HydroCAD® Stormwater Modeling - Since 1986


Modeling a roof or roof drains

To model a roof requires consideration of two separate issues:  The runoff from rainfall, and in the case of a flat roof, the possible storage and retention of water.

For all roofs, the runoff should be modeled using a subcatchment of suitable area, curve number, and time-of-concentration.  A curve number of 98 is commonly used for impervious surfaces, such as pavement or roofing.  The time-of-concentration may be evaluated by several methods, although using a value of zero is suitable for many situations.

If there is any possibility of a roof retaining or detaining water, then it should be modeled as a subcatchment (as described above) flowing into a suitable "pond."   The pond's stage-storage characteristics are easily determined by using the roof's area at several depths of inundation.

The stage-discharge curve can usually be modeled with one of HydroCAD's standard outlet devices, such as a horizontal orifice or grate.  If there are several identical outlets placed at the same elevation, these can be modeled with a single device by using an appropriate discharge multiplier.  In cases where the downspout inlet is the point of control, the downspout itself may not require direct inclusion in the model.

A simple approach for modeling a flat roof that may detain water:

1) Use a single subcatchment with CN=98 and Tc=0 to model the roof runoff.

2) Route the subcatchment into a pond.

3) Define the available pond storage by entering surface areas at two or more elevations.

4) Define a horizontal orifice for the pond's outlet.

5) Set the desired rainfall parameters and view the results.

Modeling a roof garden

The previous discussion assumes a curve number of 98 for an impervious roof surface.  To allow for the infiltration and retention effects of a roof garden, the CN value can be estimated by using the SCS equation for potential maximum retention.  Details here.

Also read about modeling rain gardens. (bioretention areas)

 
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