How to Calculate Your Roof Area
Knowing how to calculate your roof area is an essential first step when planning a new metal roof. Accurate measurements help estimate materials, reduce costs, and avoid delays during installation. At Classic Products Roofing Systems Inc., we provide guidance for homeowners who want to take a hands-on approach while emphasizing the benefits of professional measurement.
Why Calculate Your Own Roof Area?
Calculating your roof area allows you to:
- Determine the right amount of roofing materials
- Avoid over-purchasing or shortages
- Compare contractor quotes effectively
- Plan your project budget with confidence
Knowing your roof’s square footage can help protect against inflated estimates and ensure your project runs smoothly from start to finish.
What You’ll Need
- 100’ tape measurer with grout hook (in Canada building products are still sold in square feet)
- Graph paper with ¼” squares on a clipboard
- Pencil and eraser
- Pitch finder tool or smartphone pitch calculator app
Tips for Getting Started
- Tip 1: Measuring to the nearest inch along eaves and across gables can be done from the ground – just don’t forget the overhang.
- Tip 2: If you know the dimensions of your previous asphalt roofing job, you can use this info to cheat a little. Typical 3 Tab asphalt shingles have tabs that are 1 foot wide with a vertical exposure of 5 inches. Make sure to physically verify these dimensions by measuring 4 or 5 courses. Counting the shingles can give you eave to ridge dimensions from the ground.
- Tip 3: 100 Square Feet = 1 Square in Roofing Terms
Sketch the roof outline using ground measurements as shown in Figure 1 below. Knowing the longest dimension first will help you to scale your drawing appropriately. 
Figure 1 – Sketching Your Roof Plan
Measure and sketch out dormers, hips, valleys, diagonal distances and protrusions or overhangs. Chimneys, vents, skylights and pipe sizes will need to be measured to calculate trims and flashings needed for the job.
Pro Tip: You may need binoculars to count units of existing roof materials in unreachable areas. If different pitches exist on adjacent sides of a valley or hip, it’s length can be determined by treating the hip or valley as the diagonal (hypotenuse) of a right triangle on the roof pane next to it.
Pitch refers to the angle of the roof, or how steep it is. Measured by inches of rise (vertical measure) over 12 inches of run (horizontal measure) In the gable diagram in figure 2. (below) there are 4 units of rise and 6 units of run. This can be converted to units of rise over 12 by understanding that cross products in step 2 must be equal. There Therefore 4 x 12 must equal 6 x X. Step 3 shows that X must equal 48 divided by 6. Therefore X equals 8 and the pitch is 8/12.
Figure 2 – Calculating Roof Pitch
A Note About Pythagorean Theorem (Yes, You Need it in Real Life)
If you know the measurement of two sides of a right triangle, but not the third side, then the Pythagorean Theorem from 8th Grade geometry is your best friend. In Figure 2 above, rise squared (multiplied by itself) plus run squared equals the diagonal squared. In the diagram, a2 + b2 = c2. If the rise is 42 + 62 = c2, then 16 + 36 = c2 – So c = the square root of 52. Pythagorean Theorem tells us that the gable (diagonal) length is 7.21’ or about 7’ 3”.
On a simple hip or gable roof, multiplying the eave to ridge length by eave length will give the area to be multiplied by pitch factor. Ridge vent openings are best finished with coil rather than partial shingle. Using flat coil stock allows the ridge caps to lay flat and can also be formed with an extra water stop at the ridge opening.
Area for the roof in Figure 3 is 1784 ft2 The blue dash line divides the roof into 2 rectangles. There is a small area calculated as 8/12 that is actually 4/12. On a small area such as this, it is better to err on the side of the higher pitch since it should not significantly affect the total.
Figure 3 – Calculating Simple Roof Areas
Pro Tip: Pythagorean Theorem can be used to calculate the length of the valleys. A general rule of thumb is to add at least 12” lineal inches of valley material to be safe.
Most roofs in Canada would be considered on the complex side. Multiple adjacent pitches, dormers, and turrets are common. Keep in mind that an experienced metal roofing contractor will need to factor in extended labour time for areas of your roof that will require precision metalworking skills.
Start by finding the “footprint” area and multiply by the correct slope factor determined by pitch (see Pitch Factor Chart bottom of page). In Figure 4 below, the dashed blue lines give a simplified view of the “footprint”. Multiply L x W to calculate the area for each rectangular section. Note the area of 77ft2 that needs to be subtracted for the recessed area.
Once the “footprint” including overhang areas have been determined for each area, find the appropriate slope or pitch factor in the chart below. For example, a roof area with an 8/12 pitch has a slope factor of 1.202 so 2053ft2 x 1.202 = 2468ft2 total roof area.
A Note about complex roofs: Complex roofs need more accessories and trims than “easy” roofs – resulting in a slightly higher total cost/square foot.
Figure 4 – Calculating Complex Roof Areas
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 9’8” (9.62’ rounded up to nearest inch). Note that this run can also be multiplied by the hip/valley factor of 1.563 to get a valley length of 12’6”.
Record all your calculations on your drawing as seen in Fiigure 5 below.
Figure 5 – Completed Roof Drawing
Why Work With Classic Products Roofing Systems?
While measuring your roof is helpful, Classic Products Roofing Systems provides expert roof measurement, material take-offs, and installation services. Since 2004, we have been a Canadian-owned and operated metal roofing company, serving Ontario homeowners with professional guidance and high-quality roofing solutions.
Working with our team ensures accurate estimates, reduced waste, and a smooth installation experience.