Roof Slope Calculation: 2 Easy Methods and Pitch Angle Chart

Sep 27, 2023

WRITTEN BY:

Viking Contractors

CATEGORY:

Uncategorized

POSTED ON:

September 27, 2023

UPDATED ON:

May 10, 2024

Got Questions?

Are you a homeowner who’s gathering roofing estimates for a building project? You may have come across architectural terms such as roof pitch, slope, span, rise, and run. 

While they seem complex, you should familiarize yourself with them, especially the roof slope calculation. This is because it determines the type of roofing materials to use and the installation method. 

Also, the slope helps you know the cost of the roofing project so you can reduce waste. That’s why you should work with a reliable company like Viking Contractors that’ll walk you through each stage of your project, ensuring you get the best value for your money. 

 

What Is a Roof Slope?

The inclination on a roof is known as the slope of that roofing system. It’s expressed as the ratio of the vertical rise to the horizontal run and measured in inches per foot. 

The roof rise is the vertical distance between the highest and lowest points on the roof. On the other hand, the roof run is the distance between the peak of the roof and the edge above the gutter line.

For example, a 4:12 slope implies that the roof rises vertically four inches for every twelve inches of horizontal run. 

 

Roof Pitch vs. Slope 

Double gable with dark stone veneer siding, with triangle shape peaks, on a pitched roof attic at an American single family home neighborhood USA, double sash windows w/ matching dark shutters

Due to the complex nature of home construction, roof pitch and slope are used interchangeably, even though they don’t exactly mean the same thing. Contractors express a roof’s pitch as the fraction of the rise against the roof’s span. 

In this case, the span is the measurement of the roof from one end to the other. So, if the roof rises four feet over a span of twenty-four feet, the roof pitch will be expressed as 1/6.

The difference between the roof pitch and the slope is that the former is mostly used in actual home construction while the latter is relevant to home designers’ house plans. Since it’s easier to calculate roof pitch, constructors now use it as an alternative to measuring roof slopes.

 

How to Calculate Roof Pitch (Slope)

Architectural drawing plan of house project, blueprint rolls and yellow tape measure (measuring tape) on working table, Architecture and building construction industry concepts

Whether you’re installing a new roof or replacing your roof covering, calculating the roof’s pitch is critical to the success of your building project. You can determine roof pitch in two ways: measuring the angle the rafter length makes with the horizontal run or the proportion of the rise and run of the roof. 

You’ll need a tape measure, 24-inch level, pencil, and calculator for this task. However, you can use an online roof pitch calculator to keep things simple.

 

Measuring Roof Pitch in Degrees

You can calculate roof pitch in degrees using the Pythagoras theorem formula:

(Rafter length)2 = (rise)2 + (run)2

  • Divide rise by run to get the roof pitch expressed in percentage.
  • Use your calculator to find the inverse tangent (tan -1) of the figure you got in the step above.

Assuming your roof rise is 12 m, and the run is 9 m, let’s calculate the roof pitch.

  • 12/9 = 1.333
  • Tan -1 (1.333) = 53.123°

Hence, the roof pitch is approximately 53 degrees.

 

Measuring Roof Pitch as a Proportion

Traditionally, the pitch of a roof is displayed as X:12, where X is the rise. You can either measure it from the rooftop or the attic.

If you’re measuring from the rooftop, place one end of the level at the 12-inch mark, ensuring it creates an L-shape. When the air bubble in the lever is stable, use the tape measure to get the distance between the bottom of the lever and the rooftop. The figure you get becomes the vertical rise.

Assuming the rise you measured was four inches, the roof pitch will be 4:12.

If you’re not comfortable with measuring the rise on the rooftop, you can measure the rafter length within the attic. To do so, place the level at the 12 inches mark against the base of the rafter board. Ensure it’s perfectly level, then measure vertically, forming an L-shape with the level.

Generally, roofs are categorized as:

  • Flat Roof: Although no roof is completely flat because there should be provision for drainage, the pitch in this category is between ½:12 and 2:12.
  • Low Slope: This style is common in commercial buildings but requires high-quality roofing materials to prevent water damage. The pitch is between 2:12 and 4:12. 
  • Conventional Roof: Here, the roof pitch ranges from 4:12 to 9:12. It’s commonly used when constructing a room area, garage, or shed.
  • Steep Slope: This is the general name for any pitch that’s above 9:12. It’s great for snow-prone regions and harsh climates because it can shed snow and dispel rainfall, preventing damage to your house.

 

Using an Online Roof Pitch Calculator

A roof pitch calculator is a convenient way to calculate a roof’s pitch. Since you’ll be climbing on a ladder, using a pencil and paper would be the last thing on your mind. Besides, some of us aren’t good with calculations.

So, when you measure the rise and run, input the figures into the appropriate boxes in the roof pitch calculator and let the technology do the rest. Alternatively, you can opt for a photo-based roof pitch calculator that works with your smartphone’s camera.

 

Roof Pitch Chart

A roof pitch chart helps you get a better understanding of how inclined your roof will appear when it’s constructed. 

Roof Pitch

Roof Pitch Angle 

¼:12 1.15°
½:12 2.41°
1:12 4.75°
2:12 9.48°
3:12 14.04°
4:12 18.42°
5:12 22.64°
6:12 26.57°
7:12 30.24°
8:12 33.70°
9:12 36.87°
10:12 39.79°
11:12 42.52°
12:12 45°

Minimum Roof Pitch for Common Roof Types

It’s important to note that each type of roofing material has a recommended minimum roof pitch to ensure it functions optimally. We’ve summarized the acceptable range for some roof types.

  • Metal roof: ¼:12 – 3:12
  • Wood shingles: 3:12
  • Asphalt shingles: 4:12 or 2:12 with double underlayment
  • Concrete and clay tiles: 4:12 or 2.5:12 with double underlayment 

 

Conclusion

Unless you’re a constructor and possess the necessary safety equipment, going all the way to measure the slope of a building is best left for the professionals. However, no knowledge is wasted as the information we shared above will ensure you understand what’s on your building plan.

Your roof is a worthy investment, and we can help you get it right from the first shot. Contact us at Viking Contractors today for a free inspection.

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