COP v3.0:roof-drainage; downpipe-spreaders

5.7 Downpipe Spreaders 

All downpipes that discharge onto a lower roof must have a spreader to dissipate energy and ensure wide distribution of the water. A spreader should be used over multiple troughs.

For corrugate, a spreader should not discharge into a lapped trough. When using the COP calculator, discharge may be into a lapped trough of a trapezoidal or a trough profile.

The area of discharge holes from a spreader should equal the cross-sectional area of the downpipe.

The 5.7.1 Maximum Area Above Spreader Calculator enables users to determine the maximum upper roof area that a lower roof can discharge for a given combination of rainfall intensity, roofing profile, and lower roof pitch.

 

5.7.1 Maximum Area Above Spreader Calculator 

Before using this calculator, please read 5.2 Roof Drainage Design.

Calculate the maximum area above a spreader by entering the values in the designated fields.

For an explanation of each element, please click on the corresponding question mark.

For rainfall intensities, refer to NIWA’s HIRDS tool or the 5.2.2 Rainfall Intensity.

A responsive online tool for calculating Maximum Area Above Spreaders is available at https://www.metalroofing.org.nz/maximum-area-above-spreader-calculator.
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.
Select the appropriate Intensity from the Rainfall Intensity Maps, or use the Hirds-tool from NIWA.
 mm/hr
 
Select relevant options, which will determine the minimum Short-Term Intensity Multiplication Factor
 
 
The minimium Short-Term Intensity Multiplication Factor determined by the application type.
You can increase this manually for critical applications.
 
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
 bends
 
 m
 
 
 °
  rads
 m
 
Select whether runoff will drain on both sides of penetration or just 1;
 
 m
 
 each
 

For rectangular gutters you can supply custom dimensions, or use pre-supplied manufacturer data
 
 
 
You can select Standard Corrugate, input profile dimensions for Trapezoidal, or use pre-supplied manufacturer data
Illustration is for explanatory purposes only and is not to shape or scale.
 
Wetted Perimeter=49Pitch=76Cross Section Area=272Free Surface Width=42Depth=17Wetted Perimeter=85Pitch=76Cross Section Area=585Free Surface Width=76Depth=17
Illustration is for explanatory purposes only and is not to shape or scale.
 
DepthPitchCrestCapillary DepthPanDepthPitchCrestPan
Illustration is for explanatory purposes only and is not to shape or scale.
 
Wetted PerimeterPitchCross Section AreaFree Surface WidthWetted PerimeterPitchCross Section AreaFree Surface Width
 mm
 
 mm
 
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
 
 °
  rads
 °
  rads
 °
  rads
 
 mm
 
 
 mm
 
Must be less than the upstand, D
 mm
 
 
 mm
 
 x   mm
 
 mm
 
Select Manufacturer (if applicable) and Profile
 
Pitch, or centre-to-centre measurement. Can also be calculated by (Effective Cover Data) ÷ (Number of Pans).
 mm
 
Width of the pan.
 mm
 
Calculated result from (Pitch) - (Crest).
 mm
 
Width of the crest (top of rib).
 mm
 
Total depth of profile.
 mm
 
Depth of profile from the pan to the height of the capillary tube.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm²
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 
Data provided by a manufacturer, especially for irregular profiles.
 mm
 

 
 
 
 
 
 
 m
 
 m
 
 mm
 
 m
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 mm
 
 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 assessing a roof area.
 
This result is the maximum length of roof that can be drained by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
 m
This result is the maximum area that can be drained above a penetration by your selected configuration.
Be sure to consider all relevant elements when assessing a roof area.
This result is the maximum area that an upper roof area can drain using a spreader of your selected configuration.
Be sure to consider all relevant elements when assessing 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

Conditions and assumptions for maximum run:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)

Conditions and assumptions for penetrations:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Where Both Sides selected, assumes an even split of flow to either side of penetration

Conditions and assumptions for level spreaders:

  1. Mannings n assumed to be 0.014 to represent long term friction conditions
  2. Only valid for supercritical flow (most roofs)
  3. Corrugate Profiles
    1. No discharge to lap row
    2. One discharge hole per second trough
    3. Assumes flow to top of profile (no freeboard)
  4. Trapezoidal or Trough Profiles
    1. May discharge to lap row
    2. One discharge hole per trough
    3. Assumes flow to capillary groove of profile