How to Calculate the Size of Your Roof
How to Flip Houses / How to Calculate the Size of Your Roof
In this article, we will discuss how to the steps to calculating the size of your roof to help you accurately estimate roofing costs.
How Roofs are Measured & Estimated
Roofing contractors measure and estimate the size of roofs in a unit of measure called a square. A square is equal to 100 square feet or a 10ft x 10ft area.
So for example, if your home was perfect square of 50 feet wide and 50 feet long, that would equal a surface area of 2,500 square feet or 25 squares.
50ft x 50ft = 2,500 sf
2,500 sf / 100 sf per square = 25 squares
So for example, if your home was perfect square of 50 feet wide and 50 feet long, that would equal a surface area of 2,500 square feet or 25 squares.
50ft x 50ft = 2,500 sf
2,500 sf / 100 sf per square = 25 squares
Unfortunately, most homes are not a perfect square and have pitched roofs with complex shapes, valleys & dormers, which makes the exact measurement of roofs more difficult.
Calculating the Size of Your Roof
There are two different ways that you can calculate the size of your roof:
- Measuring on the Roof
- Estimating the Roof Size from the Ground
Measuring on the Roof
For the most accurate calculation you will need to get on the roof and draw a diagram of the roof shapes and measurements. Once you have the roof shapes measured, calculate the area of each shape and add up all the areas to calculate your total roof area.
Estimating the Roof Size from the Ground
While measuring from the ground isn’t as accurate as getting on the roof, it is faster, safer and gives you a rough calculation of the property roof area.
Assuming you have a rectangular shaped house, you can simply multiply the length by the width of the house.
Unfortunately, houses are not typically a perfect square or rectangle. If you have a house that is a complex shape, you will have to breakdown the floor plan into segments to calculate the building footprint area.
Assuming you have a rectangular shaped house, you can simply multiply the length by the width of the house.
Unfortunately, houses are not typically a perfect square or rectangle. If you have a house that is a complex shape, you will have to breakdown the floor plan into segments to calculate the building footprint area.
Building Footprint CalculationIn the example on the right the floor, plan has been broken down into 4 different segments. The area of each segment is calculated by multiplying the length by the width.
The 4 segments areas are then added together to calculate the building footprint. Next, you need to convert the property square footage to squares by dividing by 100. Building Footprint Calculation Area 1 = 25 FT x 40 FT = 1,000 SQFT Area 2 = 15 FT x 15 FT = 225 SQFT Area 3 = 10 FT x 20 FT = 200 SQFT Area 4 = 10 FT x 20 FT = 200 SQFT Total Area = 1,625 SQFT |
Roof Pitch
Once your calculate the roof footprint, you will need to factor in your roof pitch or roof slope. Roof pitch or roof slope is the number of inches the roof rises vertically for every 12 inches it extends horizontally.
The slope of a roof is split up into three categories.
To factor in your roof pitch, you can use the roof pitch multiplication table on the right: Most roofs are 4/12 up to 7/12, so when in doubt a safe multiplier would be 1.15 (7/12). |
In our previous example, we measured the property's footprint to be 1,625 square feet. If we assume that our roof's slope is 6/12, then we need to use the 1.118 multiplier.
1,625 sf * 1.118 = 1,625 / 100 square feet per square = 16.25 squares
16.25 * 1.118 = 18.1675 squares
1,625 sf * 1.118 = 1,625 / 100 square feet per square = 16.25 squares
16.25 * 1.118 = 18.1675 squares
Material Waste
When your roofing contractor purchases & installs your roofing materials, the roofer will buy an extra 10% of material to cover waste of scrap materials.
So in the above example, the roofer needs to purchase 16.25 squares of roofing, with an additional 10% of material that will be scrapped/wasted.
18.1675 x .10 = 1.81675 extra squares in waste
18.1675 + 1.81675 = 19.98425 squares or 20 squares
So in the above example, the roofer needs to purchase 16.25 squares of roofing, with an additional 10% of material that will be scrapped/wasted.
18.1675 x .10 = 1.81675 extra squares in waste
18.1675 + 1.81675 = 19.98425 squares or 20 squares
Contingency
A final factor that you may want to consider is an allowance for contingency which will cover any uncalculated square footages such as roofing overhangs, valleys, gables, etc.
Generally, if you have a complicated roof structure with may overhangs, valleys, gables & elements, a 10% contingency would be recommended.
19.98425 x (1+ .10) = 21.982675 squares of roofing or 22 squares
Generally, if you have a complicated roof structure with may overhangs, valleys, gables & elements, a 10% contingency would be recommended.
19.98425 x (1+ .10) = 21.982675 squares of roofing or 22 squares
Quick & Easy Rule of Thumb
So maybe all of this math is a bit overwhelming, but now you know how professional roofing contractors estimate the size of your roof.
With that said, since we now know how professional roofers estimate costs, we can now simplify this down to a quick rule of thumb.
We know that we first need to calculate the square footage of the roof footprint. Once we find the footprint of the roof, we need to factor in the roof pitch, waste factor & contingency.
Earlier, we discussed the average roof is 4/12, which added about 5% (1.05 multiplier) to the square footage. With the added in amount for waste at 10% & contingency at 10%, we’ve roughly added about 25% to the calculation.
For this reason, as a quick rule of thumb, you reasonably can take the building footprint square footage & multiply it by 1.25 to get the roughly calculated roof area.
Roof Size = Building Footprint x 1.25
Roof Size = 1,625sf x 1.25
Roof Size = 21 Squares
With that said, since we now know how professional roofers estimate costs, we can now simplify this down to a quick rule of thumb.
We know that we first need to calculate the square footage of the roof footprint. Once we find the footprint of the roof, we need to factor in the roof pitch, waste factor & contingency.
Earlier, we discussed the average roof is 4/12, which added about 5% (1.05 multiplier) to the square footage. With the added in amount for waste at 10% & contingency at 10%, we’ve roughly added about 25% to the calculation.
For this reason, as a quick rule of thumb, you reasonably can take the building footprint square footage & multiply it by 1.25 to get the roughly calculated roof area.
Roof Size = Building Footprint x 1.25
Roof Size = 1,625sf x 1.25
Roof Size = 21 Squares