Calculating the roof area of a property boils down to simple mathematics. There is no need to risk clambering around on the roof with a tape measure or falling off a ladder!
What you will need:
- Pen and paper
- Calculator
- Square footage of the house
- Pitch of the roof
The calculation
To arrive at a result, divide the roof area into squares. Only the square footage covered by the roof must be calculated. In other words, if you have a double-story home, only use the square footage of a single floor. If the property is on two levels with a total square footage of 3,000 feet, each level will have 1,500 square feet.
1. Measure the length of the roof – this is the length of the property wall of on which the bottom of the roof rests.
2. Calculate the height of the roof slope by measuring the apex at which both sides of the roof meet.
3. Calculate one side of the roof area by multiplying the length of the roof by the height of the roof slope.
Most homeowners can obtain roof size information from the building plans obtained when purchasing a property. Otherwise, your local council should be able to provide you with copies of the building plans.
Remember, accurate measurements are crucial to avoid unnecessary waste or under-estimating the amount of material needed.
Example of calculation
To calculate one side of the roof area, you must multiply the length of the roof by the slope height of the roof. The roof pitch does not matter once you have calculated these dimensions.
How to measure a flat roof
It is quite easy to calculate the roof area of a flat-roof structure.
Use a tape measure to measure the length and the width of the building from the outside. Now, multiply the length by the width to arrive at the total roof area. For example, the standard size of a two-car garage is 20ft x 20ft. In other words, the roof area will be 400ft.
Measuring from outside the building is very important to arrive at an accurate figure. Taking the measurements inside the property will give you a false result because of the thickness of the walls and will reduce the actual size of the area covered by the roof.
Roofing expert suggestions
Roofing installation experts suggest two ways in which to calculate the area of a roof. The following method does involve climbing a ladder!
What you will need
- Ladder
- Large builder’s level
- Measuring tape
- Marker
- Paper and pencil
Method
Firstly, calculate the pitch of the roof.
- Using a measuring tape, measure 12 inches on the builder’s level and make a mark on the 12-inch line.
- Place the ladder against the wall at the gable end.
- Climb to the top of the roof.
- Place the builder’s level against the gable trim. The level must be held flat against the side of the house.
- Using the tape measure, measure from the 12-inch mark on the level up to the bottom edge of the gable trim. This number represents the rise.
- Each rise measurement is recorded as “# in 12”. This is the pitch of the roof. For example, if the rise is 4 inches, the roof’s pitch is 4 in 12.
Calculating the estimated roof area
- Once you have established the pitch of the roof, divide that number by 12. So, if the pitch of the roof is 4 in 12, divide 4 by 12, giving you a yield of 1/3.
- Square the result. If the number is 1/3, it will be 1/9 when squared/
- Now add 1 to the number – 1/9 + 1 = 10/9.
- Next, calculate the square root of the new number – for example, the square root of 10/9 is 1.05.
- Using the square footage of the property, multiply this number by 1.05 to arrive at the square footage of the roof. A square footage of 1,300, multiplied by 1.05, will equal 1,365 square feet.
Calculating the true area of the roof
For exact roof area measurements, follow this guide.
- Divide the pitch of the roof by 12. If the roof pitch is 4 in 12, you will divide 4 by 12, giving a yield of 1/3.
- Now square the result. If the number is 1/3, squared would yield 1/9.
- Add 1 to your number – 1/9 + 1 = 10/9.
- Next, calculate the square root of the number. The square root of 10/9 is 1.05.
- Using a tape measure, measure the length of the house. Overhangs must be included in the calculation.
- Now measure the width of the house, including overhangs.
- Multiply the length by the width to arrive at the total area. For example, 40 feet x 30 feet equals 1,200 square feet.
- The next step is to multiply the area by the pitch of the roof – 1,200 x 1.05 = 1,260 square feet.
Hips, ridges and waste must also be included. Add 10% of the final number for a gable roof and 17% for a cottage roof. The total number will be 1,386 or 1,474 square feet, respectively.
Roof apps
There are a variety of free roof apps available online. These apps are useful, especially as some offer the advantage of calculating accessories, based on the details of the project. These include:
- Drainpipes
- Guttering
- Edge trims
- Skylights
- Adjoining buildings and walls
Roofing Calculator
An online roof calculator can estimate the area of the roof and the number of materials needed for the project.
There is a “House Base Area” which is the actual area of land covered by the house. For more complicated designs, an app such as an “Area Calculator” can be used.
To avoid miscalculations, particularly when a roof is more complex in design, it is important to measure the areas and dimensions of each section of the roof to arrive at a more accurate total area roof measurement. Remember, the calculator cannot account for complex designs and shapes based on a square footage measurement alone. Instead, the Area Calculator can calculate roof measurements of several simple shapes that, combined, comprise the total area of the roof.
References :
https://www.homify.co.za/rooms/roof