Slope: the angle of inclination of a roof surface, expressed as a ratio of the rise (in inches) to the run (in feet). The word "slope" is also used to describe the horizontal distance between two points on a land surface. The degree of slope is called its pitch. There are three types of slopes: level, rising and falling. A level slope begins at one end of the building and continues to the other without any change in height or grade. A rising slope gets steeper as it goes up. A falling slope gets flatter as it goes down. The amount of slope a roof takes is called its pitch. Slopes are usually described by their rise over their run. For example, a four-inch rise over twenty-four inches of run results in a 1:12 slope. This means that if you were to draw a line from one end of the roof to the other, the top of that line would be raised 1 inch for every 12 inches of run length. Some people call this a steep slope because it takes more effort to climb than a flat one. Other names for different pitches are ladder, mild, moderate and steep. The type of material used to build the roof affects how it will hold water. Flat roofs are made of asphalt or concrete.