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. The size of a scupper outlet must have a cross-sectional area of 1½ times that required for a vertical downpipe outlet
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
