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How Do You Calculate Roof Area On Plan

What About The Waste Factor Should I Order Extra Roofing Materials

How To Calculate Roof Area

You want to order extra material to account for waste. Waste factors vary.

10%-15% is a good rule of thumb, but your results may be different. More complicated roofs will have a higher waste factor because there are usually more cut shingles around corners, walls, and edges.

It is okay to have a few shingles left over. They can be saved in case there is roof damage later or if repairs need to be done in the future.

So for the 24 square roof example, you would want to add 2.4 squares which equal about 7 or 8 more bundles.

The Negative Roof Measuring Method

To find the area of a roof using the negative roof measuring method you will extend the roof line past the roof to form a single rectangle. You will then subtract the rectangular areas that lie outside of the roof.

First you need to extend the roof lines to form a single rectangle.

60 x 60 = 3600 sqft extended rectangle

You then need need to find the area of the rectangles outside of the roof area. You can see in Image 1.3 that the areas outside the roof layout are sectioned off into section A and section B. We will find the areas of these sections and then subtract them from our extended rectangle.

Area of section A = 20 x 20 = 400 sqft

Area of section B = 20 x 40 = 800 sqft

The total area outside of the roof layout is 800 sqft + 400 sqft = 1200 sqft.

3600 sqft 1200 sqft = 2400 sqft

You can see that using the negative roof measurement method gives us the same result as the positive roof measurement method.

How To Measure A Roof

This article was co-authored by David Bitan. David Bitan is a roofing professional, licensed contractor, and the owner and founder of Bumble Roofing based in Southern California. With over 12 years of construction industry experience, David specializes in restoring, repairing, and maintaining residential, commercial, and industrial roofs. With over 60 years of combined experience, Bumble Roofing provides easy, friendly services to structures including residential, commercial, industrial, multi-family, and government buildings along with hospitals, hotels, and churches.There are 13 references cited in this article, which can be found at the bottom of the page. This article has been viewed 356,161 times.

Measuring your roof is the first step in a successful re-roofing project. You can actually get an estimate of your roof’s measurements from the ground if you’d rather not climb a ladder and get up on the roof yourself. Otherwise, you can climb up onto your roof to take more accurate measurements. Either way, we’ve got you covered! This article will walk you through each process step-by-step.

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How Do You Calculate The Roof Area

  • 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 into the appropriate box.

  • 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.

  • 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%

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

    roof area = base area / cos

  • What Is A Roof Pitch Multiplier

    InterNACHI Inspection Graphics Library: Roofing » Calculations » roof ...

    A roof pitch multiplier, also known as a roof pitch factor, is a number that, when multiplied by the area covered by a sloped roof, gives the actual area of the roof. The math behind this number is that it is the square root of 2 + 1). Roof pitch multipliers are extremely useful in estimating the amount of material required to fit your new sloped roof, as conventional mathematics will leave you short!

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    Why Should I Measure My Own Roof

    If you plan on re-roofing your own home then you will need to calculate materials so you wont waste your own time and money. If youre like most people, youll need to hire a professional.

    Knowing your numbers will help you figure out what roofing materials are within your budget and having your square footage handy is a great way to protect yourself from inflated quotes.

    How To Measure Your Roofs Square Footage

    The square footage of your roof has a huge impact on the cost of your replacement. It affects a number of cost factors, including the amount of materials used, labor, and time.

    Be aware, the only way to get a 100 percent accurate measurement is to have a local roofing contractor come out to professionally measure your roof and give you a quote. However, I can give you the calculations to get a rough measurement of your homes square footage.

    Also Check: What Type Of Plywood Is Used For Roofing

    How To Use Trivial Arithmetic To Convert Grade To Angle Or Percent Slope

    Grade, a figure used in road building, is simply slope or angle expressed as a percentage.

    Rise / Run x 100 = Slope in Percent


    If I build a sidewalk up the slope of a hill, the building department wants to know if I should have built stairs instead. If the slope, expressed in percent or percent grade is too steep, walkers are likely to slip, fall, and end this discussion.

    Suppose my sidewalk is 100 feet long and that the total rise from the low end to the high end of the walk is four feet:

    4 ft. / 100 ft. x 100 = 4% Grade – which my inspector accepted as ok. Typical building codes specify that

    For pedestrian facilities on public access routes, the running grade of sidewalks will be a maximum of 5%.

    Keep The Slope And Complexity Of Your Roof In Mind

    How to Measure the Square Footage of a Roof

    You have to keep in mind the makeup of your roof when doing the calculation for the square footage. The steepness and complexity will change the number you multiply the footprint by.

    For example:

    For an easy up and over, walkable gable roof, youll multiply the footprint of the roof by 1.3 to get the square footage of your roof.

    For a hip roof with a low slope, you’ll multiply the footprint of the roof by 1.4 to get the square footage of your roof.

    For a steep and complex roof, you’ll multiply the footprint of the roof by 1.6 to get the square footage of your roof.

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    Roof Slope Multiplier Chart

    Since the roof slope multiplier is so important to estimating the area of a roof, weve put together a quick reference chart.

    We also have a much more complete roof slope multiplier chart that includes slopes by the half inch and slopes up to 30.5-in-12.

    If youre using the table below to help you figure out the area of a roof, you should consider getting yourself a construction calculator. That way you can instantly calculate the factor for any slope, among many other useful things. This one is very good.

    Roof Pitch

    Measuring The Roofs Pitch

    The incline or pitch of a roof can be easily measured with a level, tape measure, and a pencil. The roofs pitch is the number of inches the roof rises in 12 inches. Therefore, mark off 12 inches on the level and place it down horizontally against the roof rafter.

    Next, measure vertically from the 12-inch mark on the level straight up to the underside of the rafter, as illustrated below. That measurement is the number of inches the roof rises in 12 inches.

    Some homes will have roofs with multiple pitches. Porches and dormer windows, for example, can have a different pitch from that of the rest of the house. For an accurate surface area measurement you should make note of these additional structures.

    Using the table below, lookup the pitch/es and make a note of the number/s in the multiplier column. This multiplier number will be used to multiply the surface area calculation that SketchAndCalc will provide.

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    What Is A 4/12 Roof Pitch

    A 4/12 is a roof slope that rises by 4 inches for every 12 inches across. This forms an angle of 18.5° between the horizontal section and the roof, and creates a gentle incline that is seen as a midpoint between a low-pitch and medium-pitch roof. A 4/12 slope is enough to accommodate water runoff, but is not suitable for regions that are affected by heavy snowfall. Standard asphalt and composition shingles are the best for a 4/12 roof rubber and wood shingles are unsuitable.

    Table Of Tangent Values For Each Angle In Degrees


    Just type “tangent table” into your search engine and you’ll find a table something like the one we show below.

    To convert a slope into rise/run is to use the tangent of that slope or angle

    That number, a constant, lets us calculate rise if given run or run if given the rise amount.

    Definition of Tangent: the Tangent of any angle is defined as the vertical rise divided by the horizontal run.

    A tangent is also defined as a ratio of two lengths: the length of the side of the triangle opposite the angle divided by the length of the adjacent side of the angle.

    For a right triangle in which the roof rise is the vertical line opposite the roof angle or slope, the tangent of the roof angle is the ratio of roof rise over the length of the sloped roof surface.

    In the illustration at the start of this article, the tangent T of the 38 degree angle is the ratio:

    Roof Rise / Sloped Roof Run

    So we write the tangent of angle A as:

    Tan < A = Rise / Run

    Our sketch above shows how we calculate the rise per horizontal foot of run when we are given the roof slope in degrees .The purple sloped line is the sloping roof surface or the slope or angle of a stairway.

    Standard rise & run: Roof slope or stairway slope are properly expressed of inches of vertical rise per foot or 12-inches of horizontal run or distance traveled.

    Luckily, using the Tangent function, and given the angle of any stair or roof, we can calculate the amount of rise for any horizontal distance.

    Tan 38o = 0.7813

    Tan 45o = 1.00

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    Measuring Or Calculating Gable And Hip Lengths

    The slope factor is also used to determine gable lengths when the run length is known. For example, in Figure 4 the bottom left gable measures 16 across from eave to eave and therefore has a run of 8. When this is multiplied by 1.202 the gable distance is 98 . Note that this run can also be multiplied by the hip/valley factor of 1.563 to get a valley length of 126.

    Record all your calculations on your drawing as seen in Figure 5 below.

    Figure 5. Completed Roof Plan Drawing

    Pro Tip: A Note About Waste Factor

    Once you have figured out your roof area, you will need to also calculate how much room you need to allow for waste. For metal roofing products on a medium complex roof a waste factor of 15% is advisable. In other words for the 2468 ft2 roof in our example you should multiply by 1.15 giving you 2838.2 ft2 roofing material needed.

    Important Note: Most metal roofing products are packaged and sold by the square so for our example roof the number of squares or boxes to purchase is 29.

    Find The Slope Of The Roof

    Before you can determine the area of a roof, you need to know its slope. You can find the slope by getting up on a ladder at the rake edge of the roof and measuring with a ruler and a spirit level, or you can use a specially-designed slope finder.

    Measuring roof slope with a bubble level and a ruler. This level has no measurement markings, so the yellow tape is being used to mark 12 inches from the end of the level that touches the roof. This roof has an 11-in-12 slope.

    You could also take your measurement in the attic using the underside of a rafter, the underside of the roof sheathing, or even the ceiling, if youre sure that it follows the slope of the roof.

    Eyeballing the slope and guessing is probably not a good idea.

    The result you get from guessing like this might be close enough for government work, but if its a large roof and youre a roofer with a 10% profit margin, I wouldnt recommend it.

    There are also a number of smartphone apps that you can use to find the slope of your roof, if you have a clear view of a gable end on the building. These can be acceptably accurate sometimes.

    You dont have to get up on the roof or go to the attic to use these. Go search in your app store for roof pitch apps. Youll see.

    Measuring roof slope with a slope finder.

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    What Roof Type Is Best Suited For Your Roof Pitch

    If you have roof pitch of 3/13 or lower, it is highly recommended that you use/install a flat or low slope roofing .

    If your roof slope is above 3/12 , you can install either asphalt shingles, or a metal roof, or one of the exotic roof types: Slate, Tile, Wood, etc.

    Metal roofs have the highest wind blow-off resistance, and are best suited for ice dams prevention especially if your roof slope is below 6/12!

    Calculating The Area Of Complex Roofs

    Roof Area video

    If your roof is not the classic Gable shape, or if it has multiple sections such as hips and valleys, wed recommend splitting the roof into each section and working out the areas individually. If youre unsure of how to do this, get in touch with a roofing professional. Learn how to find a good roofer.

    Or, alternatively get in touch with us! Our customer service team will be able to help you calculate your roof area, and anything else you might need for your project. Call them on 01295 565 565, email , or use the handy live chat below.

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    Explanation Of The Formula

    Area / cosArea is the value of the Area measurement.

    Roof Angle is the value in the User Defined Column that you created.

    Cos is the math function cosine

    Pi is 3.14

    Please checkout our video tutorials on PDF Measurement , the Measurement Enhancements in Revu 12 , and Takeoffs and Estimations , in addition to the FAQ section on our Support page, and the Revu Help Guide.


    What Is The Average Quote For Getting A Roof Pitched

    There are a lot of factors that will affect how much getting your roof pitched will cost design, style, accessibility, cost of labor etc. In the USA, you can expect to pay between $3.19-$4.43 per sq.ft. for asphalt shingles. If you choose another material, such as metal or wood, the range is $7.35-$10.73 per sq.ft.

    In the UK, the price of getting a roof fitting is £72-£75 per square meter. These are only estimates though, talk to your neighbors to see what roofing costs in your area.

    Read Also: How Much To Replace Roof And Gutters

    Figure Out Your Homes Footprint

    If you use the square footage of your home, youll only get a vague idea of what the square footage of your roof is. Thats why you have to figure out your homes footprint.

    A person of average height will have a stride somewhere between 2 to 2.5 feet. To figure out your homes footprint, you can step off the dimensions of your roof by counting the steps you take to get the length of your home. You will then step off the width of your home.

    After stepping off the length and width of your home, youll multiply the two numbers to get your homes footprint.

    Length x width = your home’s footprint

    Your calculations are going to be off, but measuring this way can give you a basic idea of your homes footprint. Keep in mind, square footage of a house does not equal the square footage of a roof.

    Calculate The Simple Roof Areas


    On a simple hip or gable roof, multiplying the eave to ridge length by eave length will give the area to be multiplied by pitch factor. Ridge vent openings are best finished with coil rather than partial shingle. Using flat coil stock allows the ridge caps to lay flat and can also be formed with an extra water stop at the ridge opening.

    Area for the roof in Figure 3 is 1784 ft2 The blue dash line divides the roof into 2 rectangles. There is a small area calculated as 8/12 that is actually 4/12. On a small area such as this, it is better to err on the side of the higher pitch since it should not significantly affect the total.

    Pro Tip: Pythagorean Theorem can be used to calculate the length of the valleys. A general rule of thumb is to add at least 12 lineal inches of valley material to be safe.

    Figure 3. Calculating Simple Roof Areas

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