# Downpipe Capacity Calculator

The values for Capacity Table for Common Size Downpipes can be found in this PDF document. A responsive online tool for calculating downpipe capacity available at https://www.metalroofing.org.nz/downpipe-capacity-calculator.

Before using this calculator, please read Roof Drainage Design.

To calculate downpipe capacity, select the type of building, type of gutter and overflow (yes or no). Complete the rest of the data by changing 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 Rainfall Intensity Maps.

Note that this site address is used only for convenience if printing calculations to attach to documentation.

This address is

The address is not recorded or shared with any other parties.

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 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.

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

Minimum Fall 1:500, Maximum Fall 1:100

1: = mm per metre

rads

bends

m

Minimum 1°, Maximum 60°

°

rads

Secondary pitch only needs to be entered manually if it is different to the main Roof Pitch

°

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 scale.

Describe the product: this does not control the calculation which relies on you entering accurate data

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

°

rads

= max ( RS , RS2 )

°

rads

= min ( RS , RS2 )

Using Martindales Formula:

°

rads

= atan ( tan ( A1 ) / tan ( A2 ) )

°

rads

= asin ( cos ( A1 ) * cos ( A2 ) ) + pi()/2

= cos ( A2 ) * cos ( A1 )

°

rads

= asin ( sC7 )

= tan ( A2 ) * sin ( aD )

°

rads

= atan ( tR1 )

= tan ( aD ) * csc ( R1 )

°

rads

= atan ( tC6 )

= tan ( pi()/2 - aD ) * csc ( R1 )

°

rads

= atan ( tC6' )

°

rads

= pi()/2 - C6'

°

rads

= pi() - C6 - C6' - C5'

°

rads

= C6 + C6'

Using WSP Sketch:

=W * sin ( C5' )

=D * cos ( C5' ) - FB

=IF ( ( h1

_{max}+ h3 ) < h1_{max}, h1_{max}+ h3, h1_{max})=W * sin ( C5' )

=IF ( ( h1

_{max}+ h3 ) < h2c,h1_{max}+ h3,h2_{max})=IF ( ( h1

_{max}+ h3 ) < h2_{max},0,h1_{max}+ h3 - h2_{max})=0.5 * h1 * tan ( PI()/2 - C5 ) * h1

=0.5 * h2 * tan ( Beta - PI()/2 + C5; ) * h2

=IF ( ( h3 > 0) , ( W * cos ( C5; ) - 0.5 * h3 * tan ( C5; ) ) * h3 , 0 )

=( W * cos ( C5' ) - 0.5 * h4 * tan ( C5' ) ) * h4

=A1 + A2 + A3 + A4

=h1 / sin ( C5 )

=h2 / sin ( C5' )

=IF ( ( h3 > 0 ) , h3 / cos ( C5 ) , 0 )

=h4 / cos ( C5' )

=WP1 + WP2 + WP3 + WP4

=h2 * tan ( PI()/2 - C5 ) - IF ( ( h3 > 0 ), h3 * tan ( C5 ) , 0 )

=h2 * tan ( Beta - PI()/2 + C5 ) - h4 * tan ( C5')

=FWSW13 + FWSW24

mm

x mm

mm

Select Manufacturer (if applicable) and Profile

Describe the product: this does not control the calculation which relies on you entering accurate data

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

m²

m²

m²

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.

Be sure to consider all relevant elements when assessing a roof area.

m²

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.

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.

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.

Be sure to consider all relevant elements when assessing a roof area.

m²

#### Conditions and assumptions for flat gutters:

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

#### Conditions and assumptions for downpipes:

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

#### Conditions and assumptions for valleys:

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

#### Conditions and assumptions for maximum run:

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

#### Conditions and assumptions for penetrations:

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

#### Conditions and assumptions for level spreaders:

- Mannings n assumed to be 0.014 to represent long term friction conditions
- Only valid for supercritical flow (most roofs)
- Corrugate Profiles
- No discharge to lap row
- One discharge hole per second trough
- Assumes flow to top of profile (no freeboard)

- Trapezoidal or Trough Profiles
- May discharge to lap row
- One discharge hole per trough
- Assumes flow to capillary groove of profile

/cop/roof-drainage/outlets-and-downpipes#downpipe-capacity-calculator

Revision Category:

2 - Editing and rearrangement

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