Using roof pitch you can quickly and accurately calculate a roof’s size and the amount of materials needed for installation.
Most contractors use this measurement as a primary way to calculate roof area.
In this guide:
What is roof pitch?
Roof pitch is represented in inches rise of 12 inches run. For example a “3 pitch” or “3 in 12 pitch” or “3/12 pitch”, all mean that the roof rises 3 inches, for every 12 inches of it’s horizontal run.
Using pitch, its footprint, and basic geometry, you can quickly and with a high degree of accuracy calculate the length of the gable sides. This will allow you to get very precise area measurements.
A roof may have many other attributes, but its pitch is often important in determining the cost of replacement, as well as in choosing the correct materials. It is also a useful measurement to have if you are designing an addition for your home, cutting new rake boards, or installing skylights.
A roof can also be measured in degrees. However, it is often a difficult and an unnecessary way to estimate slope. On the other hand using pitch is simpler, faster, and often – more accurate. Nevertheless, our Roof Pitch Calculator provides instant conversion from pitch to degrees and backwards. You can also use it to instantly estimate the pitch, as well as the area, and rafter length.
However, if you want to go the long, manual route, keep reading to find out how to do this calculation yourself.
How to determine roof pitch – DIY
There are two distinctly different methods – one is more accurate, while the other is faster and easier.
The first involves climbing up on the roof, or at least the edge and actually measuring the pitch using a carpenter’s level and a tape measure.
The second is more of a guesstimating way to get the approximate pitch, while standing on the ground. Not as precise, the second method allows for a quick calculation when a ladder is not available, or when you need to get/provide a quick price quote, or quickly estimate roof size.
Method 1 – how to figure roof pitch using level and measuring tape
For this most accurate calculation, you will need a ladder, long enough to reach the roof edge, a 2 feet level, and a measuring tape. Use caution when climbing up the ladder, and do not go onto a roof without adequate fall protection. We also recommend using a ladder stabilizer to prevent it from sliding to the sides.
Once you reach the top of the ladder, position your level so that one end touches the roof surface, while the other end is in the air, and it is leveled. Now, measure the distance between the roof surface straight down from the level’s opposite edge – refer to the diagram below:
Method 2 – Guesstimate roof pitch off the ground
This process is usually rather easy, fast and pretty accurate. However, if you need to order materials for a job, we still recommend getting a more accurate pitch measurement, such as in the first method.
The basic premise in “guesstimating” is knowing (measuring) the width of the roof on the gable side, as well as measuring the exposure of your siding.
Most clapboard siding as well as vinyl siding has a 4″ exposure, before the next course starts. This means that every 3 runs of clapboard or vinyl siding, give you exactly one foot rise.
You will need to calculate the number of runs on your siding from the base of the roof (horizontal line where the roof starts) to its tip. Most homes will have a 6″ rake board running along the roof line, so this will add about 7-9 inches to your roof rise.
Let’s look at an example in the diagram below. The width of the gable side is 24 feet, plus 3 feet of overhangs (1.5 feet roof overhang on each side). This gives us a total of 27 feet. The rise is 16 full courses of 4″ clapboard siding, which equals to 5′ 4″. Add another 8″ for the rake board, and we get a 6 feet rise.
Now, we will have to do some math to actually calculate the pitch for both methods described above. But before we do that, I will quickly talk about measuring the rise if you have something other than a 4″ clapboard or vinyl siding.
If you have a brick exterior on your house, I would measure the distance of three courses of brick and then do similar calculations, as in the example with clapboard siding. Same with a cedar shingles – measure the exposure of 3 courses for more accurate total measurements of roof rise. If you have vertical siding – well, get a ladder and make actual measurements.
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Using Pitch Formula to get an accurate roof size
In the first method, everything is simple. Let’s assume we measured that the rise of the roof is 13 inches for every 24″ run. Therefore, it is 6.5″ over 12″ – well that is our roof pitch – 6.5 in 12.
The second method is both easier and more complicated at the same time. Because the ultimate goal is to calculate roof area, we won’t really need to calculate the pitch (unless you need it), as now we can calculate the actual roof size.
Now that you have your measurements, we will take a quick journey in time to your high-school geometry class, and refresh your knowledge of the Pythagorean Theorem.
It states that the sum of squares of two shorter sides of a triangle, equals the square of the longer side, or A^2 + B^2 = C^2.
In the second method, our measurements were 27 feet base with 6 foot rise. Because it is essential to have two equal triangles, we need to divide it in two – what we get is a right-angled triangle with 6′ and 13.5′ sides. Using the Pythagorean Theorem, we calculate the hypotenuse or the gable side to be about 14.77 or to make it easier, 14 3/4 feet long. Now, we can calculate the pitch, which will be a lot of fun, since our base is not a 12-based number.
The formula is as follows:
First, we need to bring all dimensions to a 12 base (because the pitch is calculated in 12 base). We take run and rise (13.5 and 6), divide them by 13.5 and multiply by 12. We get a 12″ run with 5.3333′ or 5′ 4″ rise – well – that is our roof pitch – 5’4″ in 12′.
You can also calculate roof size using pitch (without using the Pythagorean Theorem). There is a table of multipliers for each roof pitch, which were derived using the Pythagorean Theorem:
Roof Pitch Multiplier Table:
|3 pitch – 1.03||4 pitch – 1.06||5 pitch – 1.085|
|6 pitch – 1.12||7 pitch – 1.16||8 pitch – 1.21|
|9 pitch – 1.25||10 pitch – 1.31||11 pitch – 1.36|
|12 pitch – 1.42|
We excluded roof multipliers for pitch below 3 and above 12 for the following reasons. With pitch under 3, you can easily walk on the roof to get the exact measurements, as that is usually a flat roof. Also, the multiplier is so close to one, that it does not really change the total size, unless your roof is something like a 100×100′. Roofs with pitch over 12 are not very common, and besides – you can now calculate it yourself.
The above multipliers can be quickly used to calculate the length of a gable side of the roof, when you know the pitch, and the total width of that side.
All you need to do is take 1/2 the width, and multiply it by the corresponding multiplier. The same method can be quickly used the get the total area of a gable roof. For example, your roof footprint is 24 x 36′ with 7 pitch. The area will be 24 x 36 = 1224 multiplied by 1.16 = 1420 sq.ft.
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