A gutter’s discharge capacity increases with the depth of water over the outlet. The best way to increase the head is to discharge the open end of the gutter into a rainwater head or sump. Swirl at the outlet reduces its performance, so positioning of the outlet is important.
Outlets must be placed at a distance less than or equal to the outlet diameter from the nearest vertical side of the sump.
Where they are connected directly to the drain, all internal downpipes must be sealed to internal sumps by a compression ring, or similar fitting, and must have access for cleaning at the base. All sump downpipes must be able to withstand a water pressure test with an applied head of 1.5 m of water without leakage.
To avoid any water back-up if the drain capacity is overloaded or obstructed, an air-break should be provided for all downpipes to ensure that water does not back up the downpipe.
All exterior downpipes must discharge freely over a grated gully trap or into an oversize pipe which must be a minimum of 50 mm above the adjacent ground level.
Downpipes fixed at an included angle of less than 105° must have a cross-sectional area equal to that of the gutter or be sized by calculation.
Downpipes must be compatible with the roof and gutter material and must comply with the 15-year durability requirement of the NZBC.
Discharging water off an inert surface onto unpainted galvanised rainwater goods can cause corrosion. See 4.11B Normal Catchment.
Horizontally run PVC downpipes and gutters require a greater provision for expansion than metal, particularly if they are painted a dark colour. Horizontally run PVC downpipes and gutters should have a maximum length of 9 m.
When rainwater is collected into a water tank, there is often not enough distance to obtain adequate fall for one downpipe outlet. In such cases, or whenever the roof design pre-empts a continuous spouting to the tank, it is possible to have several sealed downpipes (some of which can run underground) to discharge into the tank. The outlet discharging into such pipes should be a rainwater head to avoid flooding.
Use this table to select the correct internal dimensions of common downpipe sizes for use in the online calculator at 5.4.7 Gutter Capacity Calculator.
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.
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 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
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
=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.
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.
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.
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
