Roof Pitch Calculator [Calculates Roof Slope, Angle, Rafter Length]

A Roof Pitch Calculator is a helpful tool that determines the pitch, roof angle, and rafter length. With the input of a few simple numbers, the calculator identifies the necessary information, including the percentage and degree of the roof pitch, and makes it easy to recalculate for desired values.

In this guide, we break down the steps for using a roof pitch calculator, explain the terminology, and identify the standard and best pitch for different materials and conditions. We’ll also discuss how to measure the pitch using a speed square or a level and tape measure. Our goal is to provide you with a calculator and useful information to assist you in your roofing project.

Roof Pitch Calculator

This roof pitch calculator is simple to use and provides accurate, usable values. It doesn’t require you to perform mathematical calculations or to second guess the accuracy of your computations. Use it to determine the pitch in percentage and degree, the roof angle, and the length of the rafter (without overhang) for your roofing project.

How To Use Pitch Calculator

To use the roof pitch calculator, identify and input the rise and run, and press ‘calculate’. The rise and run determine the pitch of the roof. The higher the rise, the greater the roof pitch, and the longer the rafter. If you know the roof pitch, which is the rise of the roof compared to its span, it can be used to determine the rise and run. The pitch may be expressed as a fraction or a ratio, such as x/12, or as a percentage or degree.

If you don’t know the rise and run but you know the pitch degree or percentage, input that value in the correct field of the calculator to determine the pitch ratio or fraction. Determine how many inches the roof rises per foot using the pitch and insert the values in the rise and run fields and calculate the rafter length (without overhang). The calculator contains the algorithms and accurately performs the computations, so you don’t have to do the math.

What Is Roof Pitch?

The pitch of the roof is a ratio or fraction comparing the rise of the roof to its span. The rise is measured from the top of the horizontal line running between supporting walls vertically at a right angle up to the roof’s peak or ridge board top. The span is the distance from the outside face of one supporting wall to the outside face of the other supporting wall.

How To Calculate Roof Pitch?

The pitch is not the slope. It is a ratio comparing the rise of the roof to the span. To calculate the rise, measure from a horizontal line running from supporting top plate to supporting top plate, vertically at a right angle up to the peak or top of the ridge board. The span of the supported rafters or trusses is the distance from outside to outside of the supporting top plates.

The comparison or ratio of the roof rise to its span is the roof pitch. A roof that has a rise of 5 feet and spans 20-feet, has a pitch of 5:20 or 5/20. Remember what your old math teacher used to say about fractions? Always reduce to lowest terms, so 5/20 becomes 1/4 or 1:4 or 25% since 5 goes into itself once and into 20 four times.

To calculate the rise and the run from the rise and span, multiply the pitch ratio or fraction by a value that makes the denominator or bottom value 12 – remember, what you multiply the bottom by, you must multiply the top by too.

For example:

• A pitch of 5:20 or 5/20 becomes 1:4 or 1/4 or 25%
• To find the rise and run, we need to make it out of 12”
• So, 12” ÷ 4 (the denominator) = 3”
• Now, multiply the two values in the ratio or fraction by 3” to get the rise and run, or the pitch per foot
• 3×1 = 3 and 3×4 = 12, resulting in a pitch per foot or rise to run of 3:12 or 3/12, or a rise of 3” for every run of 12”

Roof Pitch Chart

There are many styles of roof pitch charts available online or provided by roofing companies. Charts may be numerical or conversion charts, graphic or pictorial, or a combination. The information presented assists in determining and visualizing the roofline, pitch, slope, rafter length, and other useful information.

Conversion charts are tables of numbers identifying the pitch by ratio, degree or percentage, the rise to run, rafter length per foot of run, valley or hip, or other information. Pictorials come in different formats but provide a visual representation of the pitch or slope. Charts that are combinations, provide a graphic pitch or slope representation plus information about the pitch, degree, rafter length per foot, and other data.

The table below identifies the standard roofing information necessary for building a roof.

Roof Pitch Multiplier

The roof pitch multiplier or roof pitch factor is used to determine the actual roof surface from the area the roof covers. To find the multiplier use the rise over run (x/12”) and calculate the square root of 1 plus the square of the rise divided by the run. The formula looks like this: √[1+(rise÷run)²]

For example:

To calculate the multiplier using a rise to run of 4/12 or pitch of 4:12, begin with the formula

√[1+(rise÷run)²] = √[1+(4÷12)²] = √[1+(0.33333)²] =√[1+ 0.11111] =1.054

The roof pitch multiplier or factor for a 4/12 slope is 1.054.

The table below identifies the multiplier for common pitches.

To use the roof pitch multiplier or factor, determine the area a roof will cover, including overhangs, and multiply the area by the multiplier for the surface area of the sloped roof.

For example:

A 20’x30’ structure with an 18” overhang on all sides has an area of 23’x33’, or 759 square feet. If the rise over run is 5/12, the multiplier would be 1.083. Multiply the roof footprint area by the pitch multiplier, so 759ft² x 1.083 = 821.997ft². Therefore, the roof surface area would be 822ft²; a helpful value when calculating materials and costs.

What Is the Standard Roof Pitch?

Standard or medium roof pitches range from 4/12 to 9/12. Pitches greater than 9/12 are classed as steep-slopes, while those less than 4/12 are classed as low-slope roofs. A roof with a pitch less than 2/12 is considered a flat roof.

What Is the Best Pitch for a Roof?

The best pitch for a roof depends on the style of the roof, roofing material, and climate concerns. There are at least 36 styles or types of roofs used in residential construction in North America. The most common are gable, hipped, mansard, gambrel (barn), saltbox, shed or mono-pitch, flat, butterfly, and A-frame. The style affects the pitch, and the climate influences the roofing material.

Climates that result in heavy snows, rains, or winds, or all three, often dictate a medium or high roof pitch. A low pitch won’t shed moisture or snow quickly and can be damaged by the extra weight or pooling, however, the lower profile is beneficial in windy locations. High pitches also offer the potential for extra living space, plus are helpful in hot climates and in shedding precipitation. Generally, steeper roofs outperform and outlast low-slope roofs as they shed moisture and debris more quickly.

The best roof pitches are 3/12 and 4/12 as they shed moisture and debris well and offer a lower wind profile. The pitch is easy to walk and work on and can be inspected and repaired by the homeowner. They can also be finished in the widest variety of roofing materials.

Roof Pitch Angles

The roof pitch can be used to determine the angle the roof makes with the horizontal line between the tops of the supporting walls. It is the angle the roof deck slopes and can be helpful when cutting rafters and selecting roofing materials. To determine the roof pitch angle, it is easier to use a roof pitch calculator, Roof Pitch Chart, or Roof Pitch Multiplier like those above. However, if you want to do the math, here are three formulas and an example:

What Is the Minimum Roof Pitch for Snow?

The minimum roof pitch for snow often depends on location, local building codes, and design factors. Some snow areas require slopes of 30° or greater, while others accept lesser slopes. The greater the slope and smoother the surface, the faster a roof will shed snow, so the slope and roofing material are also considerations.

The minimum roof pitch for snow is typically 2:12 for residential structures, although many industrial and commercial buildings have lower pitches. A properly designed 2:12 roof will support high snow loads and the slope is enough to shed snow through gravitational creep.

Wind, temperature, and the volume of precipitation affect roof design. There are residential roofs in snow belt areas with 1/4:12 to 1:12 that survive snow loads. However, the lower the slope, the greater the risk of weight, ice-dams, leakage, and even condensation within the roof structure. A 2:12 pitch is commonly considered the minimum pitch for snow.

What’s the Minimum Pitch for a Metal Roof?

The minimum pitch for a metal roof depends upon the style or type of metal roofing. Typically, metal roofing panels that aren’t soldered or lap-sealed are limited to a minimum pitch of 3:12. Interlocking metal shingles and stone-coated interlocking metal shingles also require a minimum 3:12 pitch.

Soldered metal roofing panels or those with sealed laps are rated for slopes as low as 1/2:12, or 2.39°. Metal panels with standing seams can handle slopes as low as 1/4:12, or 1.19°. The climate can affect low-slope roofs and often affects roof design and structural requirements.

Minimum Roof Pitch for Shingles

The minimum roof pitch for asphalt shingles is regulated by the International Residential Building Code (IRC) which states the pitch must be 2:12 or greater and require a special underlayment. Climate and structural requirements also influence the pitch requirements based on the product used. Different manufacturers sell shingles rated for low, medium, and high slope roofs, and warranty products based on the slope.

Most asphalt shingles, fiber cement shingles, synthetic (composite, plastic) shingles, and wood shingles require a minimum of 4:12 pitch. There are always exceptions to the rule and are usually noted by the manufacturer. Slate, concrete, and clay tiles or shingles also commonly require a minimum of a 4:12 pitch.

How to Measure Roof Pitch

Measuring the pitch of a roof is helpful when determining the area of an existing roof deck for resurfacing. It is also useful information when planning renovations and additions and is required for home insurance in some regions. If you need to go up a ladder or on the roof, always follow safety protocols. Here are some different ways to measure the roof pitch or slope.

With Speed Square

A speed square is a useful tool and common in many builder’s tool collections. Some have built-in bubble levels which make measuring roof pitch easier, others need to be paired with a torpedo level or other short level. The easiest location to measure roof pitch is at the gable or rake fascia, also known as the eave overhang.

• You’ll need a speed square, small level, and probably a ladder.
• Place the broad base of the square upwards and rest the edge on the shingles or fascia at the 90° angle so the face is against the fascia board.
• Rest the torpedo or small level on the flat base – I cheat and tape or zip-tie it in place, so I don’t have to hold it.
• Pivot the square at the 90° angle where the base rests on the shingles until the bubble reads level.
• Note the pitch in degrees on the diagonal face of the square and the corresponding vertical rise in inches at the edge of the fascia or shingle. If the angle measure is about 18.5° the corresponding rise value will be 4”, giving a pitch of 4:12 or 4/12.

From Ground

Gauging the roof pitch from the ground provides an alternative to climbing a ladder and is useful when dealing with two or three-story homes. It should be noted there is a margin of error when eyeballing roof pitch.

• You’ll need a tape measure, calculator, notepad, and writing tool, plus you’ll need to know your eye height from the ground – so don’t bend your head back when looking at the roof deck.
• Fasten the end of the tape measure, so it is secure at the wall.
• Walk away from the building unspooling the tape until you can look up the roof and it looks like a flat plain. I tend to do a dry run without the tape to make sure there are no obstacles.
• Note the distance on the tape you are from the wall and subtract the depth of the overhang for the ‘run’ value.
• The distance from the ground to the shingles is the next measurement needed. On a bungalow that should be fairly easy, for a taller structure, it may involve some guesstimating.
• Subtract your eye height from the shingle to ground height for the ‘rise’ value. Remember, the ground commonly slopes away from the building, so you may need to compensate for the drop.
• Use the rise and run to calculate the pitch over 12. For example, if the rise is 42” and the run 96”, the pitch is 42:96. Divide the run value by 12”, which is 8. Now divide the rise by 8 to get the rise per foot. In this example, it would be 5.25:12. Adjusting for errors means the pitch is probably 5:12.

With a Level

Measuring the roof pitch with a level can be done from a ladder, on the roof, or inside the attic using a similar process.

• You’ll need a level and tape measure. Mark your level 12” from the end unless it has a scale.
• Place the edge of one end against the roof deck, shingles, rafter, or gable fascia.
• Pivot it at that end until it is level.
• Measure the distance from the roof structure surface used, to the pivot edge of the level at the 12” mark for the rise per foot.

With a Pitch & Angle Finder

Place the base of the finder against the underside of the rafter, roof deck, gable fascia, or on a short piece of board on the shingles for a smooth surface and read the pitch in inches or degree.