Technical Article #2 - Estimating the square footage of a sloped roof

While not a requirement of the Standards of Practice, there are many inspectors who provide very rough estimates of major costs that a home purchaser may be facing. Re-shingling is one of the more common costs. In order to make an educated estimate of the cost, though, you’ll have to make an educated guess of the size of the roof.

If you’re able to get onto the roof (again, not a Standards requirement), this is made relatively simple. A measuring tape can be used to figure out the dimensions of the roof surface and with a simple multiplication, you’ve got your number. If you don’t carry a tape up onto the roof, or can see the roof, but don’t walk on it, you can always count shingles.

Most roofing materials come in standard dimensions. Asphalt shingles, for example, are typically either one yard or one meter wide. Each shingle tab (on a three-tab shingle) is about one foot wide. If you count the number of tabs going across the bottom of the roof, you’ll know how wide that roof is.

The exposure on a typical asphalt shingle is about five inches. Since cutouts (the space between the tabs) lines up every other row, you can pick one cutout at the bottom of the roof and count vertically up the roof, as shown in the photo below.

Now that you’ve counted every other row, you’ve counted every ten inches. If you want to make things simple (and for the sake of very rough estimating), you might as well round this off to one foot. Now that you’ve counted the width and the height of the roof surface in feet, you can again do some simple multiplication and get the surface area.

You can see how this is simple on a gable roof, but what about on a hip roof? Do you have to measure all four sides? Fortunately, the answer is no. As long as all four sides of the roof have the same slope, a hip roof that is fifty-feet long and twenty feet from eaves to ridge will have exactly the same surface area as a gable roof with the same dimensions.

And what about those roofs that you can’t directly measure at all? You always have the option of measuring the footprint of the house at ground level and adding on a few feet for the eaves, but how can you figure out the surface area? Since the roof is sloped, we’re going to have to multiply the measurement of the horizontal footprint by some sort of factor to get the true surface area. How much do you need to bump it up by? 10%? 25%? Do you remember wondering when that high school trigonometry would come in handy? Well now’s the time.

If you know the rise of the roof and the run of the roof, we can figure out the length of the sloped surface of the roof. The illustration above will demonstrate that, but rather than get involved with a lot of square roots, let’s look at a few sample results in the table below. This table contains a number of common roof slopes, the percentage that must be added to get the surface area and, even easier to use, the multiplication factor that you need to use to figure out the surface area.

So if you figured that a house had a roof with a footprint of about 2,000 square feet and estimated that the slope of the roof was 4-in-12, you would get the surface area by multiplying 2,000 by 1.05 (i.e. adding 5%). In this case the area would be 2,100 square feet. You can see by the chart that low slope roofs are not significantly larger than their footprint area, but that as the slope goes up, the increase in area grows even faster.

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