Do you remember sitting in your high school algebra class, watching your teacher write sums and figures on the board, wondering, “When am I ever going to use this in real life?”
What if we told you that you could use those math skills to calculate the pitch of your roof? Knowing how to calculate your roof pitch can come in handy for a variety of reasons:
- It’s the first step in figuring out how many roofing materials you need to cover your roof, and how much these materials will ultimately cost you.
- It helps you figure out how many rafters you need to hold up your roof.
- You can use the information to determine whether you have the best roof pitch for your climate zone.
- You need this figure if you’re planning on adding vents, windows, or other accessories to your roof.
Here, we’ve laid out everything you need to know to accurately calculate your roof’s pitch for when you need roof repair or replacement.
Tools and Equipment
Start by gathering the equipment you’ll need to complete the task:
- Tape measure
- Level (12+ inches in length)
While most people already have these items lying around the house, a quick trip to the hardware store or your local tool lending library should help you find anything you don’t already have on hand.
If you plan to measure your roof’s pitch from the outside (more on this below), gather these additional tools and equipment:
- Safety harness, rope, and anchors
- Sturdy work boots
- A friend (to stay on the ground and hold your ladder)
Measuring From the Roof’s Surface
When you want to determine roof pitch, it makes sense that you’d physically go to your roof for answers, no?
1. Safely Get on the Surface of Your Roof
Using your ladder and safety harness, make your way to the surface of your roof. Ensure these conditions are in place before doing any climbing:
- The roof is dry and clear of debris
- No inclement weather is on its way
Your helper should stay on the ground and hold the ladder in place until you plant yourself firmly on the roof. Once you’re there, they should stay attentive and available in case you need anything while you’re on the roof.
2. Measure Vertical Rise and Horizontal Run
Does the phrase “rise over run” ring any bells for you? If you’ve ever had to figure out the slope of a line on a graph, you’ve likely used this formula to figure it out. When measuring roof pitch, you use this same formula.
Start with placing the level on the surface of your roof. Keep the edge of the level in contact with the higher point of your roof, and swing it up until the level is, well, level. If you’d like, you can use a combination square or other tool to brace the level.
Use your tape measure to find the point on the level that’s exactly 12 inches away from where it makes contact with the roof. At that spot, measure the distance between the surface of your roof and the level. This number is your “rise,” and the distance along the level (in this case, 12 inches) is your “run.”
3. Work Your Formula
Plug your numbers into the rise-and-run formula to get your slope. For example, if the distance you measured between the edge of your level and the surface of your roof was three inches, your roof pitch would be:
rise/run = 3 inches/12 inches = 3/12
Notice that you don’t reduce the fraction to its simplest form. The pitch of a roof is nearly always expressed as a fraction with a base of 12 because there are 12 inches in one foot.
In some areas of the world, roof pitch is more commonly expressed as a degree angle, rather than a ratio. If this is the case where you live, simply take the ratio you found earlier and figure out its arctangent (a scientific calculator works great for this). So, for the example we described earlier, you would do the following:
arctan(3/12) = 14.04°
Measuring From the Attic
For the less adventurous homeowner (or for anyone whose roof surface is inaccessible to them), you can also easily measure your roof slope from inside your attic.
1. Measure Rise
From inside your attic, find the highest point of your roof; this will be either the peak or the center ridge, depending on your roof style. Measure the distance from that highest point down to the ceiling joist. This is your rise.
2. Measure Run
Standing directly below the peak of your roof, measure from that point out to the edge of the attic floor. This is your run.
To ensure that you’re measuring in the appropriate direction, imagine drawing a line along the two measurements you just took and along the slope of your roof; the resulting shape should be a right triangle.
3. Do the Math
Again, plug your numbers into the formula. Reduce the fraction until it’s expressed as a number over a base of 12. If necessary, convert the ratio to degrees.
Alternative Method: Measuring Roof Slope Using the Rafters
If your roof has exposed rafters, you can also measure along a rafter length to figure out your roof pitch.
Holding the lower edge of your level against the rafter, swing the tool away from the rafter until it’s level. Measure 12 inches along the level starting from where it meets the rafter, then take a measurement for the vertical distance between that point and the rafter above it. Put your measurements into the formula, and you’re good to go!
How to Calculate Roof Pitch: Let Us Help You
As long as you’re comfortable with a little geometry, the process for how to calculate roof pitch is relatively painless and stress-free. Use this information to better collaborate with your contractors, add accessories to your roof, and generally attend to the health of your roof and your home.
If you’d rather leave your high school math skills in the past, though, we totally understand. Rather than going out to purchase a roof pitch calculator, though, let us help you out. In addition to figuring out your roof’s pitch, we can provide you with a roof inspection and help you with any repairs you might need.
Interested in hearing more about what we have to offer? Get in touch with us today!