Pitched roof angle.

The generally accepted definition of a pitched roof is at an angle of between 15 to 70 degrees.

Roof angles below 15 degrees are often classified as flat roofs while roof angles greater than 70 degrees are often classified as mansard roofs or walls.

Selecting the right pitched roof angle is critical for the type of roof tile you will be using.  Most roof tiles or slates will have a minimum & maximum roof pitch stated for guaranteeing of weather tightness and fixing methods.

It can also work in reverse, you may have to select the right tile or slate for the roof pitch angle you may be forced to use for Planning reasons for example. Shallow roof pitch angles are often somewhere between 15 and 35 degrees.  Steep pitch roof angles are somewhere between 35 & 70 degrees.