How is the angle of the roof surface usually expressed?

The angle of the roof surface is conventionally expressed as a fraction with the unit of rise over the length of the rafter. This method provides a clear relationship between the vertical rise of the roof and the horizontal run, which is essential for understanding how steep the roof will be.

This expression allows builders to easily calculate the slope of the roof, making it straightforward to determine other aspects of construction, such as the amount of material needed or how water will run off the roof. It is particularly useful in the construction trades, where practical applications and measurements are paramount.

While expressing the angle in degrees is also common, it does not provide as immediate a practical reference for construction as the rise-to-run fraction does. Other measurement methods, such as inches per foot or ratio between rafters, can offer useful information but do not capture the conventional understanding of roof pitch as comprehensively as the rise-over-run format.