COP:roof-drainage; gutters

5.5 Gutters 

External gutters must be installed with the back lower than the fascia board or cladding.

External and internal gutters must have a cross-sectional area in accordance with Cross-sectional Area of Gutter, and sumps, rainwater heads and downpipes must not restrict the flow from the gutter.

For design purposes, gutters are assumed to be level. It is not recommended to obtain fall by tapering, as it reduces the cross-sectional area of the gutter.

With the limited fall available it is not always possible to ensure that all internal or external gutters will remain dry without ponding and so, to avoid premature corrosion, consideration should be given to using non-ferrous metals. See 5.5.8 Fall.

Unpainted galvanised steel is not guaranteed for spouting and gutters.

Because dirt retains moisture and causes corrosion, ponding voids warranties.

All AZ coated steel spouting and gutters must be maintained to prevent ponding due to the collection of debris or dirt as required in 15 Maintenance .

Where their renewal within 15 years would be difficult, AZ-coated or pre-painted steel must not be used for internal gutters.

All gutters are subject to expansion and, therefore, there is a maximum recommended length before an expansion joint is needed. The maximum length is determined by the metal, its thickness and colour. It is similar to the limit recommendations for roof cladding but should not exceed 12 m. See 6.2.2 Roof Cladding Expansion Provisions.
N.B. A sump or spouting angle provides sufficient movement for expansion.

Where a spouting or gutter can move freely and independently the increase in length should be according to the specifications in 13.5.3A Sliding Washers .

Sliding washers and the spouting or gutter should be of a cross-sectional dimension capable of resisting expansion forces. Copper or aluminium spouting or guttering that has been softened by brazing is not suitable for an extended length.

Outlets are only required at twice the length module because an expansion joint can be either a sump, rainwater head or a saddle flashing. See Gutter Capacity.


5.5.1 Gutter Installation 

No fixings should penetrate the gutter, because they prevent free movement of the gutter.

Gutters must not return, be folded back under the roof cladding, or be fastened to it because that prevents free thermal movement and expansion. All internal gutters must be hooked.

The roof cladding must overhang the gutter by not less than 50 mm, with a down-turned drip edge when the pitch is less than8°. All internal gutters must have a flat sole to avoid premature corrosion caused by the accumulation of dirt and debris.

A separate over–flashing is recommended:

  • in coastal environments;
  • where the pitch of the roof is less than 8° and in a very high wind design load area; and
  • when the gutter does not shelter the profile. (see Gutter Apron)

Turn-downs are formed at the end of the cladding, over the gutter, to prevent water blowing back along the underside of the roofing. (See 5.8.2 Outlets and Overflows)

5.5.2 Gutter Support Systems 

The gutter bracket system must withstand the potential weight of a gutter full of water.
External spouting or gutters in snow load areas must be fitted with snow straps and brackets at a maximum of 600 mm centres to withstand the additional potential weight of any build-up of snow.

Snow may slide off the roof if its pitch is less than 15°. When it is likely to happen over doorways, snow guards should be provided and fixed to the structure above. (see 3.8 Snow Loads)

All bracket material should be compatible with the gutter material, and brackets for pre-painted gutters should be painted or powder-coated before installation.

Brackets must be installed to ensure a gutter gradient towards the outlets. (see 5.5.8 Fall)
The support system for internal box gutters must be flat, able to support the gutter when it is full of water, and withstand a point load of 1.1 kN.

To prevent permanent deflection of the gutter, full support for the sole of an internal gutter should be provided by either a plywood lining or by close ribbed sheets of roof cladding, both of which require an underlay to reduce condensation. Underlay is also used to protect metal from the effects of timber treatment.

Brackets should be located at all stop-ends, at both ends of sumps, and at rain-heads at a maximum of 750 mm spacings for external gutters less than 180 mm wide, and at 600 mm centres for gutters 180 - 300 mm wide.

5.5.3 Eaves Gutters 

External domestic guttering, also known as spouting, is available in standard profiles and in long lengths from the manufacturer, or in some areas made to length on site.

The useful cross-sectional area of an external gutter is governed by the back height and not by the often-high front profile.

Eaves gutter systems, including downpipes, must be designed so that water cannot flow back inside the building.

Pre-painted spouting and guttering has only a backer coat on the inside but can have the expected life lengthened by painting the internal surface prior to installation, or it can be manufactured using double-sided pre-painted coil.

Permanent leaf guards do not provide the protection claimed and assumed for metal gutters. Although they do prevent large pieces of debris from obstructing the outlets, they allow finer particles to collect on the sole of the guttering. The continual wetting of the interface between any debris and the metal will lead to early corrosion if regular maintenance is not carried out. Decaying organic matter such as leaves can produce organic acids, which will accelerate corrosion.

Vertical outlets to eave gutters or spouting must have an area equal to half the cross-sectional area of the gutter and horizontal outlets must have an area equal to the cross-sectional area of the gutter.

Eave gutters should have an outlet within 2 m of an external corner; where it is impossible, eave gutters should be given additional fall to avoid ponding. Any change of direction can negate the fall of lengths up to 6 m and the number of outlets will be determined by the catchment area.

All eaves gutters should allow free expansion to occur. Such joints can be either a sump, rainwater head, or a saddle flashing, but they should not be fastened to the gutter. See 5.5.4A Internal Gutters

5.5.4 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.
Only non-ferrous metals or alternative materials must be used, because coated steels do not meet these criteria.

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 leads to early corrosion. Splashback or run-off from copper onto coated metal must create the same problem.

Both internal and external gutters can be economically designed by positioning the outlets at quarter points as shown in Gutter Capacity . Positioning the outlets as shown can also provide additional fall.

Internal, parapet or box gutters are subject to the same requirements as eaves gutters except, being internally situated, they are designed with a greater safety factor based on an ARI of 50 years, and a minimum design rainfall intensity of 200 mm/hr.

The overall depth of the gutter should be greater than that required by the design capacity to prevent the gutter overflowing by splashing or by standing waves from winds.

For commercial and industrial buildings, internal box gutters must have a minimum depth of 70 mm. Internal box gutters for domestic buildings must have a minimum depth of 45 mm. All internal gutters must be able to withstand 1.1 kN point load.
Internal gutters must be provided with a weir outlet and discharge to a rainhead or sump to provide full drainage without ponding.
All sumps or rainheads must have an overflow, and the bottom of the overflow must be below the sole of the gutter.(see. 5.8.2 Outlets and Overflows
A freeboard allowance of 20 mm must be added to the net capacity calculation to increase the maximum depth of flow in internal gutters.
Within a building area, it is not permissible to sharply change the direction of flow of an internal gutter. Where two buildings meet at an angle, each gutter must be drained separately. This does not apply to valley gutters.
Horizontal or back outlets must not discharge directly from the gutter, because they restrict the capacity of the gutter. (see 5.6.2 Bifurcated Valleys)

When a scupper gutter-overflow is used at the stop-end or high end, the lowest level of the overflow should be located at a minimum of three-quarters of the height of the gutter. At the outlet end, a weir overflow at the level of the sole of the gutter should discharge into a sump or rain-head.

Scupper outlets should be avoided where possible. They are difficult to weatherproof, and they can inhibit expansion.

The sump for internal gutters is usually at a column with downpipe bends close to the sump. Sharp bends of less than 15˚ cause restricted flow into the downpipe and reduces the downpipe capacity. In this case, the downpipe size should be increased to that of the cross-sectional area of the gutter.

Internal gutters should have an expansion joint at the stop-end as shown in 5.5.4 Internal Gutters

When using net capacity and not the simplified calculation method, a freeboard allowance of 20 mm should be added to the net capacity calculation to increase the maximum depth of flow in internal gutters.

It is better practice to provide internal gutters with a minimum width of 300 mm for commercial and industrial buildings, and a minimum width of 200 mm for domestic buildings.

For maintenance purposes and general access, internal gutters are assumed to be capable of supporting a traffic load.

5.5.5 Fascia Gutters 

A fascia gutter is an eaves gutter with a high front that shields the ends of the profiled cladding from view.

The fascia gutter design must ensure that water cannot enter the soffit, or overflow into the building if the gutter system outlet becomes blocked.
Where a fascia gutter system is not easily replaced, and cannot be seen or provide any fall, it must be designed using non-ferrous metals or alternative materials to comply with the durability requirements of the NZBC. (see 5.5.8 Fall).
Overflow slots or an alternative overflow system must be added to concealed gutter systems where the back of the gutter is lower than the front. Overflows must be able to discharge the total amount of water from the roof catchment area. (see 5.8.2 Outlets and Overflows)

5.5.6 Concealed Gutter Systems 

Concealed gutter systems are proprietary systems that can be used with or without a fascia.

External fascia systems with internal gutters have brackets nailed or screwed to the soffit bearers or rafters providing gutter support. When the external fascia is spring clipped to the brackets, it conceals both the brackets and the gutter. The fascia has a soffit groove that accommodates a 6 mm soffit lining, and it should be installed before the roof is fixed.

The concealed gutter design must ensure that water cannot enter the soffit or overflow into the building if the gutter system outlet becomes blocked.

Where a concealed gutter system is not easily replaced and cannot be seen, or cannot provide any fall, it must be designed using non-ferrous metals or alternative materials to comply with the durability requirements of the NZBC. (see 5.5.8 Fall)
Overflows must be provided for concealed gutter systems within 1 m of either side of the downpipe to discharge through the soffit immediately behind the fascia, and they should be capable of discharging the total catchment area served by the downpipe. For overflows see5.8.2 Outlets and Overflows
Where a valley, downpipe or other water source discharges into a concealed gutter, provision must be made for an overflow within 2 m of the turbulence that is created by such a discharge.

The external fascia system can be used as a matching gable flashing and as a fascia with an external gutter or spouting.

5.5.7 Secret Gutters 

A secret gutter is used where the roof edge runs at an obtuse angle to a wall or parapet and drainage is required for the roof cladding tapering to the barge.

Because secret gutters are difficult to replace and their failure could go undetected, they must be made from non-ferrous metals and be compatible with the roof material as shown in Lockseamed Flashings
A valley gutter where the roof pitch more than 12° is regarded as a secret valley gutter. (see 5.6 Valley Gutters and 7.4.5 Valley Flashings. When the roof pitch is less than 12°, a sunken secret gutter must have a minimum depth of 45 mm and be designed in accordance with Rainfall Intensity.

Because it is hidden from view, sufficient space and fall should be provided for this type of gutter to be self-cleaning. The COP recommends that a secret gutter should terminate at a rainwater head.


5.5.8 Fall 

All metal internal gutters must have a minimum fall of 1:200 (5 mm in 1 m) and all metal eaves gutters must have a minimum fall of 1:500 (2 mm in 1 m).

For external gutters, drainage can improve significantly if there is a weir outlet, but regular maintenance is required to avoid notable ponding. With trees close by, significant ponding is likely to occur in spouting without leaf guards. For easy gutter maintenance, leaf guards should not be permanently fixed to the roof cladding.

Where rain-water remains evident in the sole of a spouting or gutter, and does not evaporate with the sun or wind within three days, it is considered permanent ponding and any warranty for AZ coated spouting or gutters will be voided.