The material of choice: why high-rise developments are choosing cast-iron drainage systems


High-rise construction encompasses some of the most challenging engineering solutions, which is why specifiers want foul and rain water pipe systems that are not only durable and safe, but also cost-efficient against Total Cost of Ownership (TCO) - and due to these challenging requirements, cast iron is the material of choice.

Cast iron’s popularity stems from the materials physical properties that make it a perfect fit for drainage systems inside high-rise buildings. It’s common in many basements to see exposed pipework, which is at risk of physical damage, either accidentally from vehicles if the system is located in a car park, or even deliberate vandalism. This is a major reason for many specifiers looking for materials with “robust strength”.

The term essentially means that systems are hard to damage, resulting in a low failure rate, and is one of cast iron’s major advantages over more “modern” materials such as HDPE. Cast iron is extremely resilient to mechanical strain, boasting over ten times the crushing strength, meaning that if a system is struck, hit or crushed accidentally, it is much more likely to survive without damage.

Physical strength isn’t its only quality; it is also far more resistant to temperature changes than HDPE systems. Due to HDPE being a Thermoplastic, it suffers from thermal expansion and has a thermal coefficient that is nineteen times higher than cast iron during a 50OC temperature rise. Coping with this expansion requires the installation of expansion joints every 5 metres, supported by anchor brackets and additional unistrut bracing which increases both the total cost of the system and, due to the increased labour time, the total cost of installation.

The cost of the additional components isn’t the only problem specifiers and installers face; the changing dimensions of the system require complex expansion calculations, that can have serious consequences and prove costly if done incorrectly. It’s not uncommon for the HDPE pipes to have arrived on site already bowed due to temperature changes; the pipes then require a significant number of brackets to ensure they are installed straight, particularly in horizontal runs.

Cast iron systems are also more adaptable; if design changes or clashes on site result in pipe systems needing to be rerouted, cast iron pipes can be dismantled and redirected with coupling joints, whereas HDPE systems that use fusion or butt welds need to be scrapped and remade.

Going up the building, these major advantages for cast iron are reinforced by its safety performance. Fires in high-rise buildings can be devastating, and a big step to helping prevent the spread of fire can be achieved by installing safer non-combustible pipes that penetrate walls and floors which, if combustible, could act as a conduit to fire spread up, across and down a building. Cast iron systems such as ENSIGN are rated A1, the highest possible fire safety rating awarded under Euroclass classification, whilst HDPE systems doesn’t even declare a fire rating, and will need fire protection such as collar or sleeve, adding further cost and complexity to the installation.

If the high-rise development includes high quality flats or apartments then acoustics will be very important to ensure the required level of comfort is achieved for its occupants. Cast iron systems outperform all plastic systems, including the more “specialist” acoustic plastic ranges, by some considerable margin and require only the most basic acoustic insulation to achieve NHBC guidelines. HDPE, in comparison, requires a much higher level of insulation which ultimately results in higher costs and a longer installation time.

Specifiers need to be aware of all the costs when comparing materials for use in high-rise buildings, ensuring all elements of the installation are calculated and included in the comparison. Materials like HDPE require extra fittings for thermal resistance, support, acoustic and fire protection, and care should also be taken when looking at which elements are in which package. This can be fragmented; for example, fire protection of the pipework can be picked up by an appointed fire specialist and therefore not included in the M&E installers remit. This can result in showing bigger savings between materials, but overall the cost is still there and needs to be accounted for; making a saving in one area can often lead to an increase in another.

High rise buildings often require larger diameter pipework of 200mm and above, and even up to 600mm for rainwater applications which are suspended at lower floor level or in basements. Here, cast iron systems with their mechanical joints and flexibility can often prove to be the cheaper option.

Another major safety element required of pipework in high-rise buildings is the ability to withstand accidental static pressure; this is a safeguard to the “what-if” scenario of a pipe blockage that can cause damage or failure, due to consequential high pressure building up in the system. Cast iron systems like Ensign are designed to withstand up to 5 bar when bracketed using standard couplings, but can go to pressures of 10 bar using high performance couplings – considerably higher than HDPE.

The life span of the drainage network over recent years appears to have become less important with the general requirement often stipulated around 30 years for Commercial and residential buildings. For developers or landlords, using products such as cast-iron drainage systems which have a proven life span twice this longevity is a smart investment, providing lower overall cost of ownership as well as a more robust and durable drainage solution.

When it comes to deciding on materials to use in high-rise construction, the numerous requirements can cause challenges. Whatever is chosen needs to be safe, durable, offer good value, a long lifespan and more, so it’s unsurprising that cast iron remains so popular with specifiers and developers. By choosing cast iron, engineers know that buildings are best prepared to deal with any drainage issues that they might face.


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Saint-Gobain PAM - Cast Iron Vs. HDPE
Cast Iron Vs. HDPE
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Saint-Gobain PAM - Cast Iron Vs. HDPE