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How To Estimate Roof Size

How To Measure Or Estimate The Total Roof Area

ROOF RAFTER CALCULATOR – Estimate Rafter Length, Cost and Quantity

If you have safe access to the roof surface you can quickly make the needed area measurements: just measure from the ridge to the lower edge or eaves, keeping your tape straight.

With a decent 3/4″ or 1″ wide 30 ft. tape measure you can extend the tape out to catch the roof eaves without having to walk dangerously close to the roof edge. Also measure the roof edge or length.

  • RW = Roof Width = dimension from ridge to eaves. In my sketch I use or the dimension of the sloping surface as RW. Remember to include in the amount by which the roof extends out over the building walls – its eaves.
  • RL = Roof Length = dimension of the roof from one gable end of the building to the other. You don’t see RL in my sketch at left. RL is simply the building width plus the amount of overhang at the two gable ends of the building.
  • RA = Roof Area = RW x RL
  • Well this is only true provided the roof surface is a rectangle. If you are working with hips and gables and you want to get precise area measurements you may find it easiest to divide the individual roof slopes up into sub-components: a rectangular part and a triangular part.
  • For a rectangle just multiply the width and length of its two sides to obtain area.
  • For a right triangle, its Area = 1/2 x
  • For a triangle such as the red example in our photo, just divide the triangle into two halves and you can still make these calculations with ease.
  • Remember to keep your dimensions all the same .
    • Roofing Squares = RA / 100

    Roof Area Calculationsmethods For Calculating The Area Of A Roof Depending On What Measurements You Already Know

    • about types of roofing materials, installation, inspection, diagnosis, repair, maintenance, & warranties

    InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

    Roof area calculation & measurement methods:

    Here we describe various methods for measuring all roof data: roof slope or pitch, rise, run, area, and other features. We include on-roof measurements, roof measurements or estimates that can be made from ground level, and several neat tricks using a folding ruler to measure roof angle or slope.

    This article shows how simple measurements can give the roof area without having to walk on the roof surface. This article series gives clear examples just about every possible way to figure out any or all roof dimensions and measurements expressing the roof area, width, length, slope, rise, run, and unit rise in inches per foot.

    We also provide an ARTICLE INDEX for this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

    How To Measure Your Roof With Google Earth

    Using your computer to get a ballpark estimate of your roof area is easy and fast. However, using satellite imagery may not be an ideal solution if your structure is in a very remote location that hasnt been 3D scanned. Thats because well need to grab an accurate representation of your roofs pitch for measuring purposes. From above with a 2D image, everything looks flat. If you were to just measure your roof area from directly above, you wouldnt be accounting for its pitch, or slope.

    Consider two homes with the same base area square footage, but one of them has a much steeper roof pitch than the other. That structure would need to be taller, and there would be a lot more roofing material to account for. If youre just measuring from above in 2D, both of these structures might look identical. If you dont account for roof pitch, youre going to underestimate your materials requirements, which could be a costly mistake.

    Satellite technology has progressed a lot in the past few years. With , you can easily find your address and a 3D replica of your structure, including your roofs pitch. Lets get to it.

    Once you load the site, youll be presented with our beautiful planet:

    While you can switch to 2D mode by clicking the button in the lower right, youll want to stay in this 3D view so you can get a good view of your roofs pitch.

    Also Check: What Is A Hip Roof

    Two: Split Tricky Shapes Into Squares And Rectangles

    Once you have these figures, return to the safety of your home. Notice in the image above, the hip roof is made of two triangles and two trapezoids. Trapezoids are tricky to get the area measurement of, so were going to split each of them into two triangles and a rectangle:

    We know enough about the overall measurements of the roof to fill in the missing values. Lets fill the figures in now:

    Keep The Slope And Complexity Of Your Roof In Mind

    Estimating Roof Pitch &  Determining Suitable Roof Types ...

    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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    How To Calculate Your Roof Area

    Knowing how to calculate your roof area can be a beneficial first step in estimating the cost of a new metal roof. In roofing we call the calculation of roofing materials needed a take-off.

    A good roofing contractor will take the time make sure that the take-off is accurate. Proper measurements and attention to roof details ensure that material shortages are avoided and prevent overages .

    Three: Determine Roof Pitch

    The pitch of a roof is the angle that it sits at and how steep it is. The pitch of the roof is the measurement of the vertical rise divided by the horizontal span, often measured out in inches. You may hear a roofs pitch is compared to the slope, but they are not one in the same. The reason the roofs pitch must be calculated before the area is because it affects the actual area size of the roof. A moderately pitched roof will run as a 6 in 12 where the roof will rise six inches vertically for every twelve inches of horizontal run.

    If you have a steep roof, you may have a 12 in 12 where your roof is at a forty-five-degree angle. The rise of the roof is the distance from the top studded wall to the peak of the roof, while the run is the distance from the outside edge of the perimeter of the studded wall to the center of the house. There are three ways by which you can measure your roofs pitch.

    The most common roof is a 6 in 12, which is a 27-degree angle with anything above this being anywhere between 30 and 45 degrees. Anything below a 6 in 12 is anywhere from a 5 to 23-degree angle.

    Also Check: How To Calculate Roof Rafters

    Download And Install Google Earth

    The first thing you need to do is download and install .

    Google Earth Pro used to be the paid version of Google Earth , but its free now.

    It includes tools that werent available on basic Google Earth, the most relevant tool being a measuring tool that allows you to measure the area of a polygon.

    You can download it at:

    How To Measure The Perimeter Of A Flat Roof

    How To Calculate Wall Height Difference When Framing Roof With Different Pitches For Fascia Board

    The perimeter of a flat roof is the total length measured around the edges of the roof. The outer edges of most roofs are not simple rectangles and squares. Most roofs have recesses which complicate measuring the perimeter of a roof.

    The formula to find the perimeter of a roof is:

    L + W + L + W + R + R = Perimeter of a roof

    2L + 2W + 2R = Perimeter of a roof

    The formula for a roof without recesses is:

    2 = Perimeter of a roof

    Example 1.3

    Lets try an example. Measure the perimeter of the roof in image 1.5.

    The perimeter of the roof is:

    2 x = 90 linear feet

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    Calculate Roof Pitch From The Ground In Three Quick Steps

    Roof pitch is one of those things that middle school math and geometry instructors can easily point to, and say, See? Learning how to work with rise over run is fun and useful, right? If at any moment you start having PTSD flashbacks to algebra or geometry class, take a deep breath. Well walk you through this slowly in three simple steps. All you need is a tape measure, and possibly a helper to hold it.

    Step One: Grab that tape measure, notepad, and pencil and head outside. Find the slope of your roof youd like to measure, and start measuring the distance from the outer edge of the eave to the point at which the plane of the roof slope is barely visible to your eye. Write that number down in both inches. This figure is the horizontal run.

    Step Two: Stand directly underneath the gutter or edge of the roof plane you want to calculate the slope for. Measure the distance from your eye to the top of the drip edge of your roof overhang. Write that number down in inches, too. This figure is the roof rise.

    Step Three: Take the roof rise figure in Step Two and divide it by the horizontal run from Step One. In the example above, the rise is 60 inches and the run is 120 inches. This reduces down to a roof pitch of 6/12. Well use this figure in just a bit, so keep it handy.

    What About The Waste Factor Should I Order Extra Roofing Materials

    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.

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    Estimating The Cost Of The Roof

    Once you know the area of the roof, you have to decide on the scope of works. Do you want to build the roof from scratch, or maybe replace the old layers? You also need to decide on the finishing material. For example, asphalt shingles will be less expensive than clay tiles.

    After you’ve made all of these decisions, it’s best to contact your local contractor and ask about the cost of both materials and labor per square meter or square foot of roof area. Remember that this unit price should include all additional elements such as fasteners, as well as the cost of removing all layers and disposing of them .

    The most typical cost of roofing falls between $2 and $4 per square foot . If the price proposed by your contractor deviates from these values, it is advisable to check with other contractors before starting the works.

    If you cannot contact any contractor, calculate the cost of materials and multiply this price by 2.5. You will obtain an approximate price including labor, assuming that 40% of the cost is taken by the materials and 60% by the labor.

    The last thing you have to do is to multiply this cost by your roof area and voilà, you now know the total cost of your roof! Remember that the real cost might differ from this value a bit due to waste, so plan ahead and assign more funds to this construction work.

    Make sure to take a look at our concrete slab cost calculator, too!

    Four: Calculate The Area Of Your Roof


    There are a few ways you can go about this, but typically the easiest way is to calculate the simple areas of the roof and then calculate the more complex areas. You can do this by finding out the estimated amount of how many shingles you will need. Measure out the length and width of each plane on your roof, including things like hips, dormers, and adjacent pitches. Multiply the length by the width to get the square footage of each plane.

    To give an example of what you would do for a shed is simply multiply the length and width of one plane but for a gable roof, which has two planes, you would measure both planes and add the totals to get the total square footage. Do not forget to factor in the slope factor, take away recessed areas, and calculate gable lengths.

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    Four Steps To Sizing Roof Drains

    Getting water off a buildings roof is important, but how do you know what size drain to use to provide adequate drainage? Using tools from the International Plumbing Code its not that hard. IPC provides four steps to size the roof drains on a building:

  • Determine 100-year, 1-hour rainfall rate used for location of building

  • Select number of roof drains and calculate the roof area sloped to each drain

  • Size horizontal storm drainage piping

  • Size vertical storm drainage piping

  • Appendix B in the IPC provides rates of rainfall for various U.S. cities based on inches per hour that may fall during a storm of one-hour duration and a 100-year return period. In other words, a storm so severe and with so much rain it will only occur once in 100 years. The goal is to plan for the absolute worst and be able to easily accommodate the typical rainfall amounts.

    Its easier to see how this works with a real world example. Assume you are tasked to size a horizontal roof drain serving 5,000 square feet of roof area on a building located in Dallas, TX. When you consult Appendix B of the 2006 IPC it shows a rainfall rate of 4 per hour in Dallas . But dont stop there always check for code amendments from the city. When you do this, you will discover that Dallas building code requires a 6 per hour rainfall rate for storm drain calculations.

    How To Calculate Rise Per Foot Of Run For This Roof Using The Number From An Angle Level

    I’ll show that even if we screw up we can still come out ok finding the angle and then the rise and run of a roof using the angle finding level.

    I read 81 deg. on my angle level. Now let’s figure run for 12″ of rise for an 81 degree slope – HOLD ON! something’s crazy here. This is a low slope roof, how can it be sloping 81 degrees? Egad! that’s nearly straight up! This is a good lesson in thinking for yourself – or performing a sanity check on calculations.

    The answer is I was holding my angle level on the wrong scale. I could have made my photos over again holding the angle level the right way, but there’s an easier trick:

    81 degrees is just 9 degrees off of dead vertical . So really I could go just 9 degrees off of flat. As “flat” is 0 degrees of slope, flat+ 9 = 9. My roof actually slopes 9 degrees. Whew!

    The Tan value for my 9 degree slope roof = Tan = 0.1583

    Find Tan 9 deg using a handy dandy calclator, or a TANGENTS TABLE or as I illustrate also at


    The inches of rise for 12-inches of run on a 9 deg low-slope roof is calculated as follows:

    Tangent is defined as a ratio: Rise / Run so all we need is a little algebra :

    0.1583 = Rise / Run

    Set run to 12-inches because we’re going to calculate the rise per foot of run.

    0.1583 = 12 / Run

    0.1583 x Run = 12″ of rise

    Run = 12″ / 0.1583

    Run = 75.8″

    That makes sense: we travel about 75 inches horizontally for every 12 inches of vertical rise on this low slope 9 degree roof.

    Total Rise = x

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    Calculating Your Roof Area: Gable Roof

    Youll first need to measure the length of your roof. This is the length of your roofs ridge from end to end. Next, youll need to measure your roofs slope height. This is the length from the eaves up to the ridge, NOT the distance between your roof deck and the apex .

    If you dont know these measurements, you can often find them on the building plans from when you bought the property. If you dont have these, get in touch with your local council, who should be able to provide your propertys building plans.

    To calculate your roof area, simply multiply your roof length by your roof slope height, and multiply this by two. This should give you the total area of your roof, not accounting for a chimney or other sections of your roof not covered by tiles.

    x 2 = Roof Area

    How To Measure The Perimeter Of A Sloped Roof

    How to Measure the Square Footage of a Roof

    The perimeter of a hip roof or a hip and valley roof run horizontally to the ground. Take a look at image 1.16 and image 1.17 for a visual example for better understanding. You can determine the length and width by simply using the dimensions from the roof plan. Therefore, you can find the perimeter of a hip roof or a hip and gable roof by using the same formula used to find the perimeter of a flat roof.

    Perimeter = 2

    If the building has no recesses the the formula is:

    Perimeter = 2

    Find the perimeter of the roof in image 1.15

    Perimeter = 2 = 110 linear feet.

    When you are trying to find the perimeter of a gable roof you must also factor in the slope of the gable. Since the gable edge runs diagonal from the ground the plan length is not the actual length. If you were to use the plan length to determine the perimeter of a gable roof your measurements would be short resulting in underestimating the materials needed such as eaves drip, rafters, and fascia board. To find the perimeter of a gable roof your formula is:

    Perimeter of Gable Roof = 2

    Actual Width =

    Thus, the formula for the Perimeter of a Gable roof is:

    Perimeter = 2)

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