Finding out what the size of your roof is starts out with finding the roof pitch. "Pitch" refers to a roof’s incline. Roof pitch or roof slope is considered the main factor in roof design. It determines the appropriate roofing materials to put atop the roof. High pitches can have shingles or tiles. Low pitch can have rolled or old rock roofs.
Roof pitches are measured by their rise and run. The run is the horizontal length from the outside edge of a perimeter stud wall to the center of the house. The rise is the vertical length from the top of a stud wall to the peak of the roof. A roof's pitch is the rise for every 12 inches it runs. Example: a 4-in-12 roof pitch means the roof rises 4 inches for every 12 inches it runs.
There are three different ways to find the pitch of a roof, both from inside and outside of the home. Using ruler and tape, you can do it anyway of these ways:
Placing a ladder near the roof, you will put a level a foot or so up on the roof, hold it level, and measure from the 12-inch mark straight down. If it’s four inches, you have a 4-in-12 pitch and so forth for other measurements, thereby determining your roof pitch.
On a ladder at the walled part of the pitched roof, place the level against the roof overhang rafter with the 12-inch mark on the rafter's bottom edge. Now measure from the end of the level up to the bottom edge of the rafter. That is the roof's rise. Knowing that will determine what your roof’s pitch since the run is 12 inches by default.
In the attic, put the level against a rafter with the 12-inch mark on the bottom. Measure the inches from the level’s end up to the bottom edge of the rafter. That measurement is the roof’s rise.
Roof Pitch Illustration
A flat roof’s total square footage with no more than a three in 12 pitch can be calculated by simply multiplying the length by the width and rounding to the nearest foot.
For steeper roofs, the best bet is to measure your house at the ground, then add in the roof’s overhang to get the best measurement. If your roof overhangs by so many inches, you’ll add the extra to the overall width and length of the measurement. Then there will be a factor of the roof pitch you multiply to figure out the square footage of tiling, shingles, wood shakes or other material you’ll need to cover it.