Piping design part 3: which pipe for what purpose?

By Fiona Ingham

Choosing the right materials calls for an objective understanding of the conditions, strengths, and weaknesses of each material.

Plumbing Africa has covered the pros and the cons of HDPE, PEX, PVC, and copper. This month, we look into the properties of cast iron pipes.


According to Saint-Gobain sales manager for southern Africa, Lefentse Masemola, cast iron pipes were first manufactured in 1864 and have stood the test of time as the material of preference for drainage systems. Cast iron pipe systems have the robustness and the strength that other materials cannot match. These systems typically last for the lifetime of a building, making them an ideal solution for main soil and rainwater stacks.

Compared to other materials, cast iron is resistant to thermal expansion and contraction, which means there is no need for expansion joints on the system. Independent laboratory tests conducted by independent third parties such as IBP in Germany and CSTB in France prove that cast iron systems are quieter than any other material system when water flows through them. “Some alternative pipe systems’ recorded results indicate they are significantly noisier than cast iron at offset points and where there are changes in direction,” says Masemola.

In high occupancy and multi-storey buildings, cast iron systems are the best alternative, as they will not add to the spread of fire across floors, will remain structurally sound for longer, and even more crucially, will barely emit any smoke or fumes — considered the fastest killer in the event of a fire. This is a vital consideration for multi-storey residential buildings where fires could break out while occupants are sleeping, increasing the risk of injury or death. They are also an ideal choice for other buildings in which the mobility of the occupants is limited, such as in hospitals, areas of public gathering and assembly, and correctional service centres.

“Cast iron systems are manufactured from up to 95% recycled content (scrap), are 100% recyclable using harmless processes, and do not lose any of their mechanical properties over time,” she explains.

Modern cast iron systems made to EN877 are lighter-weight and user friendly. Epoxy-coated fittings are also cleaner to handle and are designed to deal with the demands of modern drainage.

The plumber needs to understand the latest joining options available for cast iron systems. These include stainless steel mechanical couplings and push-fit options, which are quicker and easier to assemble, making the water tightness easy to achieve. Mechanical couplings for pressure requirements in excess of 5 bar, as well as bracketry to suit applications are also available.

Masemola says the plumber should be aware that with cast iron drainage systems there is no additional requirement for extras such as fire protection and collars, acoustic wrapping, and expansion joints.

For buildings that could change their use over time or need retrofitting (such as converting an office building to a refurbished multi-unit residential building), mechanical cast iron systems are easily demountable and can be changed to suit new design criteria, as opposed to other material options that would require entire system re-design/replacement to fit the purpose. Their best uses by building type include healthcare, commercial, stadia, public buildings, schools, hotels, and car parks.

Cast iron systems are used both above and below ground. Masemola says they can be used above ground forsanitary soil, waste and vent, rainwater systems, and suspended systems in basements. They are ideal for use below ground because of the material’s inherent strength with minimal or no maintenance. They are also ideal for bridge drainage, as their material expands virtually the same as concrete structures do, are not prone to failure with exposure to sunlight as evidenced in plastic systems (photochemical ageing), and will also not bend with temperature fluctuations, she says. They also require fewer bedding materials than other pipe system materials.


Installing a cast iron system is simple, with less room for error. However, certain areas of common mistakes still exist.

A common installation mistake is not allowing the right gradient in the horizontal system, causing areas of ‘ponding’.

Installers use unsuitable pipe cutting tools. The recommended tool for cutting cast iron is a common hacksaw, using a diamond/tungsten tipped disc. The use of tools such as snap cutters — often used by installers — are not fit for purpose because of the damage they could cause to pipe ends, which would lead to additional work to the end faces to avoid incipient rust. Alternative cutting tools include pipe cutters, band saws, and power disc cutters.

Although cast iron pipes are easy to assemble, they require proper positioning before bolts are tightened. Failure to assemble them correctly usually results in problems during air testing of the system before sign-off. Installers need to adhere to assembly instructions as provided by manufacturers. Alternatively, to further simplify installation and reduce installation time, cast iron push-fit systems can be used.

System restraining is not always correctly done, regardless of particular system material type, particularly where the system is required to cope with any pressure where a change in direction occurs. Cast iron pipes and fittings manufactured to EN877 are able to handle pressure in excess of 5 bar if correctly restrained (appropriately bracketed), with the exception of the push-fit system, which has been tested to not exceed 1.5 bar. Not using the recommended manufacturer’s brackets can put the entire network at risk.

Mistakes can happen using any material; however, mistakes made during the installation of mechanical cast iron systems using couplings are easy to rectify. Push-fit systems made to EN877 are also fully compatible with the mechanical coupling systems — as such, they can be demounted as required to suit changing or future requirements.

Cast iron is generally perceived to be more expensive than plastic pipe systems. However, different plastic and alternative material systems are also priced differently. It can be argued that the cost of a system should be considered ‘installed’, meaning, the total installed cost (including time, labour, and additional components for required system performance) and not on a system item/component basis.

Other systems have specific additional requirements that cast iron systems do not need, such as fire protection, acoustic wrapping, and expansion jointing, which are often only considered after systems have been specified.

Modern cast iron systems can be push-fit assembled with the following benefits: increased speed of installation; reduced labour and time-related costs; and no need for the additional ‘curing’ time that fusion-welded systems take before the system is tested. 

For building owners, there is a longer-term cost consideration: how much will the system cost over its entire lifetime, considering frequency of maintenance and replacement? From this perspective, cast iron is one of the most cost effective pipe materials to choose, Masemola says.

In conclusion, the superior mechanical and ‘fit and forget’ or longevity properties, combined with non-combustibility and superior acoustic properties make cast iron an ideal choice. “Cast iron systems are best used in multi-storey buildings, high-occupancy structures, and mixed-use developments,” says Masemola.


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