height.
Wall 2 Height:
Height of wall opposite the ridge.
Wall 2 Pitch:
If the pitch associated with Wall 2 is different from Wall 1, it can be changed by first clicking the checkbox.

Spacing and Material Info

Ridge Thickness:
Thickness of the Ridge material. This value is subtracted from the input span before any other calculations are made.
Rafter Spacing:
On center spacing of the Rafters
Rafter Thickness:
Thickness of the Rafter material
Hip Thickness:
Thickness of the Hip material.
Note: The Rafter Spacing, Rafter Thickness and the Hip Thickness are all needed to correctly calculate the Jack Rafters.

Two Pitch Roofs:
If the roof has two pitches, they can be set by clicking the checkbox and entering the desired pitches. If the checkbox is checked the two pitches entered here will be used for the defaults when calculating hips and jack rafters. If the hips or jack rafters are calculated using two pitches they will be denoted by the second pitch enclosed in parentheses. By default, if a pitch is used which matches either of the two pitches the other is automatically selected for hip and jack rafter calculations. If a pitch is used which does not match either pitch then that pitch is the only pitch used to calculate the hip and jack rafters.

Calculate span:
Click the Calculate Span button to calculate the runs for the input span.

Calculate Additional Info For This Span:
After calculating the span using the Calculate Span button, additional wall heights and pitches associated with the input span can be calculated by entering the desired information in the Special Wall Info section then pressing the Calculate Special Wall button.

Special Wall Info:
The special wall info section is used to calculate any additional wall heights or pitches needed for the input span. If more than one pitch is needed to calculate the hip and jack rafters click the checkbox in front of Pitch 2 and you will be allowed to change the pitch.

Output Sections

Input Span:
Shows how the Span was interpreted.

Under each wall section
Wall Height:
Input height of the wall.
Pitch:
Input pitch of the wall
Run:
Distance from the outside of building to the ridge.
Rise:
Rise from that wall. Calculated as run in feet times pitch.
Common:
Length (line length) of the Common Rafter. The ridge has already been taken into account.
Hip:
Length (line length) of the Hip Rafter. If the hip is calculated using two pitches, the second pitch will follow the hip length in parentheses. Remember, if the hip is a two-pitch hip then it does not center on the corner but is set off center to the low pitch side.
First Jack:
Length (line length) of the longest Hip Jack Rafter on the long point side. If the jack rafter is calculated using two pitches, the second pitch will follow the jack rafter length in parentheses. This calculation assumes there is a Common Rafter flush with the end of the Ridge and longest Jack Rafter is spaced according to the spacing given in the input section.
Bevel:
Saw bevel (in degrees) to use when cutting the Hip Rafter and the Jack Rafters.
Hip Pitch:
Pitch of the Hip Rafter (in degrees).

Go to Randy's Rafter Calculator