How to Calculate Roof Slope Safely

Last Edited: 4/15/24

A slopping roof top.
Are you looking to calculate your roof slope? If you are, there are numerous ways you can do it, and in this guide, we’ll explore two of them.

Calculating roof slope is done by calculating the number of inches your roof slope rises vertically for every 12 inches it extends horizontally. For example, if your roof rises 7 inches for every 12 inches of horizontal run, then your roof has a 7-in-12 pitch. The standard pitch of most residential roofs ranges between 4-in-12 and 9-in-12.

Why Should You Calculate Your Roof Pitch?

It’s good to know the measurements of your roof slope, particularly if you’re considering having any maintenance work done to your roof or want any new features such as skylights or rake boards installed. It’s best not to make any mistakes when calculating your roof slope since doing so could lead to headaches down the road.

Calculating Your Roof Slope With Roof Trusses

Luckily you don’t have to get onto your roof to determine its slope! If you have access to an attic, then you’ll have access to the underside of your roof and a roof rafter. You’ll need two tools to be able to determine the slope of your roof: a spirit level and a tape measure.

Once you have these tools, follow these four steps:

  1. Once in the attic, take your spirit level and have one end rest against your roof where the roof begins to rise. You may have a support beam that you can rest one end of the level on; if you do, then you’ll just need to hold the other end of the spirit level.
  1. Once the level is resting, double-check that the bubble in the spirit level is level. Even if the spirit level is slightly off, it can mess up the numbers drastically.
  1. Take your tape measure and measure from the end of the spirit level that is resting on the support beam to where the bubble of the spirit level lies. This will give you your horizontal measurement, and will most likely be 12 inches.
  1. Once you have your horizontal measurement, you can take the vertical measurement. Rest your tape measure level with the bubble in the spirit level and measure vertically until you hit the roof. The vertical measurement will likely be between 4 to 9 inches, and once you know this, you’ll have calculated your roof slope!

How to Measure Your Roof Slope From Your Roof

Now, don’t panic if you don’t have access to your attic. Another way to measure your roof slope is to take the measurement directly from your roof.

Here’s how to do it:

  1. Place one end of the level against the roof and measure 12 inches on a carpenter’s level and make a mark with a pencil.
  1. Once you’ve made the mark, pivot the spirit level up or down until the bubble is level with the 12-inch mark.
  1. Hold the 0 mark of your measuring tape against the level and extend it upward, so it’s perpendicular to the level, and move the measuring tape along the roof until it hits the 12-inch mark. Take note of the vertical distance and write it down.
  1.  You’ll now have the two measurements to work out your roof slope. Following the rule we defined earlier, if the vertical measurement of your roof slope is 6 inches, then the ratio will be 6-in-12.

Expert Help When You Need It

Calculating your roof slope can sometimes be quite tricky. Any projects that go wrong on the roof can have major consequences such as a serious injury or even a leak in your roof. If you’d rather have an expert take care of residential professional services or any projects on the roof, request an appointment online from Mr. Handyman or give us a call for a service you can trust.