You won’t always need a ladder, but you will need a calculator
There comes a time when you might need to know your roof’s square footage—whether you’re trying to calculate how many roofing shingles you’ll need for a project, plan to install a roof drip edge, or simply want to hang a few Christmas lights. Since roofing often has peaks, it’s not as straightforward as it seems on the floor plan. Nonetheless, with a little math, you’ll find your figure. This guide will show you how to calculate roof square footage no matter the type of roof.
Most homeowners don’t think about their roof’s square footage until they need some repairs or a roof replacement. Understanding the total size of your roof can help you:
Set a budget for roofing projects
Weigh the cost of different roofing materials
Get a more accurate quote from contractors
Know how many roofing materials to purchase
When it comes down to it, the cost of a roof replacement is largely dependent on the square footage of the roof. If you measure your roof and discover more square footage than expected, you might need to choose an alternative roofing material to stay within your budget. Similarly, roof truss costs will rise with more space. It’s important to know what to expect before you dive in.
While you can always climb on your roof with a measuring tape to get an exact square footage, it’s much easier to get your calculations from the ground. An approximate square footage is often enough if you’re looking to create a rough budget for a project. Other times, like if you’re building roof trusses or ordering shingles, you might need a more accurate figure. These steps will show you how to measure a roof.
Looking at your home’s square footage is the simplest way to get the approximate square footage of your roof. It’s not going to be totally accurate if your roof is sloped, irregularly shaped, or has some overhang, but it can give you a general idea. To get the square footage:
Measure the length of your home
Measure the width of your home
Multiply the two measurements together
If your home isn’t a perfect rectangle, you’ll need to break it up into geometric areas. Get the square footage of each area, then add the numbers together.
Once you know your home’s square footage, you’ll need to know the roof pitch to get the most accurate roofing measurement. The pitch is similar to the slope but not exactly the same. Rather, the pitch is the vertical rise of your roof divided by the horizontal run. In other words, you’re looking at how many inches your roof rises per foot. So, a roof pitch of 5/12 means your roof rises 5 inches for every 12 inches.
You may be able to find your roof pitch on blueprints or building plans. If not, you’ll need to calculate it, which requires climbing onto your roof. Keep in mind that working on a roof is extremely dangerous, and you shouldn’t consider it unless you really, really know what you’re doing. Follow these steps:
Use a measuring tape to mark 12 inches (1 foot) on a large level.
Set a ladder against your roof.
Enlist a helper to hold the ladder steady as you climb it.
Once on your roof, hold your level horizontally and place one edge firmly against the roof’s slope.
Adjust the level so it is level, with the edge still touching the roof.
Find the 12-inch mark you put on the level.
Measure the vertical distance between the 12-inch mark on your level and your roof.
Divide that number by 12.
For example, if there are 7 inches between your level and your roof, your roof pitch is 7/12.
Unless your roof is totally flat, you’ll need to account for the slope in the roof’s square footage. To do this, you’ll need to use the roof pitch to find a number called a pitch multiplier.
The pitch multiplier varies depending on the steepness and complexity of your roof. The less severe the slope, the lower the number. Take your roof pitch calculation and use an online roof pitch multiplier table to find the figure. Here are some general figures:
| Roof Pitch | Pitch Multiplier |
|---|---|
| 1/12 | 1.00 |
| 2/12 | 1.014 |
| 3/12 | 1.03 |
| 4/12 | 1.054 |
| 5/12 | 1.08 |
| 6/12 | 1.118 |
| 7/12 | 1.15 |
| 8/12 | 1.202 |
| 9/12 | 1.25 |
| 10/12 | 1.302 |
| 11/12 | 1.35 |
| 12/12 | 1.414 |
| 13/12 | 1.47 |
| 14/12 | 1.537 |
| 15/12 | 1.60 |
| 16/12 | 1.667 |
| 17/12 | 1.73 |
| 18/12 | 1.803 |
| 19/12 | 1.87 |
| 20/12 | 1.944 |
| 21/12 | 2.016 |
| 22/12 | 2.088 |
| 23/12 | 2.162 |
| 24/12 | 2.236 |
Gable roofs are one of the most common types of roofs. They consist of two sloped roof panels with two flat sides. For a walkable roof like this, your slope correction number will likely be on the lower side—think 1.3 or below.
With a hip roof, all sides are sloped. You may need to calculate pitches separately if the slopes differ. Expect your pitch multiplier to be around 1.4 or below.
Some roofs are very steep and complex. If you have a steeper-than-average roof, expect your figure to be higher than 1.6.
Once you have the roof pitch multiplier, it’s time to do some math. To get the total square footage of your roof, you’ll need to multiply your home’s total square footage with the pitch multiplier. The formula should look like this:
Home Square Footage x Pitch Multiplier = Roof Square Footage
For example, a 1,000-square-foot home with a 2/12 roof pitch (and 1.014 pitch multiplier) would have a roof measuring around 1,014 square feet. To get the most accurate number, you can split your roof into geometric sections and use this formula on each section.
Unless you have a very simple roof, you’ll likely need to make a few considerations to get the most accurate roofing measurement. These tips can help you hone your number.
When you're calculating the roof area, you may want to collect the square footage of separate sections and then add the measurements together. This is particularly helpful if different sections have different roof pitches—for example, you will most likely need to do this for a roof with dormers, hip ridges, or multiple sections.
Your home may have dormers, which are separate roof sections over the windows on the upper levels. Make sure you factor them—and any additional feature—into your square footage calculation. You do not have to factor in chimneys or pipes since your contractor will simply work around them.
Most roofs hang slightly over your home’s exterior walls. That means the square footage on your floor plan (or the square footage you measure on your own) will be slightly smaller than your actual roof’s square footage. To get the most accurate roofing measurement, measure the roof overhang and add it to your length and width measurements before calculating the square footage.
Most homeowners can estimate the approximate square footage of their roof. This is especially simple if you already have your home’s floor plans or blueprints. That said, there are times when you may need a more accurate measurement, like if you’re installing metal roofing and need to order an exact amount of materials. In this case, you may want to hire a roofer near you.
A roofing contractor will be able to safely obtain an accurate square footage, the kind you can only get by measuring each section of your roof by hand. For the average homeowner, climbing on a roof, especially a steep roof, is a potential hazard that’s best left to the pros.
You can get a general roofing measurement by figuring out the total square footage of your home. If you have a rectangular home, measure the length and width of your exterior walls. Multiply the numbers together to get the square footage. If your home has sections, you’ll need to get the square footage of each section and add them together.
Keep in mind this is a general estimate. Your home’s square footage is typically slightly smaller than your roof’s square footage. To get a more accurate number, factor in roof pitch and overhang.
Most of the time, a roof’s square footage is not the same as a home’s square footage. It’s actually slightly larger because of the slope and overhang. The only exception is if your roof is flat and flush with your exterior walls. For this reason, it’s best to avoid using floor plans to figure out roofing measurements unless you’re only looking for an estimate.
Roofs are measured in squares because it simplifies the square footage measurement. Because there’s a rounding element, it makes it easier for contractors to order materials and figure out costs. One square is equal to around 100 square feet, so a 1,000-square-foot roof would be ten squares.
