How to Calculate Roof Slope?

Your roof adds visual interest, but its real purpose lies elsewhere – from controlling water flow, protecting from elements, and increasing energy efficiency. One essential feature of roof design is understanding and calculating its slope (aka pitch). But how to calculate roof slope? Doing this correctly is important for various reasons – including protecting from rainwater leakage into your home as well as saving energy overall.

• Planning roof repairs or replacements: Understanding your existing slope is vital when selecting suitable materials and installation techniques for your specific roof.
• Select the Appropriate Roof Materials: Different roof slopes work better with certain roofing materials than others, so knowing which slope you have will allow you to select one suitable to your requirements. Calculating the slope can assist with selecting an optimal choice.
• Understanding Local Building Codes: Many municipalities have building codes that set minimum and maximum roof slope requirements for different structures in their jurisdiction.

This blog post provides the knowledge and tools you’ll need to calculate roof slope in two commonly-used methods, namely using the rise-over-run method or trigonometric functions!

Before You Begin: Understanding Roof Slope Terminology 

Let’s first define some key terms related to roofing slope calculations before discussing further:

• Rise: The vertical distance from the bottom edge of a roof deck to its highest point on an asymmetrical roof (half its span). 
• Run: Horizontal distance from vertical wall directly beneath highest point (runoff).
• Slope: Slope measures the ratio between rise and run in terms of fractions such as 3/12 or percentages such as 25%.
• Pitch: Pitch is frequently confused with slope, though technically refers to the ratio between the rise and entire horizontal span (not just half) for roofing material (including wood shakes ). But for practical purposes, they often are used interchangeably.

Calculating Roof Slope Using Two Methods

1: Rise-Over-Run Method

This is the easiest and most widely utilized approach for calculating roof slope. 

• Measuring Rise: With a level and tape measure in hand, measure the vertical distance from the bottom edge of the deck up to the highest point on the roof ridge.
• Measure the Run: For symmetrical roofs, measure the horizontal distance between vertical walls and the point directly beneath its highest point; otherwise for asymmetrical roofs measure half its horizontal span.
• Divide the rise by the run:  Dividing rise by run will yield the slope as a fraction. For instance, if a 6-foot rise has a 12-foot run length then 6/12 becomes 1/2; you could then multiply this fractional slope with 100%; hence 1/2x 100% = 50% in this example. 

2. Apply Trigonometric Functions

This method employs the arctangent (arctan) function on a scientific calculator and works like this:

• Measure both rises and runs to get an approximate rise-run ratio as discussed before.
• Divide the rise by run. Utilize your calculator’s “tan-1” or “arctan” arctangent function and input the result from step one into your calculator’s arctangent calculator function arctan-1 or arctan.
• Note that building codes often specify roof slope in terms of “rise in inches per 12 inches of horizontal run.” 
• To convert from fraction or percentage calculations into this format, multiply the numerator (rise) by 12 and divide the denominator (run).

Important Considerations

• Safety first: Only climb onto your roof if you possess adequate safety gear and experience working from heights. For added peace of mind, hire a professional roof inspector like A2Z Roofing, if measuring its slope is intimidating to do yourself.
• Accuracy is key: Accurate measurements are vital to calculate an appropriate slope angle. Employ reliable measuring instruments and double-check your measurements to maintain accuracy.
• Referring to Local Building Codes: When designing roof slopes in any part of the country, always consult local building codes before beginning work on them. Make sure your plans comply with regulations specific to your region.

Final Thoughts

How to calculate roof slope? – Calculating roof slope is an invaluable skill for those involved with maintenance, repairs, or replacement of their roofing needs. By understanding riser, run, and slope concepts as well as using one of the various methods outlined here to calculate its slope you can make informed decisions regarding any of your needs for roof work – but remember safety is of utmost importance; if any aspect of work leave you uncertain seek advice from qualified roofing professional like A2Z Roofing immediately!