Dimond 150 Half Round Gutter

(a) Thermal Movement

For guidance on expansion rate of the various materials please refer to Thermal Movement section.

Where long runs of gutter unbroken by change of direction are planned, it is recommended that steel gutters should not exceed 18 metres and aluminium and copper gutters 12 metres, without the provision of thermal expansion joints to prevent distortion.

The most practical way to accommodate movement is through the use of rainheads which will allow the gutter to move freely at the discharge end. If downpipes are fitted directly to the sole of the gutter, saddle flashings will be required at the high points to accommodate separation of the gutter runs.

(b) Flow Capacity

To confirm the suitability of a non-standard gutter to handle the expected rainfall it is necessary to determine the flow load likely from the roof, and the flow capacity of the anticipated gutter shape. 

To determine flow load from roof:

Design flow load (litres/minute) 

• Q = 1.67 x Ac x i/100

Where

• A_c = catchment area (m²) (this includes 1/2 the area of any vertical surface or the total area of any other roof that drains on to the catchment area)

• i = Expected rainfall intensity for the geographical location (see table below) (mm/hr)

To determine flow capacity of the gutter chosen:

Flow capacity (litres/minute) 

• Q_c = .0016 x Ae^1.25

Where 

• Ae = Effective cross sectional area of the gutter (mm²)

These formulae are incorporated in the table "Effective Cross Section of Gutter (mm²)" for use in the following design process.

The above formulae and table are for design of external gutters based on falls of at least 1:500 or greater.

These formulae can be used for internal box gutters provided Q is factored down by

• 0.4 for no fall
• 0.5 for 1:500 fall
• 0.6 for 1:200 fall

This method then aligns itself with AS/NZS 3500 Part 3.

Design Process for Eaves Gutters

Where eaves gutters other than the standard Dimond range are preferred, you will need to confirm the performance of the gutter shape in relation to the location it will be used in, e.g. the area of roof the gutter shape can drain per downpipe. The following is a step by step guide to confirming the suitability of the gutter chosen.

1. Place downpipes at the preferred locations around the structure. No run of gutter from high point to outlet should exceed 18m. Downpipes should be placed with 2m of valley’s discharging into the gutter. It is good practice to position the downpipe within 2m of internal or external corners to ensure the swell effect from the change in direction of the gutter, does not slow the discharge into the downpipe.
2. Calculate the roof catchment area Ac (m2) for each downpipe. Divide the roof into sections, each section served with a length of gutter sloping from a high point to the outlet. Each section or gutter length is multiplied by the rafter length. If any vertical surface can drain onto the catchment area, add half the vertical surface area to the roof area you are calculating. Also add the total area of any upper roof discharging on to a lower roof.
3. Establish the rainfall intensity for the geographical location of the structure. The New Zealand Building Code Approved Document E1 Surface Water has determined two levels of rainfall intensity. Where an overflowing gutter can result in water entering a building, the rainfall intensity shall be based on a storm with a 2% probability of occurring annually (a 1 in 50 year storm). Otherwise the intensity shall be based on a storm with a 10% probability of occurring annually (a 1 in 10 year storm). 

Enter the following table on the catchment area line and extend across to the rain intensity column. This will provide the effective cross sectional area your proposed gutter will need to have. Interpolate between columns if necessary.

The flow capacity limit for each of the standard Dimond gutters is indicated in the following table by a stepped line. Select the appropriate Dimond gutter for detailed design information.

If a custom made shape is required, choose your shape and set the dimensions to achieve the effective cross sectional area (A_e) of the gutter required by using table "Effective Cross Section of a Gutter" using the following formula:

Effective cross sectional area (A_e) 

• = 0.9 x W x D

Where 

• W = the average width measured at half the depth (mm)
• D = depth (mm)

Once D is established to achieve Ae, it is recommended a free board allowance of at least 10mm is added. Be sure to determine that it is possible to manufacture the gutter shape that you choose. Phone 0800 DIMOND (0800 346 663).

Determine downpipe size. As a general rule for eaves gutters the downpipe sizes can be calculated as follows. 

For circular downpipes: 

• The cross-sectional area should be one half the cross-sectional area of the gutter. 

For rectangular downpipes: 

• The cross-sectional area should be one half the cross-sectional area of the gutter plus 10%. 

Note: No downpipe shall be smaller than: 

Circular downpipe 

• 63mm 

Rectangular downpipe 

• have a cross-sectional area of not less than 3250mm2, and where the smallest dimension is at least 50mm. 

Ensure that the downpipe size can be accommodated within the sole of the gutter. 

Where rainheads and sumps are used both internal or external more accurate sizing of downpipes are achieved using AS/NZS 3500 Part 3.2.

(c) Overflow

Gutter and downpipe systems must be designed to accommodate any overflows that may result in water entering the structure, regardless of where the blockage occurs.

One option for eaves gutters is to ensure the top of the fascia board or cladding finishes above the top edge of the back of the gutter, including at the high point. A gap should be created between the fascia/cladding and the back of the gutter. This provides a continuous emergency overflow regardless of where the blockage occurs. 

For rainheads and sumps care must be taken to ensure the capacity of the overflow is equal to or greater than the designed flow capacity of the downpipe. In many situations the head of water above the downpipe effectively increases its performance, whereas an overflow of equal dimension to the downpipe has a slower flow capacity.

d) Design Guidelines for Box Gutters

Below is a simple outline of the main points to consider when designing an internal box gutter. 

Box gutters should be of sufficient structural strength to accommodate foot traffic and have a width that provides safe passageway (300mm plus allowance for overhang of roofing material).

The recommended minimum slope for any box gutter is 1:200.

Where steel is being considered to form the lining of box gutters, care must be taken to ensure easy inspection, maintenance and replacement is available. In most instances it is prudent to consider other materials such as rubberised membrane, copper or zinc as the relatively maintenance-free long term performance of these materials provide a more cost effective option over the life of the structure.

It is recommended that all box gutters discharge into a rainhead or sump, the depth of which can be chosen so as to permit the use of a downpipe of convenient size (the deeper the head of water above the outlet, the smaller the downpipe will need to be). The width of the rainhead or sump must be equal to or larger than the sole of the gutter.

All rainheads or sumps must have overflow systems designed to accommodate the water flow that is likely from the catchment area in the most intense rainfall for the geographical location.

An effective way to create an overflow in a rainhead is to set the front 25mm below the sole of the gutter. This will allow the water to weir over the front should the downpipe become blocked.

Where the gutter discharges into a sump positioned within a building, sufficient attention must be paid to the design of the overflow to ensure that the water flow from the catchment area is accommodated at all times. The depth of the rainhead or sump will determine the size of the downpipe required. This is due to the pressure that can be formed by the head of water above the outlet. However, the overflow system will most likely not have the head of water above it therefore it may need to be bigger than the main downpipe. The overflow drainage system must be capable of carrying all the water to the outside of the building as the overflow system will be activated only when the normal outlet is blocked.