Five Factors to Consider When Specifying Stormwater Drainage Systems

The primary objective of AS/NZS standards is to safeguard people from injury or illness and protect property from damage. Several elements contribute towards achieving this objective. Outlined below are five key factors to consider when selecting the appropriate products for the job.

Application

Ensure the correct type of product is selected for the specific application. Is a channel required, or will a sump suffice? Is this a level entry into a habitable space, a channel drain in front of a driveway or garage, or a collection point before discharge into the local authority’s stormwater system? Each application has different requirements set out in the NZ Building Code (NZBC) and applicable AS/NZS standards. For example, a level entry scenario requires a 200 x 150 mm channel and a 12 mm drip line along joinery (among other details); therefore, a standard three-sided channel would not be appropriate. Each drainage system is designed for a specific application. If unsure, consult the channel supplier to avoid costly delays and compliance issues.

Performance

Ensure the selected drainage product’s hydraulic performance meets the needs of the application. This depends on factors such as channel depth, width, and grate open area, as well as product configuration. The topography of the area plays a significant role. For instance, installing a slot drain with a 15 mm opening at the bottom of a steep driveway may cause surface stormwater to bridge the gap during heavy rainfall, limiting performance. A wider grate with more open area, such as a polymer concrete (PC) channel with a cast iron grate, would allow better water drainage. Slot drains work well in courtyards with falls towards them from both sides. It is best practice to seek advice from a hydraulic specialist.

Load Rating

AS 3996:2019 Access Covers and Grates outlines load class ratings to ensure grates perform under the expected loads. Ratings range from Class A (extra-light duty, pedestrian and pedal cyclist traffic, nominal wheel load 330 kg) to Class G (extra heavy-duty, docks and aircraft pavements, nominal wheel load 30,000 kg). Most applications require load ratings between A and D, covering commercial vehicles such as those in loading bays. Allproof conducts in-house testing to verify their products meet AS 3996 specifications, with detailed information available to assist product selection.

Environment

Material selection is influenced by the environment. Stormwater drainage products are exposed to high moisture levels. It is important to consider the chemical and physical durability of materials in specific conditions. Cast iron offers strength, long life expectancy, and cost-effectiveness. Its bituminous paint coating provides some protection, but will wear off over time, exposing cast iron to oxidation. In high-traffic areas, oxidation stabilises into a natural dark brown patina, which is an aesthetic consideration rather than a performance issue. This may be desirable in some projects but not others where consistent appearance is needed. For marine environments, Allproof recommends high-grade 316 stainless steel for superior corrosion resistance and reduced maintenance. Alternatively, glass-filled nylon (GFN) grates offer a black finish that resists environmental effects.

Grating Design

Grate design affects hydraulic performance. Larger open areas typically perform better hydraulically, but smaller individual openings are less prone to blockage. Smaller openings (8 mm or less) are advisable in pedestrian zones to prevent entrapment of high-heeled shoes; these are classified as heelproof grates. Slip resistance is also important in high foot traffic areas such as busy urban streets and commercial courtyards. AS 4586:2013 specifies slip resistance requirements and classifications. Several Allproof grates have been independently tested, achieving slip resistance ratings ranging from R9 to R13.