Have you tried estimating the size of your roof based on your home's square footage, only to realize the numbers don't seem to match the quotes you're getting from roofing contractors? Are you worried that miscalculating your roof size could lead to inaccurate estimates, surprise costs, or confusion about how much roofing material your home actually needs?
At RoofSmart, we do hundreds of inspections every year where we inspect and measure the potential customer’s roof. That way, they know they are getting the most accurate price estimate for their roof size and unique needs.
In this article, we'll explain why your home's square footage isn't the same as your roof's square footage, walk through the steps for calculating a sloped roof, show you how roof pitch changes the final number, and explain when it's worth using professional measuring tools for complex roof designs.
When trying to calculate the square footage of their roof, a lot of homeowners just provide the square footage of their house. However, this doesn’t work because the roof area depends on things like:
It’s not just about the heated living space. If you only focus on that, you’ll miss the additional space that makes up your roof.
It’s important to accurately measure the size of your roof so that your roofer can order the correct amount of roof material. You don’t want to overpay for a bunch of extra material, nor do you want your roofer to run out of shingles while working on your roof.
A flat roof or a rooftop deck is much easier because you don’t have to account for the slope. But on a sloped roof, you’ll have to figure out what the slope is.
If you don’t already know, you can figure it out by calculating the rise and run of your roof. Typically, a roof is measured by how much it rises every 12 inches. So, a roof that rises 4 inches every 12 inches has a 4:12 slope. You just need to measure how much the roof rises every foot to learn the slope of your roof.
You can do this by measuring on the actual roof, or off to the side. Here’s an example of what that looks like:
Here’s a breakdown of how to calculate your roof’s square footage:
1. Gather numbers and measurements: To calculate the square footage of your roof, you need to know the width and the length (aka the footprint), including the unheated spaces like overhangs. You need to know how steep the roof is, what the overhangs are, and the heated and unheated spaces.
2. Multiply square footage by slope: To accurately calculate the total square footage of your roof, you need to multiply the square footage (including overhangs and other additional space) by the slope. Each slope has a different decimal point to multiply by, so this is when it’s important to know the slope of your roof.
Here are the different slopes and their multipliers:
3. Convert to roofing squares: The final step is to convert your number to roofing squares, which is how roofers calculate the size of a roof. 1 square = 100 square feet, so you’ll need to divide your total square footage by 100. The number you get is the number of squares of material you need for your roof.
You can use Google Earth or another online tool and look at your house to get a sense of what the footprint is. And you can also get a sense of how that relates to the square footage you already know about.
So if you've got a one-level house with a breezeway and a garage, three-foot overhangs, and a 10:12 slope, it's quite a bit larger than just the square footage of the living space.
If your home has multiple levels, you don’t need to calculate the roof space for each floor because it’s stacked. If it's a three-story house, the only part that matters for figuring out how big the roof is is how big the roof is.
If you've got a one-level house with no garage and no space that isn't living space, then you’ll probably be able to figure it out pretty easily because you just add for your overhangs and slope, and it's somewhat close.
But most people are grossly off when they're estimating the size of the roof, especially if they have a complicated roof. Keep in mind that tree cover can interfere with Google Maps’ view of your roof, so if you have trees right next to your roof, then this might not work for you.
|
Roof Pitch |
Approximate Slope Multiplier |
Example: 2,000-Square-Foot Footprint |
|
2:12 |
1.014 |
2,028 square feet |
|
3:12 |
1.031 |
2,062 square feet |
|
4:12 |
1.054 |
2,108 square feet |
|
5:12 |
1.083 |
2,166 square feet |
|
6:12 |
1.118 |
2,236 square feet |
|
7:12 |
1.158 |
2,316 square feet |
|
8:12 |
1.202 |
2,404 square feet |
|
9:12 |
1.250 |
2,500 square feet |
|
10:12 |
1.302 |
2,604 square feet |
|
11:12 |
1.357 |
2,714 square feet |
|
12:12 |
1.414 |
2,828 square feet |
If you have a complex roof layout, the best way to make your calculations easy is to do one of the following:
These two options will take care of the measuring for you so that you don’t have to worry about it. In fact, when we send estimators out to meet with you for an inspection appointment, they measure your roof anyway to make sure that they can give you accurate numbers in the proposal.
The more complex your roof is, the more difficult it will be to calculate your roof’s square footage. Things like dormers and hip roofs, or additional spaces like breezeways, garages, or sheds, add to your total square footage, but might be difficult to account for.
You’ll have to measure all those out individually and then add them to the total.
Knowing how to calculate your roof's square footage is a great first step, but it's only possible if you understand one critical measurement: your roof's pitch. Even a small change in slope can significantly increase the amount of roofing material your project requires, affecting both your estimate and the total cost of your new roof.
While you can estimate a simple roof on your own, more complex roof designs with dormers, hips, valleys, and multiple roof sections often require professional measurements to ensure nothing is overlooked. The more accurately your roof is measured, the fewer surprises you'll encounter once your project begins.
Read our article “What Is Roof Pitch and Why Does It Matter on My Seattle Home?” to learn how roof pitch affects everything from material selection and drainage to installation methods, maintenance requirements, and the overall cost of replacing your roof in the Seattle area.