Roof Calculations of Slope, Rise, Run, Area

How Do You Calculate Rafter Length

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How to calculate rafter length?

The formulas we use are the following:

  1. For calculations based on the roof rise:

rise² + run² = rafter length² ,

meaning that:  rafter length = √(rise² + run²).

aerial roof measurement
Pitch – Is the incline of the roof represented as the ratio of rise to the span (twice the run). For example, if the roof rises 4 feet and the span is 16 feet (twice the run), then the pitch is 4/16, or 1/4.
But they do tend to space rafters in one of several industry-standard increments, typically either 12, 16 or 24 inches apart. These spacing intervals are measured “on center,” meaning that the measurement is taken from the center of one rafter’s horizontal surface to the center of the next rafter.
Maximum allowable span for a 2×6 rafter:- the maximum allowable span for 2×6 rafter is 16 feet 5 inches when spaced 12 inches apart from centre, 14 feet 8 inches at 16″ OC & 12 feet 8 inches at 24″ OC with best quality No.
 

* Sky Roof Measure provides industry leading takeoff and  Roof  estimating Reports for professional construction contractors. Please Email Us : info@skyroofmeasure.com

Estimate Roof Size
The length of the rafter is given from the long point of the ridge cut at the top down along the top edge the board to the same plumb cut on the rafter’s notch. The next result is the run. This is the horizontal measurement from the outside of the wall to the inside of the ridge.
Rafter span: this is the distance between the supporting wall and the centre of the roof measured horizontally. The span is NOT the distance between the supporting wall and the peak of the roof measured along the diagonal. Rafter: This is the lumber that runs from the centre (peak) to the supporting wall.
2-grade 2×6 joists can span up to 10 feet 9 inches from beam to beam when spaced the standard 16 inches apart with a maximum live load of 30 inches per square foot. In comparison, No. -1 grade lumber can span slightly further to 10 feet 11 inches under the same parameters.
 
In general, a 2×8 will span 1.5 x 8, so 12-feet. Based on all factors though, a 2×8 joist span is 7′-1” to 16′-6”, and a rafter 6′-7” and 23′-9”. In this guide, we’ll explain what span means in terms of construction, factors that impact the span, and how much weight a 2×8 can support.
 
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* Sky Roof Measure provides industry leading takeoff and  Roof  estimating Reports for professional construction contractors. Please Email Us : info@skyroofmeasure.com

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How to get the most accurate measurements in your roof report

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Measure the width and length of each plane of the roof (including dormers), and then multiply those numbers to get the correct figure for the square footage of that area. Note any skylights, chimneys or other parts of the roof that would not require materials. You can subtract those measurements from your total area.

Aerial Measurement Technology

It extracts measurements that may have at an accuracy rate of 95% or higher. Contractors benefit from the time savings that come with not having to visit the property, as well as the added safety: it’s possible to gather the measurement with no climbing.

Using this method of remote measuring also enables roofing contractors to hire professional salespeople, who can focus on selling rather than learning how to measure a roof. They can use the technology to capture measurements accurately and reassure homeowners that your company uses the latest tools and methods available.

However, aerial measurement technology has its disadvantages. One is cost: depending on the size and complexity of the roof, the measurement report can cost $50 or more. Report turnaround time can also be a turnoff for contractors. In some cases, especially during high-volume situations such as following a hailstorm, the wait for turnaround may reach 24 hours or more. In a world where consumers want instant gratification, delaying an estimate by 24 hours could result in losing the job.

High Resolution Imagery = Better Accuracy
As we mentioned before, one of the benefits of drones is their ability to capture high-resolution imagery. Below you will see the differences in ground sample distance between satellite, aerial and drone imagery.

  • Satellite Imagery: Usually 30-50 centimeters per pixel resolution
  • Aerial Imagery: Usually 7-9 centimeters per pixel resolution
  • Drone Imagery: Sub-centimeter per pixel resolution

Measuring by Hand

Getting a roof measurement by hand is the tried-and-true method trusted by roofing contractors for years. By measuring themselves, a contractor can be confident in the numbers and know that the measurements will be completely accurate. This means that orders for materials will be more precise and there will be less waste on the job.

On the other hand, manually obtaining a roof measurement also comes with drawbacks, some of which might outweigh the benefits. The first one is time. Manual roof measurement requires the contractor or sales representative to drive to the property, climb onto the roof, and potentially spend up to an hour gathering measurements, depending on the roof’s complexity.

The second concern is safety. Climbing onto the roof requires contractors to carry equipment—including a tape measure, pitch gauge, tablet and pen—up a ladder and onto the roof. While the contractor is focused on capturing measurements, it’s possible to lose sight of key safety precautions such as the location of fall hazards and leading edges. Physically walking on the roof can also cause damage and loss of granules.

Measure a roof safely

Without the proper training and equipment, it isn’t recommended for homeowners to climb on their roofs.  If you want to get the project started yourself, opt for the safer alternative that will save you time and keep you safe by ordering an aerial roof measurement report.   

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Roofing Calculator – How to Measure a Roof

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How to Calculate Total Square Footage of Your Roof. To find your roof’s total square footage: Measure the length and width of each plane on the roof (including dormers) then multiply length times width. Add the square footage of each of the planes together.

How do you 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(%)]

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

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

Tips on measuring for roofing

Measuring your roof’s dimensions is one of the most important aspects of any roofing job, so make sure your tools are up to par. An accurate ruler, tape measure, and a sturdy ladder are all required.

The problem is that not everyone is comfortable with climbing a ladder to do this. What’s more, there is no guarantee that the measurements will be correct. That error can cost you more money, time, and effort.

That’s why it’s best just to hire or consult a professional roofing contractor. They will have the tools, knowledge, and experience to help you accurately measure your roof and figure out how much materials you need with minimal waste.

How to Measure a Roof for Shingles

Don’t worry too much if you don’t know how to measure a roof for shingles. Your roofer will measure the surface of your roof to determine the total square footage, which will dictate how many shingles must be purchased to fully cover the roof’s planes.

But if you’re curious how the measuring process works, think back to your geometry classes in school. To calculate a rough estimate, follow these steps:

  1. First, you must measure the length and width of each plane of the roof, including dormers. If the planes aren’t rectangles, this may be complicated.
  2. Next, to calculate the square footage of each rectangular plane, multiply the length by the width. For example, if a plane is 130 feet long and 100 feet wide, it’s 13,000 square feet.
  3. Finally, add together the square footage of each plane to calculate the total square footage of the roof. So if there are two planes that both measure 13,000 square feet, the total square footage of the roof is 26,000 square feet.