This section considers the design of water drainage from the time it hits the roof cladding to the time it enters the downpipe. As the design considerations are similar, this also includes the discharge from gutters and troughs within the roof plane, valleys, internal gutters, and external spouting.
The Roof Drainage section gives guidance for compliance with NZBC Clause E1 Surface Water. It describes how to drain rainwater effectively from roofs and gutters.
The Code of Practice provides several interactive calculators:
Safeguard people from injury or illness, and property from damage, caused by surface water
Functional Requirement
Buildings and sitework shall be constructed in a way that protects people and other property from the adverse effect of Surface Water.
Performance
Surface water from an event that has 10% probability of occurring annually that is collected or concentrated by buildings, shall be disposed of in a way that avoids damage or nuisance to other property.
Surface water resulting from an event with a 2% probability of occurring annually shall not enter buildings.
The height of the watertight portion of a gutter or profile above the design water level at maximum rain intensity. This is to allow for wave action or impediments in the sole of the channel that may otherwise cause overflowing.
Gutter
A channel formed to collect and carry water away from a roof, variously described as internal, external, box, eaves, valley, secret, hidden, and raking. Spouting specifically refers to an external gutter.
Hidden Gutter
An internal gutter that runs parallel to the roof pitch. This is commonly found beside penetrations positioned close to the eaves.
Internal Gutter
A gutter inside the building envelope. In the COP terminology, this applies specifically to those that run transverse to the roof pitch. See also Hidden Gutter, Secret Gutter.
Raking Spouting
Spouting that runs at an oblique angle to the roof.
Secret Gutter
An internal gutter that runs at an oblique angle to the roof pitch. This is commonly found where a roof runs into a raking wall or barge.
The objective of roof drainage systems is to maintain a weatherproof building, to minimise the risk of injury or inconvenience due to flooding, and to avoid potential monetary loss and property damage — including to the contents of buildings.
Roof drainage design requires consideration of:
Type of gutter (external, internal, valley, or roof gutter),,
rainfall intensity,
catchment area,
gutter fall,
gutter-cross-sectional area and wetted surface area, and
outlet and downpipe capacity.
This section details specific requirements for the sizing of all drainage components.
The effective catchment area for a gutter is determined not only by the plane area of the roof itself but also by the walls adjacent to the roof. When a wall is discharging on to a roof, half the surface area of that wall (up to a maximum height of 10 m), must be added to the catchment calculation.
The COP calculations are based on the plane area of the roof (which is the sloping surface area of the roof), not the plan area (which is the area covered by the roof).
Wind action can influence effective catchment area, and the COP assumes the worst case scenario, i.e., rain striking the roof at an angle perpendicular to the roof plane.
Rainfall intensity can be taken off the maps for 50-year average return intervals (ARI). When the co-ordinates of a site are known, site-specific values can be obtained using NIWA’s HIRDS tool at https://hirds.niwa.co.nz/
As NZBC E1 requires that rainwater from events having 2% likelihood of occurring annually shall not enter buildings, the COP uses figures for 50-year Average Return Interval, rather than the 10% probability figures published in E1/AS1.
Use NIWA’s HIRDS tool for the most accurate rain intensity figures. The HIRDS tool shows figures for historical rainfall intensities and predicted rainfall based on the anticipated effects of climate change, expressed as Representative Concentration Pathways (RCP) levels.
The increased rainfall intensity in a worst-case scenario is typically 11 – 13% higher than historical levels, mostly occurring under the least intense (RCP 2.6) value. To ensure design calculations account for expected climate change, use the most appropriate RCP level.
A very stringent pathway that assumes CO2 levels peak at 2020 and go to zero by 2100.
RCP4.5
An intermediate scenario where CO2 levels peak by 2040, then decline due to the decreased availability of fossil fuels.
RCP 6.0
A stabilisation scenario, where CO2 emissions peak around 2080, then decline with the deployment of various technologies and strategies.
RCP 8.5
Worst-Case scenario where CO2 emissions continue to rise throughout the 21st century. Thought by some to be based on an overestimation of projected coal outputs.
Rainfall intensity figures quoted on the NIWA site are for maximum intensity over a ten-minute duration. Intensity may vary within this period, and roof drains can overflow quickly when demand exceeds capacity. A 1-minute rainfall intensity can be as much as 4.2 times higher than the 10-minute intensity.
To account for short-term rainfall intensity, various factors should be applied to internal and external gutters, and to drains depending on their location and consequence of overflow. See 5.3.2.3A Short-Term Intensity Multiplication Factors.
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.
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.
In gutters where overflow can enter the structure, it is necessary to have freeboard to allow for wave action, obstructions, and other unforeseen circumstances. 5.4.7 Gutter Capacity Calculator allow for these minimum freeboard values.
The term “gutters” can be applied to all roof drains, but “spouting” refers specifically to external gutters.
Types of gutter:
External gutters – positioned outside the building envelope.
Concealed Fascia-Gutter Systems – gutters installed directly behind a fascia.
Internal Gutters – formed inside a parapet wall or where two connected gables meet at an internal draining point.
Valleys – where two roof planes meet at an angle of less than 180°.
Roof Gutters – where a penetration obstructs and concentrates the flow of water, often into a single pan.
Secret Gutters – where a roof discharges into a raked barge.
The definition of gutters in the COP includes the troughs of a profile adjacent to an obstruction (such as a penetration) or where a secret gutter is required, i.e., at the barge line of a swiss gable roof.
NZBC clause B2/AS1 requires spouting to have a durability of 5 years. In practice, this is rarely commercially acceptable. However, with sound design and reasonable maintenance, a spouting life of 10 years or more is usually achieved when using the same material as the profiled metal roof.
Spouting that is difficult to access for replacement should be specified in more durable, compatible materials.
E1/AS1 does not prescribe a need for a building to have spouting, it merely requires that concentrations of water gathered by structures does not enter the building or cause damage or nuisance to other property. This is traditionally achieved by using gutters and downpipes to discharge roof catchments into stormwater drains.
Minor wall projections such as bay windows and boxed penetrations are treated as part of the wall catchment and are typically excused from requiring spouting and downpipe, provided the plan view surface area of individual projections does not exceed 5 m2.
Small outbuildings such as garden sheds up to 10 ㎡ are also traditionally exempted from requiring spouting and downpipes providing the discharge does not interfere with neighbouring buildings.
Spouting should be installed with the back lower than the fascia board or cladding to allow for draining of overflow water through the gap between the gutter back and the fascia.
A 2 mm gap between the back of the gutter and the fascia will give a discharge area equal to the diameter of a 75 mm downpipe for every 2.2 m of gutter run.
This gap is only totally effective if the spouting is correctly maintained and the gap is free of debris. A designed outlet is preferred, either a gutter bracket creating a minimum 6 mm space stop end, a weir, a raised outlet above the spouting sole, a slotted front, or a low fronted gutter.
A weir stop-end, or an outlet with a top edge above the sole of the gutter, can be used to increase outlet capacity.
All gutters are subject to expansion. Maximum gutter-length is determined by the type of metal and its colour. Where gutters have an allowance for expansion (such as an external gutter on a typical gutter bracket or an internal gutter with sliding clips), lengths should be restricted to 25 m in steel and 12 m for copper or aluminium.
An expansion joint can be either a sump, rainwater head or a saddle flashing. Gutters that are directly through-fastened to the fascia or eaves purlin will not be free to move and should be restricted to a maximum of 12 m. Through-fastened gutters are not recommended as they are difficult to replace.
The spouting bracket system must withstand the potential weight of a gutter full of water. In snow load areas, spouting may be fitted with snow straps and brackets at a maximum of 600 mm centres to withstand the additional potential weight of any snow build-up.
Brackets should be made of compatible material or non-ferrous metal. Brackets for unpainted spouting should be hot dipped galvanised or have an AZ150 or Z450 or equivalent coating. Unpainted Z275 brackets are not acceptable nor are painted or unpainted electro-plated brackets. Brackets for pre-painted spouting must be painted or powder-coated before installation, where they must be colour-matched. In severe marine areas, brackets should be epoxy primed before powder coating or should be formed from non-ferrous metal.
Brackets for external gutters should be located close to all stop-ends, at both ends of sumps and rain-heads at a maximum of 750 mm spacing for gutters less than 180 mm wide, and at 600 mm for gutters 180 – 300 mm wide. Brackets must be installed to provide a 1:500 (2 mm per metre) minimum gutter gradient towards the outlets.
When the back of a gutter is cut down to allow the valley to discharge into it, the gutter capacity is affected. In these cases, gutter calculations should allow for 20 mm less water height, and a min 3 mm spacer should be attached to the back of the gutter (or fascia) at the internal corner to maintain the clearance between the gutter and the fascia.
Concealed gutter systems are bespoke or proprietary systems that run inside the fascia.
The concealed gutter design must ensure that water cannot enter the soffit or overflow into the building if the gutter system outlet becomes blocked.
Overflows must be provided for concealed gutter systems within 1 m on either side of the downpipe to discharge through the soffit, immediately behind the fascia, and be capable of discharging the total catchment area served by the downpipe.
When internal gutters are difficult to replace and their failure could cause major disruption to the building below, they must be made from materials that will last 50 years to comply with the NZBC; metallic coated steel is not recommended for internal gutters that are difficult to replace.
Common internal gutter materials are butyl or other membranes, fibreglass, or non-ferrous metal. Where butyl gutters are used, the metal and flashings should be separated from wet contact with the butyl rubber.
Suitable non-ferrous metals include 0.9 mm aluminium, 0.6 mm stainless steel, and 0.6 mm copper. Contact between coated metal products and copper or stainless steel must be avoided because it will lead to early corrosion. Splashback or runoff from copper onto coated metal can have the same effect.
All internal gutters must have upstands that are hooked or returned. Gutters that return under the eaves are not recommended as this design makes removal for replacement more difficult.
To prevent permanent deflection of the gutter, support for the sole of an internal gutter should be provided by either a plywood lining or by close ribbed sheets of roof cladding, separated by a layer of roofing underlay. Internal gutter support must be strong enough to support the weight of water when at capacity, and if over 300 mm wide, be able to support foot traffic.
Internal box gutters must have a minimum depth of 50 mm at their lowest point, including freeboard. A width to height ratio of 2:1 plus freeboard gives maximum flow as it minimises wet surface area for a given cross-sectional area.
A sharp direction change in flow of an internal gutter will affect discharge capacity. Where two buildings meet at an angle, each gutter must be drained separately, or a specific discharge capacity calculation must be applied.
Internal gutters should have an expansion joint at the stop-end.
Outflows from internal gutters may be scuppers or weirs.
A scupper is formed where an internal gutter discharges horizontally through the side or end wall of a gutter through a restricted opening. If a scupper is the same dimension as the gutter, standard calculations for internal gutter sizing may be used. A scupper in the side of a gutter counts as a right-angle bend when using the Gutter Drainage Calculator. When scuppers have a restricted opening, the size of the opening, not the size of the gutter, determines the effective size of the gutter and its maximum catchment capacity. Scupper apertures are vulnerable to blockage and it is recommended that they are fitted with an overflow to alert the building inhabitants of a problem.
To calculate gutter capacity, select the type of building, type of gutter, and overflow (yes or no). Complete the rest of the data by changing the val 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
1: = mm per metre
rads
bends
m
°
Secret gutter offset from Main Pitch (plan)
m
m
Illustration is for explanatory purposes only and is not to scale.
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.
Illustration is for explanatory purposes only and is not to shape or scale.
Illustration is for explanatory purposes only and is not to shape or 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 freeboardMay be gross of freeboard with overflow along gutter
mm²
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboardMay be gross of freeboard with overflow along gutter
mm
Data provided by a manufacturer, especially for non-rectangular profiles. Must be nett of freeboardMay be gross of freeboard with overflow along gutter
=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.
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
Valley gutters must discharge into a rainwater head, sump, or an eaves gutter. The discharge point must be within 2 m of a downpipe if the catchment area exceeds 50 m².
When the roof pitch is less than 12°, the valley should be made in one piece or the joints must be sealed. To ensure snug fitting, the valley angle should be matched to the pitch of the valley support. Having the valley too open will result in a diminished capacity, and too sharp an angle will make installation difficult.
A change of roof pitch in a valley run will usually result in the change of angle in plan view. The change is acceptable, but the freeboard of the lower valley must be at least 20 mm to allow for turbulence.
Where opposing roofs of different pitches discharge into a valley‚ an asymmetrical valley is required. As these have reduced cross-section area compared to a symmetrical valley at the same (lower) pitch, it is often necessary to increase the valley dimensions. Increasing the depth has the biggest effect on capacity. Greater depth can be gained by using 10 mm ply valley boards, standing purlins on edge, or fitting valley boards flush with the rafter. The consequences that a deeper valley will have on the capacity of the gutter it discharges into must also be considered.
A valley baffle is recommended where the difference in roof pitches exceeds 10°. Valley baffles are also helpful in wooded locations to minimise lodging of debris under the roof overhang.
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.
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
Rainwater heads are situated outside the building envelope and sumps are internally located.
They both serve to increase the head of water entering a downpipe, and to provide an overflow capacity to safely discharge water when downpipe capacity is compromised or exceeded. The overflow should be obvious so discharging water warns the occupant that downpipe capacity has been exceeded or the primary downpipe is blocked.
Rainwater heads must be at least as wide as the gutter and have an overflow (normally a weir type). The cross-sectional area of the overflow must be at least equal to that of the required downpipe size for the catchment being served. The lower edge of the overflow must be at least 25 mm below the sole of the gutter, and the upper edge must be at least 25 mm below the upper edge of the gutter.
Sumps must be at least the same width as the gutter and have an outlet positioned below the sole of the gutter to increase the head of water at the outlet.
Internal sumps must have overflows. These are often a secondary pipe overflow, with the outlet height positioned above the level of the primary outlet. Internal sumps must also be sealed against the sides and sole of the gutter.
An internal sump should have a guard that prevents debris from blocking the outlet. A removable aluminium expanded-metal box can be fitted at a minimum of 40 mm below the sole of the gutter. Because the top is flat, it is unlikely that the entire surface area of the outlet can become blocked, so it is preferable to balloon-type guards. A leaf guard should have a horizontal surface area of at least four times the size of the downpipe outlet area and should be installed at roughly mid-height of the sump depth. Gratings can cause sump blockage, and this can reduce the outlet capacity.
Gratings or guards should be designed so that any debris will float, and hail, or obstructions, such as a tennis ball, will not wedge and block the guard. Gratings or guards should be cleared of accumulated debris regularly as part of normal maintenance.
Overflows must discharge clear of the building to clearly show that downpipe capacity has been exceeded; it should be an obvious indication that the gutters need maintenance.
The overflow opening of a rainwater head from an external gutter must have a cross-sectional area equal to that of the downpipe. The bottom of the overflow must be no higher than 25 mm below the bottom of the spouting.
Where the position of an outlet of a parapet wall gutter is on an outside wall, any scupper outflow should discharge into a rainwater head.
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.
Downpipe retaining brackets, otherwise known as stand-off clips as astragals, are used to align the downpipe between the dropper and and discharge sump. Normally they are situated near both ends of the downpipe, with intermediate brackets placed in high traffic areas or where span is greater than 3 m.
Downpipes must be compatible with the roof and gutter material. Discharging water off an inert surface onto unpainted galvanised surfaces can cause corrosion. See 4.11 Inert Catchment.
Horizontally run PVC downpipes and gutters require a greater provision for expansion than metal, particularly if they are painted a dark colour. Provision for expansion with PVC gutters and downpipes should be made according to the manufacturer's recommendations.
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
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 5.8.1 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.
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 shape or scale.
Illustration is for explanatory purposes only and is not to shape or scale.
Illustration is for explanatory purposes only and is not to shape or 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
