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How To Calculate Roof Surface Area

Calculate The Complex Roof Areas

Calculate roof / tile surface in ArchiCAD

Most roofs in Canada would be considered on the complex side. Multiple adjacent pitches, dormers, and turrets are common. Keep in mind that an experienced metal roofing contractor will need to factor in extended labour time for areas of your roof that will require precision metalworking skills.

Start by finding the footprint area and multiply by the correct slope factor determined by pitch . In Figure 4 below, the dashed blue lines give a simplified view of the footprint. Multiply L x W to calculate the area for each rectangular section. Note the area of 77ft2 that needs to be subtracted for the recessed area.

Once the footprint including overhang areas have been determined for each area, find the appropriate slope or pitch factor in the chart below. For example, a roof area with an 8/12 pitch has a slope factor of 1.202 so 2053ft2 x 1.202 = 2468ft2 total roof area.

A Note about complex roofs: Complex roofs need more accessories and trims thaneasy roofs resulting in a slightly higher total cost/square foot.

Figure 4. Calculating Complex Roof Area

Start The Project With A Roof Area Calculator

Calculating the surface area of a sloped roof can be time-consuming without the right tools. You want a roof area calculator that provides accurate estimates to help you order materials such as shingles, membrane, roofing, or ceramic tile.

Fortunately, if you know the roofs pitch, or have access to the homes attic to measure its incline you can arrive at an accurate roof area calculation using SketchAndCalc.

Roofing professionals can also use SketchAndCalc to provide fast accurate quotations over the phone. Experts recommend that a pitch measurement is taken before using a roof area calculator.

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.

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

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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.

Calculating Your Roof Area: Gable Roof

Roof Area Calculator  Surface Area Multiplied by Pitch

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

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How To Measure Or Estimate The Total Roof Area

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

    Keep The Slope And Complexity Of Your Roof In Mind

    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.

    Read Also: Can You Raise A Roof On A House

    The Type Of Roof You Want

    The type of roof you want will have a big impact on the cost of your roof replacement. There are a variety of different types of roofs for you to choose from based on your budget and the look you want.

    If youre on a tight budget and looking for an affordable roof, an asphalt shingle roof might be right for you. But if youre looking to upgrade to a longer-lasting and sharp looking roof, theres the standing seam metal roof.

    Asphalt and metal are two of the most commonly seen roof types, but there are also premium shingle roofs that come at a premium price. These roofs give you a unique look that really stands out. These premium roofs include cedar shake, slate, and tile.

    The premium shingle roofs can get expensive quickly and be more than triple the cost of an asphalt shingle roof.

    Calculate The Roof Area

    How to work out surface area of a hip roof

    Now that you have all of your roof measurements, you can calculate its surface area. Simply multiply the length and width of each rectangular plane, and then add the combined area of the hips. Then multiply this number by the roofs slope factor, and then multiply this final result by 1.10 to allow for error.

    In this example, our imaginary home’s hip roof would cover a surface area of + , while the lean-to roof would be equal to . This gives you a total surface area of 935 square feet. Multiplying this by the slope factor of 1.03 equals 963.05 square feet. Multiply this by 1.10 to obtain 1059.4, which is a good estimate of your roofs surface area.

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    The Labor And Time To Complete The Job

    Labor and time are two of the main factors a roofing contractor considers when calculating the price of your new roof. If anything takes time and labor, it will show up in the final cost of your roof replacement.

    The labor cost of each man on a crew is based on their skill level, experience, and how much they can get done in a day. The labor cost will vary from contractor to contractor.

    The time it takes to complete the job is also a factor. The more time a roofer has to spend on your roof and property will always affect the price of a new roof.

    If the roof is complex and hard to access, it take longer and more labor to complete. From tearing off your old roof to cleaning up your property after the job is done, if it takes manpower and time, its going to impact the cost of your new roof.

    Expressing Roof Slope In Degrees

    The rest of the English-speaking world uses degrees, which seems simpler, but it takes a few extra steps to figure it out mathematically.

    It also doesnt immediately convey as much information about the roof on a practical level as the rise-in-run ratio expression.

    To express the slope in degrees, you divide the rise by the run and find the inverse tangent of the result.

    But you still need to know the rise and the run, the X-in-12, in order to find the roof slope in degrees, unless youre using a slope finder that tells you the slope in degrees.

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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

  • Measuring Or Calculating Gable And Hip Lengths

    Roofsurface calculation

    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.

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    Notes On Calculating The Area Of A Roof

  • Ifthe roof has areas with different slopes, as on a gambrel roof, youll have to figure out more than one slope and use the appropriate roof slope multiplier for the different areas.
  • As a shortcut, the roof slope multiplier for any slope can be determined by finding the square root of ² + 1).Divide the rise by the run. Square the result. Add 1 to the result of that. Find the square root of that result.
  • If you knowthe roof slope in degrees, simply find the secant using a scientific calculator. For example, if the roof slope is 45°, then sec = 1.414213. Thats your roof slope multiplier.
  • 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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    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.

    Calculating Your Catchment Capacity

    Measure the Surface Area of a Roof

    Roughly speaking, 1 millimetre of rain over 1 square metre of roof equals 1 litre of water.

    This capacity can be calculated using the following formula:

    Annual rainfall x Roof surface area = Roof catchment capacity.

    This is an important figure to understand so you can design a Rain Harvesting system and choose a tank size that will meet your water volume requirements.

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    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

    Calculating The Area Of Complex Roofs

    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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