Roof slope is a very important aspect and it is considered the primary factor in roof design. The slope of a roof has an effect on the interior volume of a building, the drainage, the style, and the material you use for your covering. For example, if you noticed water collecting on your roof the problem is probably related to the slope. The style is affected too because the framing of the roof changes the slope.
The slope of a roof is often referred to as the pitch. The slope, or pitch, of the roof is determined by the vertical rise in inches for every horizontal twelve inch (12") length (called the "run"). A roof with x rise/12 run slope means that for every 12 inches horizontally (run), it rises x inches. Below are some of the common roof slopes and the terms which classify them.
Steeper sloped roofs are generally more visually pleasing and tend to last longer as well. However, they also cost more because a steep sloped roof requires a taller chimney and more lumber for framing. On average, a 12/12 roof can cost up to 50% more than a roof with a 4/12 slope. Many find that it is worth it though because the roofing material is estimated to last up to 50% longer and will require less maintenance in the long run.
Lastly, roof slope helps determine the appropriate materials for the roof. A roof with a 4/12 pitch will allow products such as shingles or tiles. However, these materials do not work well on low-slope roofs. Likewise, a single-ply membrane or a built-up roof will not be appropriate for a high-slope roof.