What is Rafter?

Rafters are the sloping beams that form the main structure of a roof, supporting the roof deck and shingles.

Calculating rafter measurements can be complex, involving trigonometry and precise measurements.

A rafter calculator simplifies this process by automating these calculations based on input parameters such as roof span, pitch, and overhang.

How to Use the Rafter Calculator?

1. Enter the Roof Span
Start by inputting the roof span in feet. This is the horizontal distance between the outside edges of the two opposing walls that the roof will cover. Measure this distance accurately, as it forms the basis for all other calculations.

2. Select the Roof Pitch
Choose the appropriate roof pitch from the dropdown menu. The pitch is expressed as a ratio of rise to run, typically given as X/12, where X is the vertical rise in inches for every 12 inches of horizontal run. Common pitches range from 2/12 (a relatively flat roof) to 12/12 (a very steep roof).

3. Specify the Overhang
Enter the desired overhang in inches. The overhang is the portion of the rafter that extends beyond the exterior wall. A typical overhang is 12 inches, but this can vary based on architectural design and local building codes.

4. Calculate
Click the “Calculate Rafter” button to process your inputs. The calculator will then perform the necessary trigonometric calculations to determine various important measurements.

5. Interpret the Results
The calculator will display several key pieces of information:

• Rafter Length: This is the diagonal distance from the ridge (peak) of the roof to the wall plate, not including the overhang.
• Total Rafter Length: This includes the rafter length plus the overhang, giving you the full length of lumber needed for each rafter.
• Roof Angle: This is the angle of the roof slope measured from horizontal, useful for setting your saw to make accurate cuts.
• Birdsmouth Angle: This is the angle of the notch (called a birdsmouth) that needs to be cut where the rafter meets the wall plate, ensuring a secure fit.
• Ridge Height: This tells you how high the peak of the roof will be above the top of the wall plates.

6. Additional Considerations

• Always double-check your measurements and results before cutting any materials.
• Remember that these calculations assume a simple gable roof. More complex roof designs may require additional calculations or professional assistance.
• Consider factors like lumber sizes, local building codes, and snow loads when planning your roof construction.
• For very large or complex projects, it’s advisable to consult with a structural engineer or experienced contractor.

7. Adjusting Your Design
If the results aren’t quite what you’re looking for, you can easily adjust your inputs and recalculate. For example, you might try different pitches to see how they affect the rafter length and roof angle.

8. Using the Results
Once you’re satisfied with the calculations, use these measurements to guide your lumber purchases and cuts. The rafter length will determine how long each piece of lumber needs to be, while the roof and birdsmouth angles will guide your saw settings for precise cuts.

By using this rafter calculator, you can save time, reduce waste, and ensure more accurate construction. It’s an invaluable tool for both planning and execution stages of roof framing, helping you to create a sturdy and well-proportioned roof structure.