Having a water tank at home is very important these days, but there are so many types that people can become confused about which one to choose. Tanks are made in plastic, steel, fibreglass and concrete, and while all of these are suitable for a house or office, they are not interchangeable. Each material behaves differently once it is holding thousands of litres, and the differences show up in weight, in how the tank gets to site, in what happens to the water inside it, and in what goes wrong years later.
Whichever material you are considering, one thing applies to all of them: you must be aware of whether the tank complies with the requirements of AS/NZS4020. Confirm that first. After that, the comparison comes down to practical questions, and this guide works through the four materials one at a time.
Steel Tanks
Steel is a common material used in manufacturing water tanks. Steel rainwater tanks are tough, but they are also prone to corrosion, which is why steel tanks are mostly coated with rust-resistant coatings such as zincalume. The coating, rather than the steel itself, is doing much of the work of keeping the tank intact.
Brand new steel tanks usually need to be flushed before they can be used. This is because these tanks may leak an excess concentration of zinc into the water. That is not deadly, but it could affect the taste of stored rainwater, so the first fill is one to run off rather than drink.
Galvanised Steel and Fabricated Steel
The most common material used in manufacturing steel tanks is galvanised steel, which is basically applying zinc to steel. These steel tanks are commonly joined using a hot iron to combine lead and tin on the joint of the steel. Like other types of steel, galvanised steel is not resistant to corrosion, so manufacturers also need to apply rust-resistant coatings to these tanks.
Steel tanks fabricated for water storage do not have corrosion issues like galvanised steel tanks. That comes at a price: they cost more than plastic coated tanks and galvanised steel tanks. If you are comparing steel quotes, it is worth establishing which of the two you are being quoted on, because the cheaper option is the one carrying the corrosion problem.
Read the Steel Warranty Carefully
Steel normally has a warranty period of about 20 years. The manufactured product, meaning the steel water tank itself, only has a 10-year warranty on a pro-rata basis. Those are two different numbers covering two different things, and the second one is the one that applies to the tank you are buying. Pro-rata also means cover reduces across the term rather than holding at full value to the end of it. Should you choose this type of tank, it is very important that you read the warranty guidelines carefully so you know what is covered, for how long, and at what proportion.
Concrete Tanks
Another popular material used for manufacturing tanks is concrete, and its defining characteristic is weight. Concrete tanks are usually very heavy, difficult to transfer and difficult to install. Normally, an 18,000 litre concrete tank would weigh about 8.81849 tons, and that figure dictates almost everything else about how these tanks are handled.
Large concrete tanks are often installed underground, while smaller tanks can be installed above ground. Small tanks are commonly delivered complete, while medium and large tanks are poured on site. We have looked at this material in more depth in the truth about concrete tanks.
Installing and Repairing Concrete Tanks
Concrete tanks are delivered in three pieces and the openings are covered by a rubber or synthetic sealing ring. A sealant is applied to the inside part of the tank at the join, and this is what stops the water from leaking out. The seal is a serviceable part rather than something inherent to the structure.
New concrete tanks may affect the taste of stored water and may leak lime, which increases the pH level of stored water. It is advisable to flush these tanks before using them. Cracking and leaking are the common longer-term issues. The repair is to drain the tank completely, or down to the portion of the crack, and use a sealant on the damage. Sometimes a plastic liner is needed to repair extensive damage, and such cases can be a bit expensive.
Fibreglass Tanks
Fibreglass tanks are manufactured using a food-grade coating on their interior surface. It usually takes time for this coating to cure, so production also takes a bit of time. But the wait is worth it.
One good feature of fibreglass is that it is very rigid, which means the wall of the tank can be thin and still manage water pressure. Fibreglass tanks are often manufactured with resin mixed with a hardener. Along with the glass fibres, the catalyst or hardener is sprayed onto a mould by an operator, and doing this makes these tanks sturdier. It also makes the process dependent on individual skill, which results in variation from tank to tank.
Where Fibreglass Struggles
Like concrete tanks, fibreglass tanks are prone to cracking and leaking. Fibreglass also allows light to pass through it, which makes algal growth possible. Manufacturers deal with this by using a black covering inside these tanks, or by painting them on the outside, and some tanks are designed with sufficient pigment to address the issue. It is a solvable problem, but it is worth noticing that it is a problem the material creates and then has to be corrected for.
Plastic Tanks
Plastic tanks are the most popular type of water tank in New Zealand. Mostly, these tanks are made of food-grade polyethylene plastic to comply with AS/NZS4020. Polyethylene is not prone to corrosion, which means a longer life for plastic tanks, and it also means they are lightweight, easy to transport and easy to install. We have looked at the material question in more detail in are poly water tanks safe.
Plastic tanks are also inexpensive compared to other tanks. There is no need for heavy equipment to install them. They are manufactured and delivered in one piece, so you do not need sealants during installation, and that removes the joins that other materials have to seal and later re-seal.
Why Plastic Suits New Zealand Conditions
Polyethylene is a tough material, which allows people to bury it at depth or install it above ground. This is especially applicable to corrugated water tanks. These tanks are also manufactured to withstand earthquakes, which tend to happen often in New Zealand.
Plastic prevents light from passing through, which inhibits the growth of algae, the problem fibreglass often encounters. As with all types of water tanks and rainwater tanks, plastic tanks need to be flushed after installation to make sure there is no dirt or taste distinction. And at the end of their service life, polyethylene tanks can be recycled.
Choosing Between the Four
Set side by side, the four materials sort themselves fairly quickly. Steel is tough but fights corrosion and carries a warranty worth reading closely. Concrete is durable but heavy, awkward to install, and prone to cracking. Fibreglass is rigid but varies with the skill of the operator and lets light through. Plastic is light, arrives in one piece, does not corrode, blocks light and can be recycled, which is a fair part of why it is the most popular choice here. You can use a polyethylene tank for laundry, washing the car, the garden and a whole lot of other applications. For a closer look at the two most common options, see concrete or plastic compared, or browse the full water tanks range.