COP v3.0:roof-drainage; capacity-calculations

5.7 Capacity Calculations 

Calculating drainage capacities of gutters, downpipes, and valleys involve various factors such as rainfall intensity, roof pitch, gutter size, downpipe size, valley angle, etc.  The NZMRM COP provides online calculators to derive the maximum allowable roof area drained under various scenarios.

The values for  5.4.2 Capacity Table for Common Size Downpipes and 5.2.4.2C Maximum Valley Catchment in m² for Areas Having a 50-year Rainfall Intensity <150 mm/h can be found in this PDF document. Responsive online tools for calculating gutter capacity, downpipe capacity and valley capacity are available at www.metalroofing.org.nz/cop/roof-drainage/capacity-calculations.
Note that this site address is used only for convenience if printing calculations to attach to documentation.
This address is not factored into calculations - you must determine intensity from Rainfall Intensity Maps or NIWA's HIRDS tool.
The address is not recorded or shared with any other parties.
Illustration is for explanatory purposes only and is not to scale.
 
vc.A_g DBwFB
Enter 1:X or mm per metre- the calculator will automatically convert
Minimum Fall 1:500, Maximum Fall 1:100
 
1:=  mm per metre
  rads
 
 °
  rads
 °
  rads
 °
  rads
 °
  rads
Select relevant options, which will determine the minimum Short-Term Intensity Multiplication Factor
 
 
Select the appropriate Intensity from the Rainfall Intensity Maps, or use the Hirds-tool from NIWA.
 mm/hr
 
 bends
 
 m
 
For rectangular gutters you can supply custom dimensions, or use pre-supplied manufacturer data
 mm
 
 mm
 
 
 mm
 
 x   mm
 
 mm
 
 
 mm
 
 
 mm
 
Must be less than the upstand, D
 mm
 
 
Illustration is for explanatory purposes only and is not to shape or scale.
 
image/svg+xmlimage/svg+xmlimage/svg+xmlCross-Sectional AreaNett of FreeboardWetted Perimeter
Illustration is for explanatory purposes only and is not to scale.
 
image/svg+xmlWFBthingD
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm²
 
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboard
 mm
 
 
 
 
 
 
 
 m
 
 m
 
 mm
 
 m
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
The minimium Short-Term Intensity Multiplication Factor determined by the application type.
You can increase this manually for critical applications.
 
 mm
 
 
 
 
 
 
 
 
 
 m/s
 
 m³/s
mm
 
 
 
 
 
 
 
 
 
 
This result is the maximum capacity that can be drained by an element of your selected configuration.
Be sure to consider all relevant elements when assesing a roof area.
 

Conditions and assumptions for flat gutters:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions.
  2. Equations valid for gutters with min gradient 1:500, max gradient 1:100.
  3. Bends are accounted for by local loss coefficients (0.5 for each 90° bend).

Conditions and assumptions for downpipes:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Any grates must not restrict flow or site-specific design is to be completed - typically double the number of outlets
  3. Gutters must have fall for downpipe sizing to be valid
  4. Calculations consider weir, orifice and friction effects
  5. Orifice discharge coefficient of 0.61 assumed
  6. Weir coefficient of 0.65 and 75% of outlet perimeter assumed available for weir flow
  7. Minimum pipe gradient of 20% assumed for friction conditions

Conditions and assumptions for valleys:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Minimum height of Type A valley returns to be 16 mm
  3. Minimum freeboard of 20mm mm for valleys below 8°
  4. Minimum freeboard of 15mm for valleys 8° and steeper