Roof Runoff Coefficient

Multiply the area of your roof by the total obtained in step 2 as outlined in the following equation.

Roof runoff coefficient.

Runoff coefficients when calculating the amount of runoff that can be harvested from a roof it is common to include a runoff coefficient. Alternatively the runoff coefficient is the factor by which depending on the type of surface the effective impermeable area is calculated. Using the runoff coefficient scientists and hydrologists can calculate how much water passes over a given area per second. The runoff coefficient describes the percentage of rainfall which runs off a particular surface during a storm event.

The runoff coefficient c is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. This coefficient accounts for the fact that some roof surfaces are more efficient than others at collecting rainwater. It is a larger value for areas with low infiltration and high runoff pavement steep gradient and lower for permeable well vegetated areas forest flat. Surfaces that are relatively impervious like streets and parking lots have runoff coefficients approaching one.

Rational method runoff coefficient applications. The physical interpretation of the runoff coefficient for a watershed is the fraction of rainfall on that watershed that becomes storm water runoff. Thus the runoff coefficient must have a value between zero and one. According to the rational method the widespread implementation of the green roofs facilities causes a reduction of the runoff coefficient of about 20 essentially originated by the increase of the permeable surfaces accordingly to the sustainable stormwater management principles.

Green roofs can help to reduce the surface run off entering the drainage system. For example it is 0 9 for metal or tile roofs and up to 0 3 for green roofs. Surfaces with vegetation to intercept surface runoff and those that allow infiltration of rainfall have lower runoff coefficients. The higher the value the greater the quantity of water which will need to be dealt with by the drainage system.

The average runoff coefficient is in this case equal to 0 45. Green roofs storm water run off. In summer a green roof can typically retain between 70 80 of the runoff. Typical values are given below.

A runoff coefficient is a number that relates the rainfall rate and runoff rate. There are two ways you can find the runoff coefficient.