Roof Slope Calculator | How to calculate roof pitch?

You can find several automatic roof pitch/slope calculators on the internet. Still, if you understand the basic concepts behind it, you will determine the slope yourself and appreciate that it is pretty straightforward.

Roof Slope Calculator

Roof Slope (X)
Roof Angle (A)
Rafter Length (L)
Pitch (K/S)

What is the slope or pitch?

If you know the rise and run of a roof, it is a cakewalk to find out the roof pitch/slope. If you already know a bit of trigonometry, you will instantly know what I mean.

Don’t worry if you don’t know trigonometry or are afraid of mathematics, as I will explain everything from a layman’s point of view.

In a right-angled triangle (90-degree triangle), there are three sides: base, perpendicular, and hypotenuse.

The rise is nothing but the perpendicular side’s length, and the run stands for the base length.

The slope of the third side is determined by the size of the other two sides. If you increase the rise or decrease the run, the slope/pitch becomes steeper.

Similarly, if you drop the height or increase the run, the slope becomes flatter.

Read more: The average cost installing a new roof.

Different Types of Roof Slopes

Roof pitch
Roof slope

Depending upon the slope of a roof, we can categorize the rooftops into five types. Some are steeper, some are flatter, while others are too steep or too flat.

Flat Roofs

In real life, there are no perfectly flat roofs. Roofs need a slight slope to avoid waterlogging and ensure smooth water runoff. Such shelters have a pitch of less than 2/12, which is about up to 16.7%.

Low Pitch Roofs

Low-pitched roofs have a slope of 3/12 to 4/12. It may be challenging to maintain such roofs and may require specific material to avoid leakages. They can be about 17% to 33% steep.

Traditional Roofs

Conventionally, sloped roofs have a pitch of 4/12 to 9/12. They range from 34% to 75% steep. It is reasonably easy to walk on such ceilings safely, and you can construct them easily.

High Pitch Roofs

High-pitched roofs have a slope higher than 10/12 to 21/12, ranging from 76% to 175%. Since they are so steep, you will require additional fasteners to hold them steady.

Non Existent Roofs

Theoretically speaking, it is possible to have a slope above 21/12 or 176%, but such roofs do not exist in the real world as they serve no purpose. They will look like steep walls with a minimal gap.

Read more : Most common roof styles and shapes.

Calculating Roof Pitch

The roof slope determines its steepness. We can determine the slope as fractions or angles. If you know the roof slope, it will help you decide the suitable installation method, type of roofing material, and approximate budget.

Moreover, the pitch can be a critical factor in determining snow load in cold climates.

For the sake of simplicity, the slope is expressed in inches per foot. Since a foot has 12 inches, you get pitches like 4/12 or 7/12.

For Example, if a roof has a rise of 6 inches and a run of 12 inches, we can say that the roof slope is 6:12, 6/12, 6 inches per foot, or 6 in 12. When we mention the pitch as a ratio, we separate it with a colon (:). To say it as a fraction, we put a slash sign (/).

Depending upon the method of calculating slope, there are four ways to find the roof slope. We can find out the rise and run or the angle to find the pitch.

Measure Slope by Climbing the Roof

In this method, you need to climb the roof to measure the rise for any 12-inch run. We will need two instruments to mimic as a base and perpendicular of an imaginary triangle.

We will fix the triangle base as 12 inches and then check what the rise is. You will require a tape measure and a level instrument of at least 12 inches.

Hold the level and check if it indicates a flat reading. Now measure the height or rise from a 12-inch point to the point that touches the roof below. The value you see on your tape is the rise or perpendicular.

For Example, if the value is 5 inches, then the top has a slope of 5/12 or 5:!2.

Measure Slope from the Attic

In this method, you need not climb the roof. You can go into the attic to find the rise. This time you will measure the height from the opposite side.

Here again, ensure to level the 12-inch level by touching it to a rafter. A 12-inch point measures the rise upwards till it joins the roof by using the tape measure. The only difference is that you are taking the same rise value from below the ceiling.

Measure the Complete Run and Rise

In this method, you will have to make a little effort in measuring the full height and width of the roof. Here, you can imagine a right-angled triangle where the base is half of the total roof width.

Let us take an example. If the attic height is 4 feet 2 inches, it will be 50 inches (4 x 12 inches plus two inches). Similarly, if the total width of 20 feet, we shall divide it by two and convert it to inches.

In this case, it will be a run of 10 feet or 120 inches (10 x 12 inches)

Now, you have got the rise and run as inches. The slope is rise/run = 50/120. To standardize, you need to convert it in terms of 12 inches or 5/12. If you cannot standardize it into 12 inches, the roof slope calculator can do it for you. You can also solve the fraction and multiply by 12.

For Example, if you got a figure of 7/28, it will be 0.25/1 or 0.25×12/12 or 3/12. Similarly, if you got 11/47, it will be 0.23×12/12 or 2.8/12.

Measure Slope using a Speed Square

Using a tool like speed square can expedite the calculation. You can quickly measure the roof pitch by setting the level on the edge of the speed square.

Holding it and checking the point where it meets the rafter edge will get the roof slope’s angle on the speed square in degrees. You can convert degrees into a standard 12-inch fraction.

Steps to convert roof pitch from degrees to standard fraction form

First, you have to convert the angle to slope in decimal and then convert it into a fraction by multiplying it by 12. Here, you will use the tan (degrees) formula and then multiply by 12 to convert it into inches per foot.

For Example, if the roof angle is 35 degrees, then tan (35 degrees) = 0.70. Multiply by 12, and you get 8.4 inches per foot.

Steps to convert roof pitch from inches per foot to angle in degrees

Sometimes, you may need to convert the roof slope in angle form. Here you will use the slope inverse tangent formula. First, make the slope rise and run the figure into a fraction. E.g., the rise of 4 and run of 12 means the slope in a fraction is 4/12. Now divide it to get the decimal value of 0.33. Now the angle is ArcTan (0.33) or 18.42 degrees

Standard Roof Pitch table

You can make a standard roof pitch table by starting from a slope of 1/8 inch per foot up to as high as 24 inches per foot.

The table also shows the gradient in percentage, degrees, and radians. The table can be a quick reference for the slopes nearest to your rooftop figures for quick analysis.

Relation between Roof Pitch and Roofing costs

If you plan to install or replace your roof, the existing or proposed slope will have a definite impact on overall costing. Hence, it is vital to choose a suitable pitch because, in general, steeper roofs cost more.

Low slope roofs may cost 10% more to install, whereas a steep slope can result in 20-30% cost escalation. The roofing costs may increase further if the roof pitch is too steep.

Read more: Building code requirements for roof sheathing replacement.

Interesting Facts about Roof Pitches

  • There is no universal roof pitch. Factors like architecture, material, climate, and cultures affect how flat or steep a roof is.
  • The standard roof slope in the US is 4/12 to 9/12.
  • In the UK, the typical roof pitch is 40-50 degrees.
  • It would be best if you avoided 45-degree slopes for stability during storms.
  • Contemporary houses have near-flat roofs. They have slopes of 1/40.
  • Italian roofs have 16 to 25-degree roof pitches.
  • The minimum roof pitch suitable for snow-shedding is 6/12 or 7/12 (about 30 degrees).
  • A roof is walkable if its slope is less than 6/12 or 26.5 degrees. You can walk 7/12 roof pitch with caution. You can walk even 8/12 to 10/12, but it is not advisable and it calls for extreme caution. Roofs beyond this range are not walkable.

Some people apply the Pythagoras theorem, which makes it unnecessarily complex to understand. All you need is the slope, and the above four methods are sufficient to find it.

Please keep it simple and understand the logic behind it.

That is all about the roof pitch calculation. Remember, the basic idea is to standardize it into inches per foot.

The angle determines the pitch based on the rise and run sizes.

No matter what method you choose, measure the rise and run to get the slope, directly or indirectly.