Roofing reports

How to Measure the Roof of a Round Building

How to Measure a Quonset Roof

  1. Measure the horizontal width of the Quonset roof to determine the diameter. …
  2. Multiply the diameter dimension time 3.14, referred to as “pi,” and then divide the result by 2 to determine the circumference measurement of the Quonset roof.

Turret Roof Difficulties - Absolute Roof Solutions - Vancouver - YouTube

How to Measure the Roof of a Round Building

1 . Measure the outer diameter of the building.

2 . Measure the amount of horizontal overhang between the building’s outer wall and the roof’s outer edge.

3 . Multiply the amount of overhang by two to include the opposite side of the building.

4 . Add the total overhang to the diameter. For example, a building diameter of 20 feet and an overhang of 1 foot on one side results in a total roof diameter of 22 feet.

5 . Measure the length of the rafters from the peak to the lower edge.

6. Calculate the total surface area of the roof using the following formula: pi times the radius times the rafter length, also called the slant height. To find the surface area using the diameter in the previous example and a rafter length of 20 feet, use the following calculations and use 3.14 as an approximation for pi:

Radius = 22 / 2 = 11.

Surface area in square feet = 3.14 * 11 * 20 = 690.8.

7 .  Determine the vertical height of the roof by finding the square root of the result of subtracting the squares of the rafter length and the radius. Using the previous results, the answer would look like this:

Rafter length squared = 20 * 20 = 400.

Radius squared = 11 * 11 = 121.

So 400 – 121 = 279.

The square root of 279 is approximately 16.7. In this example, the vertical rise of the roof is 16.7 feet from the outer edge to the peak of the roof.

Rafter Framing for a Conical Roof | JLC Online

Measuring Roof Slope and Pitch


Slope is the incline of the roof expressed as a ratio of the vertical rise to the horizontal run, where the run is some portion of the span. This ratio is always expressed as inches per foot.

Slope Ratio

A roof that rises 4 inches for every 1 foot or 12 inches of run is said to have a “4 in 12” slope. If the rise is 6 inches for every 12 inches of run, then the roof slope is “6 in 12.”

The slope can be expressed numerically as a ratio. The slope ratio represents a certain amount of vertical rise for every 12 inches of horizontal run. For example, a “4 in 12” slope can be expressed as the ratio of 4:12. A “6 in 12” slope is expressed as 6:12.

The DIYer's Guide to Measuring for Replacement Windows - Bob Vila


Pitch is the incline of the roof expressed as a fraction derived by dividing the rise by the span, where the roof span is the distance between the outside of one wall’s top plate to another.

Pitch Fraction

Historically the word “pitch” meant a ratio between the ridge height to the entire span/width of the building or ratio between the rafter length to the building width. And back then, the ridge was typically in the middle of the span. This is no longer the case in modern building practices. The ridge can be placed anywhere in the span, from directly middle to either span endpoint.
A roof that rises 8 feet over a 24-foot span was said to have a “1 to 3” pitch. If the rise is 4 feet over a 24-foot span, then the roof pitch was said to be “1 to 6.” 
The pitch can be expressed numerically as a fraction. The pitch fraction represents a certain amount of vertical rise over the entire span. For example, given a roof with a rise of 4 feet and a span of 24 feet, the pitch is “1 to 6” pitch, which can be expressed as the fraction of 1/6. A “12 to 24” pitch is expressed as 1/2.
Satellite roof measurements

How Do You Calculate Shingles For a Roof

How Many Roofing Shingles Do I Need?

  1. Measure the length and width of each plane on the roof (including dormers) then multiply length times width.
  2. Add the square footage of each of the planes together.

The average bundle of shingles covers 33.3 ft2, so three bundles of shingles are needed per square. Asphalt shingles range in price from about $25 per bundle for a standard 3-tab style to around $50-$75 or more for an upgraded architectural style.

How many square feet does a bundle of shingles cover?

33.3 sq. ft.
Calculating the number of bundles, you need is simple if you are using shingles that come three bundles to a square. Each bundle covers 33.3 sq. ft. of roof area—which is close enough to the 32 sq.

How many shingles are in a square?

When shingles come three bundles to the square, there are 29 standard-sized shingles (12 in. by 36 in.) in each bundle. Figuring out the roof area is the first step to determine how many bundles you’ll need to order.

How Shingle Quantities are Measured

When you have a bundle or square count for the main roof area, you will then add additional shingles to account for waste, starter shingles, and extra shingles for hip and ridge caps.


1. Measurement method

The most accurate way to calculate how many bundles of shingles you’ll need is to get up on the roof and measure each roof plane. If all the roof planes are rectangles, you simply need to multiply the length times the width of each plane to get the square footage; then you will add up the square footage of each plane. Many times, the roof may be too steep to walk on without safety equipment, so you will have to do the estimate from the ground. If this is the case, measure the length of the building at the ground level and estimate any rake-edge overhangs. Next, from a ladder, use a stiff, wide blade measuring tape to measure from the edge of the eaves to the ridge.


2. Sheet-count method

If the sheathing is still exposed you can use the sheet-count method, which is sometimes preferred more than the measurement method. This method is fast, and you can usually complete it from the ground. The caveat is you can use this method only on roofs sheathed with 4x8ft. structural panels. Each of these panels is 32 sq. ft., and you can easily count the full panels from the ground. Another way to tally them up is by estimating the relative size of ripped and crosscut sheets along the edges of the roof to the size of a full sheet. Diagonally cut sheets along hips and valleys are a bit more of a challenge to size, but typically you can assign them a relative size, like half or quarter sheet, and it will be close enough

Calculating the number of bundles, you need is simple if you are using shingles that come three bundles to a square. Each bundle covers 33.3 sq. ft. of roof area—which is close enough to the 32 sq. ft. a sheet covers, which means you can order one bundle for each sheet of roof sheathing.

If you are working with other bundle counts per square, simply divide the number of sheets of sheathing by three and you’ll have the total number of squares needed to cover the roof – this is because three sheets of sheathing equal roughly 100 sq. ft. (one square).


3. Shingle-count method

This method makes it easy to measure when the old shingles haven’t been stripped off yet or if you’ll be doing a layover (meaning shingling over existing shingles).

To start, measure the length of the eaves of each roof plane, either directly from on top of the roof or from the ground by measuring the length of the house and adding in the width of the rake overhangs, if any. Alternately, if the existing shingles are standard three-tab, determine the eaves’ length by counting the number of tabs along the ridges and eaves to determine the length in feet (note that one tab is equal to 1 ft.).

Count the existing courses of shingles from eaves to the ridge to get the length of the rakes. The exposure on each course of shingles is five inches, so multiply the number of courses by five inches and then divide by 12 to get the length of the rakes. Make sure you check that the existing shingles are not metric size and are the standard 12-in. by 36-in. shingles. To get the area in square feet just multiply the length of the eaves by the length of the rake and get the details.


Climbing around on a roof is not safe, so we do not recommend measuring the planes of your roof yourself. Plus, it is important that the measurement is accurate if it will be used to purchase roofing materials. So instead of going to the trouble yourself, contact a reliable roofer for an estimate. The roofer can use professional tools and methods to measure your roof and calculate the total square footage, and then provide estimates for shingles, underlayment, and other materials based on that total.

Aerial measurement

Calculate Roof Area

Installing a new roof is a capital intensive affair that requires you to establish your budget in advance. Thankfully, it is easy to estimate the costs by calculating the roof area and then using the sum to determine the cost of shingles per square foot.

How to calculate the roof area

  1. Begin with calculating the area of your house in a plane parallel to the ground. If your house is rectangular, all you need to do is multiply the length and width of the building. If the shape of your house is more complex, simply enter the total area (after measuring the exterior dimensions) into the appropriate box.
  2. Determine the roof pitch of your house. Roof pitch is the slope created by the rafters. If you don’t know how to calculate it, head to our roof pitch calculator. You can input it either in degrees, as a percentage or as a ratio of x:12.
  3. Once you know these values, you will be able to calculate the roof area. Begin by converting the roof pitch to an angle expressed in degrees, using the following formulas:

pitch(%) = x / 12 * 100%

pitch(deg) = arctan [pitch(%)]

  1. Then, use the following equation to find the total area of the roof:

roof area = base area / cos[pitch(deg)]

Satellite roof measurements

How to Estimate Roofing Materials

Although roofing projects are estimated and sold by the square, roofing material is often not sold in 1 square increments.

How Many Bundles of Shingles Will You Need

The average bundle of shingles covers 33.3 ft2, so three bundles of shingles are needed per square. Asphalt shingles range in price from about $25 per bundle for a standard 3-tab style to around $50-$75 or more for an upgraded architectural style.

Since transportation is a major cost factor, expect prices to vary by location. A typical bundle of shingles weighs about 75 pounds, while some premium products can weigh over 100 pounds per bundle, so consider delivery of materials.

How Many Rolls of Felt Will You Need

Roofing felt is sold by the roll. The average roll of 15-pound roofing felt covers about 400 ft2, or 4 squares, while the average roll of 30-pound roofing felt covers about 200 ft2, or 2 squares.

Rolls of felt are 36″ wide x 144′ long for 15# and 72′ long for 30#. Consider other underlayment options as needed, such as rubber or tar products.

How Many Nails Will You Need

Nails will also be required for any roofing project. The average 3-tab shingle needs four nails per shingle. High wind areas and other types of shingles may need more. 320 nails will be needed to install a square of standard 3-tab shingles, assuming four nails per shingle and 80 shingles per square.

Remember, be wary of too-good-to-be-true estimates that promise cheap and fast results. Investing in name brand materials and quality service will save you time and money in the long run.

How to measure your roof area from the ground

A more accurate way to calculate the area of the roof than using Google Earth is to get outside to estimate the roof pitch and the base area of the property. Using these two figures, we can get a good idea of the shingle roof’s square footage and estimate your asphalt shingles’ needs and costs. This DIY technique can be helpful, especially if you are not comfortable getting on your roof, or if you have limited access to it.

Ground measures only work well for a gabled roof, since there are usually just two main rectangular pitches to calculate area for. For more complex roof shapes, like hip roofs, you’ll need to work with a roofer or measure from on top of the roof itself to calculate its total area.