In the design and detailing of a roof drainage system, consideration must be given to a range of factors.

These include:

• rainfall intensity
• roof catchment area
• gutter size/capacity, falls and outlets (sumps, rainwater heads, nozzles)
• downpipes (size, quantity and placement)
• overflow consideration
• material selection, and
• jointing.

Stramit recommends a minimum fall of 1 in 500 for all gutters as this is a design requirement for the gutter and downpipe selection table . Good fall reduces the risk of leaf and debris built-up that could otherwise effect durability.

All fastening screws must conform to AS3566 – Class 3. For connecting brackets use the following.All fastening screws must conform to AS3566 – Class 3. For connecting brackets use the following.
 
ALL STATES (excl. WA)

WA

Gutter overflow needs to be considered when designing and installing gutter systems. The overflow devices should have adequate capacity and the roof drainage system must be in accordance with AS 3500.3.

Drainage from copper or lead products (including roof flashings) should not be allowed to discharge on to zinc/aluminium or aluminium/zinc/magnesium alloy or COLORBOND^® steel components. Similarly, lead or copper components should not be installed in contact with zinc/aluminium or aluminium/zinc/magnesium alloy coated steel. Each of these combinations will lead to premature corrosion.

Drainage from copper, COLORBOND^® and zinc/aluminium or aluminium/zinc/magnesium alloy coated steel, translucent (or other inert material) should not be allowed to discharge onto, or into, galvanised products.

Stramit's rainwater products are manufactured from G550 and G300 Zinc Aluminium or Zinc Aluminium Magnesium coated steel (AZ150/AM125) or galvanized (Z275) in accordance with AS1397, and COLORBOND^® steel with a coating conforming to AS2728.

Other coatings, grades and materials may be available, subject to enquiry.

NSW/ACT

QLD/NT

VIC/TAS/SA

WA

In all Australian states except Western Australia, overflow measures are not required for eaves gutters fitted to a verandah, or where the eave is greater than 450mm wide, with either no lining or sloping away from the building.

Slotted Stramit gutters give some overflow provision, when used with the Stramit^® Snap Clip. A higher overflow volume can be catered for by providing the Stramit^® Gutter Spacer or the Stramit BAT^® Clip.

The Roof Run Length tables give the maximum sloped roof run length which can be used for the overflow through the slots, and back of gutter. These values are based on independent testing.

Where the Stramit^® Gutter Spacer or BAT^® Clip is used, they need to be installed as recommended in the installation leaflets provided with the product.

Alternative overflow measures

Overflow may also be addressed through alternative building design methods such as:

• Unlined eaves, where appropriate to the house design, to eliminate the issue.
• Gutter installed so that the gutter front is 10mm below the top of the fascia.
• Back flashing, where gutter support brackets allow for installation of back flashing (e.g. external brackets).
• The detailing and sizing of the selected overflow method/s is normally completed by the designer/installer, but must be adequate for the situation and must meet the relevant performance requirements of the NCC and Australian Standards.

The Building Code of Australia requires that where high-fronted gutters are installed provision must be made to avoid any overflow back into the roof or building structure by incorporating overflow measures or similar.

This requirement does not apply where the gutter is connected to a verandah or where the eave is more than 450mm wide, with either no lining or raked with a slope towards the gutter. Overflow design must be based on the 100 year ARI 5 minute duration rainfall intensity.

Methods of providing for overflow in the design and installation of roof drainage systems with high front gutters may include:

• slotted gutter front to allow for water overflow through the slots visible on the front face of the gutter
• a gap between the fascia and the gutter back, either by inserting a packer between the back of the gutter and the fascia or by employing proprietary systems and trade solutions
• specific overflow measures including an inverted downpipe drop/pop nozzle at high points in the gutter but set at a level below the fascia top
• stop ends cut down to a lower level to act as a weir; the stop end weirs could be hidden at the high point of the gutter and designed as part of an expansion joint
• rainwater heads with overflow weir, or
• holes, slot, or weir at downpipes.

Stramit's rainwater products have been designed and/or tested to all appropriate loadings and design action effects. These include wind, atmospheric corrosion, rainwater flow, rainwater mass, foot traffic loads, dead loads and ladder loads. The performance information for each product indicates those action effects accounted for in each case.

NSW Gutters Performance Table (PDF)

QLD/NT Gutters Performance Table (PDF)

VIC/TAS/SA Performance Table (PDF)

WA Performance Table (PDF)

Rainfall intensity values in the tables below are for 20 and 100 year ARI, 5 minute durations and have been derived from the National Construction Code 2016. It should however be emphasised that the extent and longevity of records in Australia are limited and any such data therefore carries with it a degree of uncertainty.

The 20 year ARI values should only be used for external eave gutters. For internal/box gutters and overflow design use the 100 year ARI values included in the tables. Specific data for any location can be obtained from the Australian Bureau of Meteorology.

NSW/ACT

QLD/NT

VIC/TAS/SA

WA

In all Australian states except Western Australia, determining the maximum sloped roof run length involves taking into account the additional length of roof which contributes to the flow in any one position when there is a roof penetration or spreader.

In these positions, the effective roof run length would be longer than the distance from the ridge to the eaves. A simplified method of finding this length is shown in the illustration. In this case, the maximum roof run length is 14m for a 10m length of roof due to the penetration 4m down from the ridge.

If the catchment area is known, the roof run length can be found by dividing the area by the length of gutter it feeds into.

 ROOF RUN LENGTH (all states excl. WA) PDF

Stramit^® Gutters require the correct proprietary Stramit brackets (or snap clip/stiffener bracket combination) for support at spacing no greater than those shown in the following table.

NSW/ACT

QLD/NT

VIC/TAS/SA

WA

Gutter runs in excess of 20 metres require expansion joints.

While there may be variations between states, contractors who install guttering systems are generally required to hold an appropriate licence. The work is required to comply with the appropriate codes and standards. Statutory warranties normally apply and consumers have a right to lodge a complaint with the appropriate authority.

During the installation of the roof drainage system, particular attention should be given to the following:

• The use of compatible materials for drainage system components, leaf-guard system components and fasteners/sealants to connect and seal the components.
• The position of the gutter in relation to the fascia.
• Installation of the specified gutter and downpipes, and ensuring that downpipes are installed in the correct locations and numbers.
• Gutter fall, ensuring sufficient fall in the direction of the downpipes.
• Overflow must be allowed for and specific components installed where required.
• All debris and loose waste materials (swarf, fasteners, etc.) must be cleaned off at the end of each day and at the completion of the installation, to prevent blockages of the drainage system or deterioration of the individual components, and
• Any protective films should also be removed as part of the installation process.

Stramit recommends installers follow the installation code for metal roofing and wall cladding as outlined in the Standard Australia Handbook - HB39.

Stramit's rainwater products are available in COLORBOND^® steel colours. However should painting of Zinc Aluminium / Zinc Aluminium Magnesium coated steel products be required, use the following procedure.

A weathering period of two weeks following installation will make painting easier. Clean the rainwater product immediately prior to painting. Dirt can be washed off using water with mild detergent. Any grease marks should be wiped away with paint thinners.

In benign locations good quality acrylic paint will give satisfactory performance. First use a low-gloss water-borne acrylic primer. Finish with water-borne acrylic gloss, or your choice of gloss level.

Use only neutral-cure silicone for sealing joints when using Stramit's rainwater products. Take care to avoid pockets in joints which may hold moisture and potentially reduce durability.

Consideration should be given to handling and installation issues as part of site induction safety procedures. Specific consideration should be given to pack handling , avoidance of cuts, trips, slips and falls, long section handling, particularly in windy conditions, section cutting procedures and surface temperature on sunny days.

Personal Protection Equipment (PPE) should always be used, including cut resistant or leather gloves when handling and transporting product and appropriate footwear when handling long lengths.

Stramit's rainwater products manufactured in the COLORBOND^® range of colours are supplied with a protective strippable coating. This should be removed at the last possible stage during the installation process.

It is possible to selectively move the coating to one side to avoid fastenings and joints before finally remove the coating from the installed product. Do not leave the products with strippable coating intact exposed to direct sunlight for more than a week or it can become difficult to remove.