Allowance for Short-Term Intensities

The COP drainage calculator multiplies the ten-minute maximum intensity by a factor to allow for short-term fluctuations. This minimum factor varies by gutter location as follows.


Short-Term Intensity Multiplication Factors

ApplicationGutter MultiplierDownpipe Multiplier
With OverflowNo Overflow
Internal Gutters Residential3.12.13.1
Internal Gutters Commercial2.21.52.5
External Gutters — no Overflow2.51.72.5
External Gutters — with Overflow111
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Short-term Intensity Factor Explanation

These are minimum factors; higher factors may be applied at the designer’s discretion.

  • Valleys, Penetrations, and Internal Gutters Residential have a minimum factor of 3.1 because failure of these gutters is likely to cause damage to internal elements. Where a 2% probability of flooding is unacceptable, a higher figure should be used.
  • Internal Gutters Commercial have a minimum factor of 2.2 as failure of these gutters is less likely to cause severe damage and water run time may be longer. Short runs and steep pitches will reduce run time. (At 250 mm/hr intensity and 3 degrees pitch, rain will take 2 minutes to travel 15 metres). For short runs, steeper pitches and where the probability of flooding of 2% is unacceptable, a higher figure should be used.
  • External gutters no overflow have a minimum factor of 2.5, providing the building has a soffit. Otherwise, they should be treated as an internal gutter.
  • External Gutters with overflow have a minimum factor of 1, provided the building has a soffit, as occasional overflow is not likely to cause damage. To qualify as drained, the back of the gutter must be below the fascia height and it must have a gap of at least 3 mm between the gutter and the fascia or cladding. This gap must be maintained in all areas, including internal angles. External gutters to buildings without soffit must be provided with a 10 mm drainage gap or be designed as an internal gutter.

For convenience, ARI maps are included in the calculation section which includes tables for gutter and valley capacity for different rainfall intensities.


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Revision Category: 
3 - Substantial change to recommendation
Revision Detail: 

Updated Short-term Intensity Factor Explanation.