How to Calculate Roof Pitch

Detail of house roof structure covered with brown metal tile sheets.

Our Simple Guide to Calculating Roof Pitch

When it comes to roofs, the term “pitch” has two different meanings. For the large majority, when they are talking about pitch, they are referring to how steep a roof is. Roofing contractors and carpenters tend to refer to this as the slope of the roof and consider pitch as an actual measurement to describe the overall dimensions based on the rise and run of your roof. These calculations are invaluable when considering roof repairs and replacements.

You can easily find a roof slope calculator online – but there is quite a benefit to ensuring your numbers are accurate. Before you begin, it is a good idea to measure 12 inches on your carpenter’s level and mark it clearly. Many levels already have a ruler, but this makes it easier to see the mark. You can also lock your tape measure in an extended position before you start.

Measure the Roof Slope from the Rafters

The easiest way to measure the slope of your roof would be to access the truss system or measure the rafter length in your attic. This also tends to be the more accurate method, as uneven roof tiles aren’t a problem. The overhang of a rafter at the end of your roof or the barge rafter at the side of the gable will also work.

  1. Place one end of your level against the underside of the rafter, and adjust the other end of the level until the bubble is centered between the two lines, indicating that it is “level.” Locate the 12-inch mark you made on the level earlier.
  2. Now measure the vertical distance from the level to the rafter by placing the tape measure against the level at a 90-degree angle (straight up toward the rafter). Slide the tape measure along the level until it reaches the 12-inch mark, then extend the tape measure up until it touches the underside of the rafter. Confirm that the level is still level before writing the vertical measurement down.
  3. You divide the rise (vertical measurement from tape measure) by the run (horizontal measurement from level) to get your slope. So, if the amount on your tape measure was 6 inches, your roof’s slope would be 6/12 (also expressed as 6:12 or “6 in 12”).

Measuring Roof Slope from the Outside

How to calculate roof pitch from the outside starts by confirming that your roof is safe to walk on. You’ll need a ladder to get safely up to the roof, as well as a carpenter’s level, tape measure, paper, and a thick marker. 

It is a good idea to place all these in a bucket so your hands stay free and nothing can roll or slide away. Do not walk on the roof if it is wet or icy, clearly damaged, or very steep. It is also a bad idea to walk on slate roofs. Now you can begin.

  1. Climb the ladder and place one end of your level against the roof, then pivot it up or down until the bubble is directly between the two lines – indicating that it is “level.” You may need to lay down a flat board if your roof is very uneven.
  2. Now measure the vertical distance from the end of the level that is NOT against the roof, down to the roof. Slide the tape measure along the roof until it reaches the 12-inch mark you made on the level. Note the vertical measurement on the tape measure and write this down.
  3. Use these two values to calculate the roof slope. Roof slope is indicated as “rise over run,” or the vertical height on the tape measure over the horizontal length measured by the level. So, if your vertical measurement was 9 inches, the slope of your roof would be 9/12 (also expressed as 9:12 or “9 in 12”)

Measuring the Roof’s Pitch Angle Based on Slope

You will need to start by finding the slope of your roof using one of the two methods described above. Technically, the pitch of your roof is the ratio of the total vertical roof rise to the total horizontal run from wall to wall (span). You generally won’t need to measure the pitch of your roof, but it is a good idea to understand what it is. 

This method is also how you calculate roof pitch in degrees. If the total vertical rise of your roof (from the top of your walls to the ridge or apex of the roof) is 8 feet, and the entire span (width or length across the ridge) is 32 feet, then your pitch would be “8 to 32.” 

This can be simplified to “1 to 4” or “1/4.” You may need to do this section by section for more complex roofs, and it is highly advisable to use one of the many free online roof pitch calculators available today. 

Detail of house roof structure covered with brown metal tile sheets.

You Need a Professional for an Accurate Readout. Call JET Now!

Trying to calculate the slope of your roof by yourself can be a headache. There are a few different ways to measure the slope of your roof, but the most common is inches per foot. This is the number of inches that your roof rises for every foot that it extends. Another way to measure the slope of your roof is with degrees. 

The angle in degrees of your roof will determine how steep or flat it is. A flat roof has a very low angle, while a steep roof has a high angle. If you’re not sure how to measure the slope of your roof, you can always ask the professionals at JET Contracting for help. We have over 20 years of combined experience in the industry and can help you get the job done right. From gutters to siding and more – contact our team to learn about how we can help you.

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