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Topic: Roof Framing.

Read The Ubiquitous Jack-Rafter.

Finding the Length of Jack-Rafters.

You can calculate the unadjusted hip/valley-jack length for your roof by using the following equation.

O.C./Cos(roof pitch angle) * Tan(plan angle)

O.C. represents the "on center" spacing of your members. Common spacing is 16" and 24", but 19-3/16" is becoming popular as well.

Roof pitch angle is the angle at the common rafter makes with your roof. If you have an 8/12 roof, the angle would be 33.69°. You can find any roof angle by Tan^-1(roof pitch/12).

Plan angle is the angle that the hip/valley makes with the "corner" of the roof. A standard hip roof makes a 45° angle.

If you were working with a standard hip roof with an 8/12 pitch with 16" you could find the first jack length as follows;

• O.C./Cos(roof pitch angle) * Tan(plan angle)
• 16 / Cos (33.69) * Tan (45)
• 19.23 * 1
• 19.23 or 19-1/4"

This measurement is also called the "Common Difference". This means the adding this amount to itself would give the "next" jack length.

• 19.23 + 19.23 = 38.46 or 38-7/16"

Another cool thing is you can find the length of any jack in any position by using (O.C./Cos(roof pitch angle) * Tan(plan angle)) * (Position).

If you wanted to know the length of the third jack, "position" would be 3. 19.23 * 3 or 57.69

You can calculate the unadjusted hip/valley-jack length for your roof by using the following equation.

O.C./Cos(roof pitch angle) * Tan(plan angle)

O.C. represents the "on center" spacing of your members. Common spacing is 16" and 24", but 19-3/16" is becoming popular as well.

Roof pitch angle is the angle at the common rafter makes with your roof. If you have an 8/12 roof, the angle would be 33.69°. You can find any roof angle by Tan^-1(roof pitch/12).

Plan angle is the angle that the hip/valley makes with the "corner" of the roof. A standard hip roof makes a 45° angle.

If you were working with a standard hip roof with an 8/12 pitch with 16" you could find the first jack length as follows;

• O.C./Cos(roof pitch angle) * Tan(plan angle)
• 16 / Cos (33.69) * Tan (45)
• 19.23 * 1
• 19.23 or 19-1/4"

This measurement is also called the "Common Difference". This means the adding this amount to itself would give the "next" jack length.

• 19.23 + 19.23 = 38.46 or 38-7/16"

Another cool thing is you can find the length of any jack in any position by using (O.C./Cos(roof pitch angle) * Tan(plan angle)) * (Position).

If you wanted to know the length of the third jack, "position" would be 3. 19.23 * 3 or 57.69