Timber Rafter Size Calculator
This calculator helps users determine the correct size of timber rafters needed for their specific building projects. It takes into account various factors such as the total load, including the weight of the roof and potential snow load, the span of the roof, and the properties of the timber used.
Formula of Timber Rafter Size Calculator
To calculate the size of timber rafters, the following formula is utilized:
Where:
- S is the size of the rafter.
- T is the total load on the rafter.
- L is the span of the roof.
- F is the snow load factor.
- D is the deflection limit.
- C is the fiber stress factor for the timber.
General Terms and Conversion Table
Understanding common terms used in rafter calculation is vital. Below is a table defining these terms and providing necessary unit conversions:
| Term | Definition |
|---|---|
| Span | The distance between two supports where the rafter is placed. |
| Load | The total weight supported by the rafter, including roof material and environmental loads like snow. |
Conversions:
| Inches | Millimeters |
|---|---|
| 1 | 25.4 |
Example of Timber Rafter Size Calculator
Consider a scenario where you need to calculate the rafter size for a roof with a span of 20 feet, a total load of 35 pounds per square foot, and a snow load factor of 1.5. Using the provided formula, the steps to determine the appropriate rafter size are as follows:
- Identify and plug in the values into the formula:
- Total Load (T) = 35 pounds per square foot
- Span of the roof (L) = 20 feet
- Snow Load Factor (F) = 1.5
- Assuming a deflection limit (D) of 360 and a fiber stress factor (C) of 1000 (common values for typical constructions)
- Apply the values to the formula: S = sqrt((T * L * F) / (D * C)) S = sqrt((35 * 20 * 1.5) / (360 * 1000))
- Calculate the result: S = sqrt((1050) / (360000)) S = sqrt(0.00291667) S = 0.054
- Convert the result to a more useful unit (e.g., inches if the result is very small in feet): Since the result, 0.054 feet, equals approximately 0.65 inches, this would be the calculated size of the rafter in terms of its depth, necessary to support the given load over the given span with the specified conditions.
Most Common FAQs
The maximum span varies based on rafter size, load, and timber type. It’s best to consult specific tables or use the calculator for precise data.
Higher snow loads increase the total load on the rafter, requiring a larger rafter size to safely support the additional weight.
No, different types of timber have different strength characteristics, which must be considered when calculating rafter size.