The illustration at page top and used again below, courtesy of Carson Dunlop & Associates, summarizes the ranges of roof pitch or slope for flat, low-slope or conventional or "steep slope" roofing. Our photo at right illustrates a roof whose slope has become irrelevant after the building collapsed. I suspect the roof in the photo was a bit steeper before the building fell in.
Actually most flat roofs are not dead flat and in good design also include slope towards their drains.
Flat roofs (0" to 2" in slope) are flatter than low sloped roofs and pitch just enough to drain water. In our photo (left) the roof slopes less than 1" per foot - notice that dark ponding area at the center of the photo.
Roofers express roof slope as "rise" or "pitch", measured in inches of vertical rise per foot of horizontal distance or "run". So a 3-inch rise roof, also described as a 3 in 12 roof, means that for every 12" (or foot) of horizontal distance, the height of the roof increases by 3".
Our sketch (left) shows the relationship between horizontal distance or "run" and roof slope or "rise". [Click any image to see an enlarged, detailed version.]
While roof slope is typically expressed in "rise", it can also be expressed in degrees or in percent of slope. A 3-in-12 roof rises 3" for every 12" of run. That's the same as a 14 degree slope, or a 25 % slope. Why is the roof slope 25%? 3" of rise per 12" of run is the same as 1" of rise per 4" of run or 1/4 = 25%.
Steep slope roofing is defined as any roof pitched greater than 3 in 12 or 14 degrees or 25%. Roof slope affects roof life (steeper roofs drain better so may be more leak resistant), as well as roof installation cost (steeper roofs are harder or even impossible to walk-on without using roof jacks or scaffolding, increasing labor costs to install roofing).
Our photo (above-left) shows a steep slope slate roof. You can see by eye that this is certainly more than 3" of rise for every foot of horizontal distance or run of the roof slope, and you'll also see that the slate roofers were using roof jacks to work on the roof surface.
Low slope roofing is defined as any roof pitched at 3" in 12" or 14 degrees or 25% slope or less. Low slope roofing in other texts refers to roofing systems for pitches below 4" in 12" of slope. See LOW SLOPE ROOFING and see ROLL ROOFING, ASPHALT, also see MODIFIED BITUMEN ROOFING for examples of low-slope roofing designs & materials.
Flat roofing is roughly horizontal or "flat" but in fact very few "flat" roofs are really flat, either because it is difficult to build a dead flat surface over a building, or more importantly because even "flat" roofs need to drain water to avoid ponding and leaks.
So most "flat" roofs have at least a little slope either towards one or more roof edges or towards roof drains. So "flat" roofs are really "low-slope" roofs most of the time. To avoid ponding and leaks, flat roofs typically have a nominal drainage slope of 2% to 4%.
Most metal roofing systems can be installed on slopes of 3:12 and greater and standing-seam systems from 2:12 and greater.
Special standing-seam systems designed for slopes as shallow as 1/2 :12 require field crimping machinery and have sealant in all seams. The height of the ribs at seams and whether they are protected with a sealant affect how weathertight a roof will be under extreme weather.
Clay roofing tiles are installed on slopes as low as 4/12, restricted to 6/12 in some jurisdictions. Our photo (above left) shows a low slope clay tile roof in San Miguel de Allende, Mexico. This roof was built without sufficient pitch (about 2/12) and it leaks badly during heavy rains, as you can see by our photograph of the roof's under-side (above right). Raising the high end of this shed roof a few inches will improve the roof drainage and stop the leak problem.
Where clay tiles are installed on low slope roofs (less than 4/12) for aesthetic reasons, install a waterproof membrane on the roof surface below the tiles.