A proper level of slope is needed on horizontal roofs where little to no slope seems to exist, according to the view of the naked eye. In reality, most flat-roofed buildings have some type of slope to ensure proper drainage. Without this slight sloping effect, the weight of the water would become too great during rain storms and the roof would collapse under the weight. A typical roof slope for proper drainage will be approximately 1/4 inch for every 12 inches of roof.
The proper level of slope is necessary when drain pipes are constructed to run horizontally along the side of building before connecting to a vertical drain pipe. This will prevent the water from building up in the drain pipe and either overflowing from the side of the building or possibly building up on the roof area. This is especially true of roofs where all four walls of the building extend above roof level, creating a flat enclosed roof surface.
The slope for horizontal piping can be made based on two primary factors. The first of these is the square footage of the roof itself. The larger the square footage of the roof, the greater the slope. The second consideration to take into account is the size of the pipe being used. As the size of the pipe goes up, so too does the capacity to hold more water at one time, thus creating the need for a greater level of slope to drain the water off faster.
The capacity of pipes often differ and this plays a role in determining how much slope is needed. The city of San Diego requires that roofs be sloped based on the roof's square footage and the size of the pipe. For instance, a roof that is 1,028 sq. ft. and is drained by a 3-inch diameter pipe only needs a 1/8-inch slope. However, a roof that is double that size at 2,055 square feet will require a 1/2-inch slope with just 3-inch pipes. By contrast though, a larger roof with a square footage of 2,350 will only require a 1/8-inch slope if the the pipe size is increased to a 4-inch diameter. A half inch slope is required for a roof double that size.