Estimating square footage on your roof is a useful skill and needed in many roofing projects, whether you’re renovating or replacing your roof or calculating roofing materials. Accurately determining the size of your roof allows you to plan, budget, and make sure you have the right materials for the job.
While it may seem complicated at first, estimating roof area is actually straightforward.
How to Calculate Roof Area
There are several ways to calculate the roof area, depending on the type of roof you have. The first step is to determine the overall size of your home and the roof pitch (the roof slope height).
Taking Measurements from the Ground
One of the easiest ways to estimate your roof area is by taking measurements from the ground. This method doesn’t require you to climb onto your roof, which is safer and useful if you’re uncomfortable with heights.
You’ll need a measuring tape, ladder, and potentially a helper to ensure that you get accurate measurements.
1. Find the Length and the Width of Your Home’s Exterior Walls
Start by measuring the length and width of one side of your home’s exterior walls. You can do this from the ground, outside your home. These measurements will give you the basic dimensions needed to calculate the area of your roof.
You can measure these dimensions with a normal tape measure. If your home is not a perfect rectangle, or you don’t have a classic gable roof, break it down into simpler rectangular roofs and measure each rectangular section separately.
2. Calculate Your Roof’s Pitch
Roof pitch refers to the steepness or slope of your roof. Understanding the roof’s pitch is essential to accurately calculate the surface area because a flat roof will have a different area than a steeply sloped one.
To determine the pitch, you will need to inspect the roof from below. If you have access to the attic or loft, you can use a level and tape measure for a more precise measurement.
Roof pitch is often represented as a ratio, such as 4/12, meaning that for every 12 horizontal inches, the roof rises by four inches. The steeper the pitch angle, the larger the surface area of the roof.
3. Use the Pitch Multiplier Table
Once you have the pitch of your roof, use a pitch multiplier table to adjust your measurements. This multiplier compensates for the added surface area caused by the slope.
- 4/12 = 1.054
- 6/12 = 1.118
- 8/12 = 1.154
- 12/12 = 1.414
- 18/12 = 1.732
For example, if your roof has a 6/12 pitch and the length of the roof is 20 metres, you would multiply the length by 1.118 to find the actual surface length of the roof. Your actual length would be 22.36 metres.
4. Find the Surface Area of Your Roof
After adjusting for pitch, you can multiply these dimensions by the pitch factor to determine the surface area. For a classic gable roof, multiply the length and width, then multiply by the pitch factor.
Roof area = (length × width) × pitch
For example, if the length of the roof is 20 meters, the width is 10 meters, and the pitch factor is 1.118, the calculation would look like this:
Roof Area = (20 × 10) × 1.118
Roof Area = 200 × 1.118 = 223.6 square meters
This is the total surface area of the roof.
Taking Measurements From the Roof
For more accurate measurements, it’s better to get up on the roof and take direct measurements – particularly if you have a complex roof shape, like hips, valleys, and dormers.
1. Determine the Type of Roof
Before you start measuring, identify your roof type, as different roof designs require slightly different calculations.
- Gable roof: A simple pitched roof with two sloping sides.
- Hip roof: A roof with slopes on all four sides.
- Mansard roof: Two steep slopes on each side.
- Flat roof: A roof with little or no pitch.
2. Start Measuring
Once you’ve identified your roof’s type, you should measure the length and width. For complex roofs or those with roof features, you’ll need to measure each section. Ensure that all measurements (length, width, and height) are in the same units (metres or square footage).
3. Calculate the Area of Each Section
If your roof has multiple sections, break it into smaller sections. Measure each section separately to calculate the area for each.
For example, if you have a classic gable roof with two equal slopes, you will calculate the area for each slope separately and add them together. For a hip roof, for instance, you will measure each of the four slopes individually.
4. Add Up Each Section
After calculating the area for each section, add them together to get the total area of the roof.
Always round up your final roof area calculation to make sure you have enough material to cover your roof. It’s better to have a little extra than to run out in case of inaccuracies or mistakes.
An Example of Calculating Your Roof Area
Let’s say you have a simple gable roof with the following measurements:
- Length = 15 meters
- Width = 10 meters
- Pitch = 6/12
The pitch multiplier table shows us that 6/12 is 1.118. Now, use the formula:
Roof area = (length × width) × pitch
Roof area = (15 × 10) × 1.118
Roof area = 150 × 1.118 = 167.7 square meters
So, the total roof area is approximately 167.7 square meters. If you need to convert your square metres to square feet, use an online converter or simply multiply the square metres by 10.764.
Calculating the Area of the Roof
Estimating roof areas might seem intimidating, but with the right approach and tools, it’s a fairly simple DIY job. Whether you’re measuring a shed roof or your house, it’s good to get an accurate estimate. If you’re unsure, speak to a roofing contractor or professional.
FAQs
How Do I Estimate the Size of My Roof?
To calculate the size of your roof, measure the length and width of your home’s outside walls, making sure that you adjust for the roof’s pitch. Use a pitch multiplier table to calculate the roof’s area.
What Is the Formula for Estimating Roof Area?
The formula for determining roofing is:
Roof area = (length × width) × pitch
This basic formula gives you a general estimate of the roof’s surface area. For more complex roofs, you’ll need to break them into smaller sections and calculate each one separately.
What Is the Average Measurement of a Roof?
A typical roof in Australia will generally be between 120-200 square metres, depending on the design and location. Smaller homes will be around 111-167 square meters, while rural buildings could be as large as 558 square meters.