Roofing measurements are tricky, especially when it comes to calculating the roof's incline. You've got rise. You've got run, slope, span, and pitch! What’s the difference between all these complicated architectural terms? And for the average homeowner gathering roof estimates, do the differences even matter? Keep reading to find out...
Table of Contents
- Basic Incline Measurements
- Advanced Measurements
- Common Examples
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You can do all the digging you’d like for the scientific answers to these contractor debates, but at the end of the day the exact similarities and differences between all the measurements doesn't matter too much for the average homeowner.
Whether slope is different than roof pitch or the same doesn't help you determine how many roof square is on the home so you can order the correct amount of roofing materials. And if you're a home owner, it doesn't help you make sense of all the calculations that are jotted down on your roof estimate.
Some of you may want to know the exact scientific differences, however. So if that's you, keep reading below. But if you want to know the basic similarities and differences between slope and pitch, you can just click here.
Basic Incline Measurements
Let’s bring this thing back to 5th grade math class, shall we?
Completely flat surfaces have slopes of 0, while completely vertical surfaces have what's called an undefined slope. Everything in between has an inline, and thus, a slope and pitch. That's what we're here to calculate and understand, of course...
But before we're able to figure out the pitch of your roof, we need to get a handle on the definitions of rise, run, and span. These three measurements are the basic vertical and horizontal calculations needed to calculate both slope and pitch.
For more of an in-depth, mathematical overview of slope you can check out this resource from Columbia University.
What is rise?
Rise is the vertical change between two points. It's how much something goes up between point A and point B. And in the roofing industry, rise is a measurement you'll use regardless of whether you're calculating pitch or calculating slope. Rise is the first number in both pitch fraction and slope ratio, which are used to display any given roof's pitch or slope.
What is run?
Mathematically, run is the horizontal change between two points. It's how much distance changes between point A and point B. In roofing, run is the distance from the roof's ridge (or peak) and the edge of the roof right above your gutter line.
On a perfect gable roof, your roof's run will be exactly half its span (explained below). This is the perfect example, but unfortunately we don't live in a perfect world for measurements any longer...
The differences between span and run are important because they determine whether you're calculating the pitch or slope of your roof. In exact terms, run is used to calculate slope while span is used as the horizontal measurement in pitch calculations.
What is span?
Span is the end-to-end measurement of a roof.
It’s generally displayed in feet and can easily be calculated using a 100 foot tape measure. Span used to be a more important roof measurement than it is today because it gave you an overall measurement of the roof's length in feet. But as homes get more complex, roof styles do too...
For example, measuring a mansard roof isn't quite as simple as measuring a gable roof.
Modern roofs aren't straight up and down any more and many times they're seperated into different sections, with dormers and house additions thrown in there to make everything extremely confusing. It's become much more difficult to calculate your roof's incline using just one measurement for length, which is what used to be a big benefit of span.
Now that we understand rise, run, and span, we can dig into slope and pitch. After that, it'll be possible to determine whether or not the differences between the two even matter (and if so, which we should be using).
What is slope?
The exact slope of a roof is calculated by the difference between rise and run and expressed as a ratio.
For example: if the roof has 4 feet of rise for every 12 feet of run, the slope would be expressed as 4:12. This is the slope ratio.
What is pitch?
Mathematically, the pitch of a roof is different calculation than roof slope. Pitch compares the roof’s span to the rise while slope compares rise and run. To calculate pitch, you simply divide rise by span and display your result as a fraction. You'll often hear experts express pitch as X over Y.
A pitch factor (also known as a pitch multiplier) helps roofers calculate for waste, which is the extra materials they'll need because of the given roof pitch. For example, if a roof's slope is 8 over 12, then it's much steeper than 4 over 12. To help calculate the area for this part of the roof, an experienced roofer would first calculate pitch, then multiply the pitch by it's corresponding pitch multiplier. Doing so will account for extra material that's needed because of the steep pitch- and thus- the larger roof area!
Pitch Multiplier Chart
If you already know the pitch for a specific section of your roof, you can use the pitch multiplier chart below to help calculate waste so you (or your contractor) orders the right amount of materials for installation.
Below is a comparison of the technical differences between pitch and slope.
|Divide rise by span||Divide rise by run|
|Displayed as a fraction||Displayed as a ratio|
|More common roofing measurement||Less common roofing measurement|
Using Slope & Pitch Interchangably
Because home construction is now more complex, remodeling as a whole is too! And contractors have been forced to adapt. In doing so, most (if not all) contractors are now using slope and pitch interchangeably, especially when writing roofing estimates.
When reading about pitch and slope, the most common explanation you'll find is actually a hybrid of the two. You'll see a measurement referred to as pitch (and expressed as a fraction) but the real measurement is slope (rise over run). Historically, there's a few reasons for this...
A Hybrid Definition?
First, pitch is simply more accepted terminology.
Second, slope is easier to calculate for non-gable roofs.
A hybrid pitch-slope combo; a measurement that's technically slope but displayed as and called pitch.
You don't need to be! You've got our permission (and the permission of remodeling author Russell Burgess) to consider pitch and slope the same exact thing!
Though this simplification may make architects pull their hair out, slope and pitch are widely used as synonyms by both roofing manufacturers and roofing contractors alike. And if the differences don’t matter when we order your roofing materials, then there’s no use wasting time trying to be as specific as possible.
Now that we've got that out of the way, we can get to the important stuff. Roof angles! That's what roof pitch and slope are both out to determine anyway, isn't it?
Roof Angle Chart
Whether you call or slope or pitch, calculating the steepness of your roof is all about figuring out what angle it is.
Once you know your pitch, you can use the chart below to get a better visualization of your roof's incline:
Flat Roof: 0/12
Flat roofs don't have a pitch, so if you needed to express the pitch of a flat roof you could simply use 0 or 0/12. Alternatively, you could use 0* or "zero degrees."
Low Slope: 1/12 and 2/12
Low slope roofs are often confused as flat roofs, but this isn't the case. They do have a slope, it's just not very pronounced. And because the slope is so gradual, you need specific low slope roofing materials. 1 over 12 is the lowest of the low slopes.
You'll often find a 1/12 pitch on back patio porches or at the top of an older gambrel roof. 2 over 12 is a little steeper than a 1 over 12 pitch, but not by much. In degrees, low slopes are between 4.76 degrees and 9.46 degrees.
Standard Pitches: 3/12 through 9/12
Standard pitch roofs are the common roofing pitches you'll find on the main living areas of residential homes. Unlike low slope roofs, areas that have a standard slope can use regular roofing shingles without worry. Because there's enough of a slope, you don't have to worry about water drainage as much as you would with something like a 2 over 12 low slope roof.
In degrees, standard roof pitches are between 14.04 degrees and 36.37 degrees.
Minimum pitch for shingles
3 over 12 is the minimum slope a roof needs in order to qualify for roofing shingles.
Some roofers still opt for low slope materials with a 3 over 12 pitch, but this is not entirely necessary and just adds to the cost of your project.
Steepest standard pitch
A 9/12 roof pitch (36.37 degrees). is the steepest standard slope.
Anything above a 9 over 12 is considered steep slope.
Steep Slope: 10/12 and above
Any pitch that's at least 10/12 (39.81 degrees) is considered steep slope. This includes 10 over 12, 11 over 12, 12 over 12, and pitch where the rise is greater than the run.
An important note for roofing contractors: when replacing roofs with steeper pitches, it's important to consider the safety of your crew. All roofers should be harnessed and strapped in!
If you're interested in seeing some real life examples of various roof pitches, we've created diagrams of some of our recent roofing installations for you.
To browse these examples, choose a roof pitch below that you'd like to check out: