Selecting a rainwater tank – materials

First published December 2007, updated April 2011

Drought is a part of normal life in many parts of the world and given the threats posed by climate change, place with traditionally low rainfall may become even dryer and long established rain patterns give way to major rain events – a feast or famine scenario. A way to better utilize the feast to help through the lean times, or in other disasters where mains supply may be cut or contaminated is to install a rainwater tank 

Here in Australia, the corrugated metal rainwater tank is somewhat of a national icon. In South Australia, around 30% of homes have rainwater catchment systems. Water tanks haven’t been an overly attractive home addition until recently; although seeing the old style tank does make me feel very nostalgic! These days, tanks come in all sorts of shapes, colors, materials and sizes to suit any home and space.

Even if having a full size water tank isn’t an option for you, rain barrels are a very cheap and easy to install option. These look like a pickle barrel, are mostly made of plastic and hold up to 80 gallons – you can pick them up for around a hundred dollars. Ever gallon of water saved counts!

In regard to larger tanks where construction is a more important aspect, in this article we’ll take a brief look at the various materials commonly used, their advantages and disadvantages.

Poly (plastic) rainwater tanks

Poly tanks are made from polyethylene; a UV stabilized, food grade plastic. The tanks are light and you only need a sand base to place them on. They come in a wide variety of colors, usually molded into the plastic, and have a long serviceable life. Many poly tanks carrying a 25 year warranty, although many claim 15 years is a more realistic lifespan. They are also usually the second cheapest of the options covered in this article.

One of the major disadvantages of polyethylene is the material is made from petrochemicals. Even after their serviceable life has ended, there’s still a great big hunk of plastic that will take generations to break down and will release toxins as it does so.

However, polyethylene tanks can still be easily recycled after 15 years, so it’s just a matter of breaking the tank up and then carting it away to a recycler.

Some poly tanks are made with a vertical seam – this is a weak point that may cause splitting and subsequent water loss – so a seamless tank is probably a better choice. Polyethylene water tanks and fire don’t really mix either as they’ll just melt should the flames get too close. This can be a real problem if you’re in a rural area and you need that water to fight a fire.

The other issue is the long term effects of drinking water stored for such a long time in this material. Polyethylene tanks are relatively new on the market, so there’s no serviceable life studies been performed in relation to these issues as far as I know.

I had a polyethylene rainwater tank at my original place in the outback where temperatures would get up to around 46C (115F) degrees Celsius in the shade and below freezing during winter. The tank performed well over the couple of years I had it before selling the property, but there was a bit of an odd taste to the water on hot days (it was placed in full sun). A subsequent poly tank I acquired for my current property has also stood up well to similar extremes – and without the odd taste being added to the water.

Just on that point – before purchasing a poly tank, check the warranty for temperature stipulations as some manufacturers will void the warranty if conditions where the tank is installed can get extremely hot.

Steel tanks

Three materials are most popular – Galvanized steel, Zincalume®  and Colorbond®  (the latter two may be called by other names in different countries).

Galvanized tanks have been around for over 150 years and are usually the cheapest type of tank. Hot-dip galvanizing is a process used to coat steel or iron with zinc. The Zinc helps slow down corrosion, but depending on environmental factors, a galvanized tank may last well under 5 years, particularly if the roof of the structure capturing the rainwater is made from Zincalume. This is due to electrolysis.

Zincalume®  has been around for about 30 years and was originally used for roofing. It’s a a mix of 55% aluminium, 43.5% zinc and 1.5% silicon bonded to steel. There’s a lot of conflicting information around about lifespan, but the general consensus seems to be about 10-15 years.

Colorbond is Zincalume with a conversion layer applied to the surface of the steel to improve adhesion; then a polyester primer baked on, followed by a top coat of paint that is also baked on. It’s not unusual to find a 20 year warranty on these tanks, but a deep scratch to the paint can be enough to accelerate the corrosion process.

Some metal tanks now also have polyethylene linings to further help slow down corrosion – escaping plastic altogether can be a difficult thing to do these days.

If you do buy a steel based tank, look into installing extra sacrificial anodes to further delay corrosion.

Concrete water tanks

Concrete rainwater tanks can be installed either above or under ground. The latter is a good option if you’re short on space as they can be constructed in such a way to allow for load bearing, for example, under a driveway.

Given the material, they are very heavy and often poured on-site or delivered in sections that are then basically cemented together. Again, a polyethylene liner may be used. Without a liner, the tank will leach lime and over time you’ll have a slightly alkaline water. With concrete being porous, without a liner water will penetrate into the concrete over time which may cause corrosion problems in relation to steel framework.

Concrete is also an energy intensive product that requires a great deal of heat and water in its production. Additionally, the components need to be mined – but the same goes for any material.


This is another long-lasting option that can be installed above or below ground. Fiberglass tanks resist corrosion and are not generally affected by chemicals.

As fiberglass tanks tend to allow more light in than other types of tank materials, this can encourage the growth of algae, so they should be painted or gel coat applied. Fiberglass can also tend to be brittle, leaving it prone to cracks – something you don’t want, particularly in an in-ground situation.

Choosing a tank material

Choice is wonderful, but as you can see, there’s advantages and disadvantages with each type of tank, particularly when it comes to environmental impact – so it’s really a matter of gauging your needs and budget and then choosing the lesser of the evils. In regard to the financial side of things, bear in mind not just the initial cost, but how many times the tank will need replacing over X years. This also plays a role in the amount of resources used. 

Something worth checking into are rebates on rainwater tanks and/or associated plumbing – many governments now offer cash-back schemes.

Even with the various disadvantages of each material, given the length of serviceable life of most tank options and the tens of thousands of gallons of water you can collect over that time; installing a rainwater tank is still a very green move.

A brief note on rainwater tank regulations

Regardless of the material you settle on, before buying and installing a tank you should check with your local authorities as in some places you will need a special permit and in others they may be totally banned – which is absolutely ridiculous in my opinion. Even in Australia, the driest inhabited continent on Earth, some local governments had bans in place until relatively recently – but the Millennium Drought thankfully sorted out most of those short-sighted councils.

Australia wasn’t the only country with crazy laws regarding rainwater harvesting. Up until 2009, in Colorado in the USA, it was illegal for households to capture rainwater as rights to water were allocated in the state; i.e, that water that fell from the sky was owned/leased by other parties.

I’m pleased that sanity prevailed in that case also; but no doubt there are still some regions where similar silly legislation is still in place. If you’re unfortunate enough to live in such an area, consider organising protest action rather than letting the status quo remain.