If you’re planning work on a roof and don’t have the building plans, taking preliminary measurements from the ground allows you to approximate the roof area and estimate some costs. Measuring hip roofs, dormer roofs or roofs on hillside homes from the ground requires experience and special equipment. But, you can measure a gable or flat roof from the ground with the help of an assistant.
Method 1: Counting asphalt shingles to measure your roof area from the ground.
Most asphalt shingles are three feet wide with three tabs. Each tab is got 5 inches of reveal. So by counting these in digital photos you can come up with pretty reliable dimensions. If there is any doubt on the shingle dimensions, you can calibrate the method by measuring a typical shingle in a lower, accessible location.
Method 2. Pythagorean Theorem.
Measure the horizontal run of the roof surface from the ground (‘a’ in the third figure below). Then count the number of shingles to the peak of the roof. Multiple the number of shingles times the width of each shingle (which you can measure at ground level) to get ‘b’ in the figure below. Then calculate the line length of the roof (‘c’ in the figure) using the Pythagorean Theorem; c = square root (a squared + b squared). It is also easy to calculate the roof angle by taking the inverse tangent of b divided by a.
With these simple tricks you can measure your roof area from the ground.
How Roof Pitch is Calculated
To accurately calculate the area of your roof, you must first determine its slope, or pitch. A roof’s pitch is determined by how much it rises vertically for every foot it runs horizontally. Thus, a moderate “6 in 12” roof pitch means that the roof rises 6 inches vertically for every 12 horizontal inches it runs. A “12 in 12” pitch is a steep, 45-degree angle roof. See the image below for more explanation on rise, run and pitch.
Three Easy Ways To Measure Your Roof’s Pitch
1. On a ladder beside the roof, place the level a foot or so up the roof, hold it level, and measure from the 12-inch mark on the level’s bottom, straight down to the roof. If this distance measures 4 inches, you have a 4 in 12 pitch; 8 inches and you have an 8 in 12 pitch.
2. On a ladder at the gable end of your house, place the level against the gable trim, flat against the side of the house. Now measure from the 12-inch mark of the level up to the bottom edge of the gable trim. This distance is the roof’s rise.
3. In the attic, place the level against a rafter with the 12-inch mark on the bottom of the rafter. Measure from the end of the level up to the bottom edge of the rafter. That is the roof’s ris