How To Measure Roof Slope & All Other Dimensions From The Ground
All of the roof data: rise, run, and slope can be calculated from the ground if the building sports horizontal siding such as that shown in our photograph below.
Roof rise : first we will obtain the total roof rise by counting siding courses. We measure siding width & then we count courses at the building gable end. If we start counting siding at a horizontal lilne even with the lower roof edge or eaves and count up to the ridge, we’ve got a close guess at the total roof rise.
The number of siding courses from the roof triangle base to the roof peak x siding course width = total roof rise =
Roof length : Measure or step off building gable end width.
Roof width : This data allows us to calculate the roof triangle as we know two sides and of the three sides of a right triangle . Let = the vertical rise in the roof and = the roof length . The third side of the triangle, its hypotenuse or the sloping surface of the roof, or side is which is calculated as follows:
a2 = b2 + c2 – the square of the length of the hypotenuse equals the squares of the lengths of the opposite sides of a right triangle and .
Given a2 we use our calculator to take the square root and bingo, we have the length of the sloping side of the roof.
Given that we now know all of the lengths of our triangle we can easily obtain roof slope too if we need it.
RA = x
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.
Straight To The Answer
Divide your roof pitch by 12. Roof pitch is given as the number of inches in height change over the distance of one foot. Dividing by 12 gives the ratio of inches in rise per distance. For example, a standard roof pitch is likely to be about 4. So for a pitch angle of 4, divide by 12 and get 1/3.
Square the result of Step 1. So keeping with our roof pitch of 4 this gives us 1/9.
Add 1, then take the square root. Adding 1 gives us 1 + 1/9 = 10/9, then taking the square root gives us about 1.0541.
Multiply by the square footage of a single floor of your house. So continuing our example, if you’ve got a two story house with a total square footage of 2,700 square feet, with each floor having 1,350 square feet, multiply 1,350 by 1.0541 to get 1,423 square feet. So you’d need 1,423 square feet of roofing.
Things You Will Need
-
Square footage of your house
-
Pitch of your roof
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Getting A Rough Estimation Of The Size Of Your Roof
Your home buying documentation should have the exact size of your roof inside, however if you do not have this information, or another document that details the exact size of your roof , then youre going to have to make the roof estimation yourself . While it may seem intimidating, the process isnt that difficult: youll have to know the overall square footage of the ground floor, as well as the pitch of your roof. If the roof has varying pitch sizes then each area with a different pitch will have to be separately measured and added all together in order to find the total roofing area.
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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Hire A Professional Contractor To Measure The Tiles
As you can see, measuring a tile roof is not as difficult as many would have you believe. However, because it can be difficult to walk on a roof and measure it, it can be dangerous work for inexperienced people. For a simple roof, I generally figure 1 percent as a waste factor. To make life easier and safer, its best to hire a professional contractor to measure your shingles and the best experts.
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.
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Technical Reviewers & References
How To Measure Roof Pitch
Knowing the pitch of your roof is useful when designing an addition, installing skylights, or cutting new rake boards. Here’s an easy, safe way to determine the pitch from inside your attic.
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How To Measure The Perimeter Of A Sloped 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)
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.
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How To Calculate Roof Area
Before you begin work on any roofing project, its vital to have an accurate measurement of your roof area. This will allow you to work out everything from how many roof tiles you need to project cost and timeframe for completion. Thankfully, calculating your roof area is a quick and easy process once you know how. In fact, for the classic gable roof that most British homes have, you can work out the size of your roof with just two measurements.
How Many Bundles Of Shingles Per Square
A package of roofing shingles is called a bundle. Laminate or architectural shingles, one of the most common types of roofing shingles, are generally packaged in three bundles per square.
So in the 2400 square foot gable roof example, you would need 72 bundles of shingles .
Here are a couple more examples:
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Select Your Measuring Tools
Now youre ready to go ahead and actually measure a roof.
At the top left, click on Tools. In the drop-down menu, check the box next to Ruler. This opens a box with a little menu across the top. Click on Polygon.
The default unit of measurement should be feet and square feet. If it isnt, or if it is but you want a different unit, you can change it by clicking on the little unit fields there.
How Many Roofing Shingles Do I Need
To estimate how much roofing material you need, such as bundles of architectural shingles or rolls of synthetic roofing underlayment, you need to know the total square footage of your roofs surface.
How to Calculate Total Square Footage of Your Roof
To find your roofs total square footage:
Example #1: Shed Roof with One Roof Plane
For example, this shed roof has one roof plane.
Multiply length times width :
A x B = 40 x 30 = 1200 sq. ft. for the total square footage of the roof.
Example #2: Gable Roof with Two Roof Planes
This gable roof has two planes.
So to derive the total square footage of the roof, you would:
For example:
- Plane 1 : 40 x 30 = 1200 sq. ft.
- Plane 2 : 40 x 30 = 1200 sq. ft.
- Plane 1 + Plane 2 = 2400 sq. ft. for the total square footage of the roof.
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How To Measure A Roof With Google Earth: Easy Step
Roof Online Staff
Keep in mind that for measuring roof areas, this measuring technique is only usefully accurate for low-slope or flat roofs. Satellite images cannot account for the extra surface area that a pitched roof has compared to a flat roof with the same footprint area. And satellite images will not help you find the pitch of a roof.
If you want to measure the area of a pitched roof, see How to Find the Area of a Sloped Roof.
And even flat roofs usually have a little bit of a slope, so you might want to use a roof pitch multiplier once you have your area figured out.
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.
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Roof 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.
Roof Slope Multipliers: Convert A Flat Or Projected Horizontal Building Footprint To Sloped Roof Coverage Area
Roof Slope or
The numbers in the table above can be calculated as follows:
For a 12-inch unit-length roof we calculate the hypotenuse dimension to obtain the roof true width for each roof slope. In the table above the roof slope or rise gives us the vertical dimension of a right triangle. The horizontal dimension is fixed at 12 inches.
A complete table of roof slopes expressed as rise / run, slope, and grade is given at ROOF SLOPE TABLE, TYPES, WALKABILITY
Continue reading at ROOF MEASURE by FOLDING RULE or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.
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Below, there is a simple triangle, and the math used to work out how to make a proper measurement. Once you work through the basic triangle, we will review a typical roof, and how to apply some simple math to calculate how much material you will need for a roof used in our hypothetical example.
1 roof square = 100 square feet The length times the height of a triangle is twice its area . So if you divide your answer of a product of length times height by two, you will get the area of a triangle. /2 = Area
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In Figure above: l=30 feet h=12 feet /2 = 180 square feet
Now that the basic concept has been covered, lets take a look at a more complex roofing system. In this overhead view you see both a hip-end section, and gable ends. With a more complex roof such is this one, it is highly recommended that you make a basic sketch. By doing so, it will be easier to mark your measurements, and calculate the necessary materials for the project. For this example we will break this roof up into sections labeled A through E.
In the figure B. above, a sketched portion is a basic triangle. As we did in the first example, measure the length of the eaves, and the vertical line from the eaves half way point to the peak. Multiply these numbers, and then divide the answer by two./2 = 270 square feet
S1 = /2 = 135 square feetS2 = 50 × 15 = 750 square feetS3 = /2 = 135 square feet
