COP v3.0:roof-drainage; types-gutter

5.2 Types of Gutter 

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.

5.2.1 External Gutters (Spouting) 

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.

5.2.2 Concealed Fascia Gutters 

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.

See 5.3.3 Overflows.

5.2.3 Internal Gutters 

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. Internal Gutter Design Features 

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.

Scupper outlets should be avoided where possible. They are difficult to weatherproof, and they can inhibit expansion. At the outlet end, a weir overflow should discharge into a sump or rainwater head.

5.2.4 Valleys 

A valley is a gutter at the internal intersection of two sloping panes of roof cladding. Valley Fixing 

Valleys should not be positively fixed, except at the head, because that would inhibit expansion and can produce noise.

Alternative means of securing the valley gutter to the substrate include:

  • A clip system allows for thermal movement and security.
  • A compatible washered nail or screw or a galvanised nail, provided they do not penetrate the sole of the gutter. Valley Design 

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. Internal Valley Angle

Roof PitchInternal Angle
60°104° Maximum Valley Catchment in m² for Areas Having a 50-year Rainfall Intensity <150 mm/h

Roof Pitch10°12.5°1520°25°30°
A 3-fold  1218294170106146
B standard  2534476399140184
C Deep6086152180215251321389452
D Tile    1722334557
Free Board: 15 mm for pitches 8° and above
                      20 mm for pitches below 8°

 For other pitches, rainfall intensity, and valley shapes refer to the 5.7 Capacity Calculations tool. Internal Corners 

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. Bifurcated Valleys 

The maximum recommended catchment area for a bifurcated valley is 10 m². Changing Angles in Valleys 

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. Asymmetrical Valleys 

Where opposing roofs of different pitches discharge into a valley an asymmetrical valley is required. They may be designed so the side under the flatter roof is at the same height as the steeper side and 20 mm freeboard is required

5.2.5 Roof Gutters 

Roof gutters may be secret gutters where a barge or a wall is at an acute angle to the roof.

Penetrations concentrate water flow from numerous troughs to a single trough. These troughs can be considered as roof gutters and designed to accommodate the required water flow.





5.2.6 Secret Gutters 

A secret gutter is used where the roof edge runs at an angle of less than 90° to a wall, barge, or parapet.





Secret gutters should be wide enough to allow for cleaning and must be designed in accordance with Internal Gutter Design Features