When constructing a pitched roof, one of the essential components is the rafter—the sloped beams that support the roof’s covering. Correctly calculating the rafter length is crucial for ensuring structural stability, proper alignment, and overall roof strength. The rafter length depends on the roof pitch, span, and overhang, which must be carefully measured to determine the right length for each rafter.

This guide will help you understand how to calculate the rafter length for your roof, with clear formulas and examples to guide your project.

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Key Terms

• Rafter: A sloped structural beam that supports the roof covering (shingles, tiles, etc.).
• Roof Pitch: The slope of the roof, usually expressed as a ratio (e.g., 4:12), indicating how many inches the roof rises for every 12 inches of horizontal run.
• Span: The horizontal distance between the two outside walls of the building, typically measured at the peak of the roof.
• Overhang: The portion of the rafter that extends beyond the walls to provide roof coverage.

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How to Calculate Roof Rafter Length

To calculate the length of a roof rafter, you need to know three things:

1. The pitch of the roof (angle of the slope).
2. The span of the building (width of the roof from one side to the other).
3. The overhang (the extension of the roof beyond the building walls).

Step 1: Calculate the Horizontal Run

The horizontal run is the distance from the center of the roof peak (ridge) to the exterior wall. This is calculated using the roof’s span.

Horizontal Run = (Span ÷ 2)

For example, if the span of the roof is 24 feet:

• Horizontal Run = 24 ÷ 2 = 12 feet

Step 2: Determine the Roof Pitch

The pitch of a roof is usually expressed as a ratio of the rise to the run (e.g., 4:12). The first number (rise) indicates how many inches the roof rises for every 12 inches of horizontal run. The second number (run) is always 12 inches (a foot).

For example, a 4:12 pitch means the roof rises 4 inches for every 12 inches horizontally.

Step 3: Use the Pythagorean Theorem to Calculate Rafter Length

To calculate the rafter length, you’ll need to use the Pythagorean theorem:

Rafter Length² = Horizontal Run² + Rise²

• Horizontal Run is the horizontal distance (calculated above).
• Rise is the vertical distance, calculated from the pitch (rise per 12 inches of run).

For example, if you have a 4:12 pitch (rise = 4 inches for every 12 inches of run) and a horizontal run of 12 feet (144 inches):

• Rise = 4 inches per 12 inches of run = (4 ÷ 12) × 144 = 48 inches

Now, use the Pythagorean theorem:

• Rafter Length² = 144² + 48²
• Rafter Length² = 20736 + 2304 = 23040
• Rafter Length = √23040 = 151.7 inches

To convert inches to feet:

• 151.7 inches ÷ 12 = 12.64 feet

So, the rafter length would be approximately 12.64 feet.

Step 4: Add the Overhang

Once you have the basic rafter length, you may want to add the overhang—the distance the rafter extends beyond the wall. For example, if you plan for a 2-foot overhang, add that to the rafter length.

Final Rafter Length = Rafter Length + Overhang

Example with 2-foot overhang:

• Final Rafter Length = 12.64 feet + 2 feet = 14.64 feet

So, the total rafter length with an overhang would be approximately 14.64 feet.

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Roof Rafter Length Formula

Here’s a simplified formula for calculating the rafter length:

1. Horizontal Run = (Span ÷ 2)
2. Rise = (Pitch Rise ÷ 12) × Horizontal Run
3. Rafter Length = √(Horizontal Run² + Rise²)
4. Final Rafter Length = Rafter Length + Overhang

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Roof Rafter Length Calculation Example

Let’s go through a full example of calculating a rafter length for a roof with a 6:12 pitch.

Scenario:

• Roof Pitch = 6:12
• Span = 30 feet
• Overhang = 2 feet

Step-by-Step Calculation:

1. Calculate Horizontal Run:
   • Horizontal Run = Span ÷ 2 = 30 ÷ 2 = 15 feet
2. Determine the Rise (for a 6:12 pitch):
   • Rise = (Pitch Rise ÷ 12) × Horizontal Run = (6 ÷ 12) × 180 = 90 inches
3. Use the Pythagorean Theorem:
   • Rafter Length² = Horizontal Run² + Rise²
   • Rafter Length² = 180² + 90² = 32400 + 8100 = 40500
   • Rafter Length = √40500 = 201.2 inches
   • Rafter Length = 201.2 ÷ 12 = 16.77 feet
4. Add the Overhang:
   • Final Rafter Length = 16.77 feet + 2 feet = 18.77 feet

So, the final rafter length is approximately 18.77 feet.

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Roof Rafter Length Calculator Chart

Here’s a table to help you estimate rafter lengths for common roof pitches and spans.

Roof Pitch	Span (feet)	Rafter Length (feet)	Rafter Length with 2′ Overhang (feet)
4:12	20	9.22	11.22
4:12	24	11.07	13.07
6:12	20	10.77	12.77
6:12	24	13.00	15.00
8:12	20	12.64	14.64
8:12	24	15.16	17.16

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Frequently Asked Questions (FAQs)

1. How do I calculate the rafter length if I have the roof pitch and span?

• Use the Pythagorean theorem:
  Rafter Length = √[(Span ÷ 2)² + (Rise per 12 inches of pitch × Span ÷ 2)²]

2. Can I use a different unit of measurement (e.g., metric)?

• Yes! You can use any unit, but ensure that all your measurements (span, rise, and overhang) are in the same unit (inches, feet, or meters). For metric measurements, the conversion factors are:
  • 1 foot = 0.3048 meters
  • 1 inch = 2.54 cm

3. How do I account for the overhang?

• Simply add the overhang measurement to the calculated rafter length. For example, if the rafter length is 12 feet and you want a 2-foot overhang, the total length will be 14 feet.

4. What if I have an irregular roof pitch (not a simple 4:12, 6:12, etc.)?

• For irregular pitches, you can still use the same Pythagorean theorem approach. Just use the exact rise measurement for your roof pitch, which may require you to calculate the slope angle based on your roof’s design.

5. Do I need to account for ridge board thickness in the rafter length?

• If you are using a ridge board, you may need to slightly adjust your calculations. The ridge board’s thickness should be subtracted from the overall rafter length, but this is typically a small adjustment.

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Conclusion

The Roof Rafter Length Calculator is an essential tool for anyone constructing or designing a pitched roof. By accurately calculating the rafter length, you ensure that your roof is both structurally sound and aesthetically pleasing. Using the Pythagorean theorem and knowing your roof pitch, span, and overhang will allow you to create a properly sized roof that fits your building design perfectly. Always check local building codes and regulations to ensure your roof meets safety and construction standards.