Rainwater harvesting systems: how they work and how to install them

Collecting rainwater is no longer just for environmental pioneers: it has become a legal requirement in parts of Belgium, and a profitable investment everywhere else. Here’s how a complete system works, what the law requires depending on your region, and why this naturally soft water is also a game-changer for your appliances.

How does a rainwater harvesting system work?

The principle remains the same, whether the system is small-scale or comprehensive: collect, filter, store, then redistribute.

Water falls onto the roof, runs off into the gutters, and then flows down into the cistern via the downpipes. Before entering the cistern, it passes through a filter that traps leaves, moss and the coarsest debris. This coarse filtration does not make the water drinkable: it simply prevents the cistern from turning into a quagmire over the course of the seasons.

The tank, whether buried in the garden or installed in the cellar, then stores this water. This is where the pump comes in – the component that really sets one system apart from another. There are three types, and they do not all serve the same purpose.

The domestic water pump supplies water directly to indoor taps, toilets, washing machines and garden taps, whilst maintaining sufficient pressure in the pipes. The pressure booster unit goes a step further: it continuously monitors the mains pressure and starts up automatically as soon as it falls below a set threshold, ensuring a steady flow even when several taps are in use at the same time. Finally, the submersible pump is not used to distribute rainwater but to drain a flooded cellar or crawl space, a useful function to complement a rainwater harvesting system, particularly in homes with basements.

One health and safety point that must never be overlooked: the rainwater system must remain physically separate from the drinking water system, with no direct connection between them. If the cistern is empty, an automatic switch-over system to mains water takes over, ensuring that toilets and washing machines are never left without a water supply.

Is a rainwater tank compulsory in Belgium?

The answer depends entirely on the region, and there is a significant difference between the three.

In Flanders, the installation of a water tank is a legal requirement for all new builds, rebuilding projects or significant extensions. The minimum capacity remains set at 5,000 litres, but the rules will be tightened at the end of 2025: roofs larger than 80 m² must now have a rainwater tank with a capacity of at least 7,500 litres, and those larger than 120 m² must have one with a capacity of 10,000 litres. A few exemptions apply, particularly in the case of green roofs.

In Brussels, this requirement has been in force since 1 January 2014 for all new buildings: the regional town planning regulations stipulate a minimum capacity of 33 litres per square metre of roof area, calculated on the basis of the projected horizontal area of the roof.

In Wallonia, there is no blanket regional requirement. Local authorities retain control: many require a septic tank as a condition for granting planning permission, with requirements varying from one local authority to another. Checking with your local authority remains the only reliable way to find out which rules apply to a specific project.

What tank capacity should you choose – is it compulsory or not?

Even if there is no legal obligation to do so, it is worth giving careful consideration to the sizing of the tank: a tank that is too small will overflow at the first heavy downpour, whilst an oversized tank ties up funds that will never be fully utilised.

The basic rule is based on the horizontal surface area of the roof. For a roof measuring 100 m², a 3,000–5,000-litre tank generally meets the needs of a household with a toilet and washing machine connected to the system. For a roof area of between 100 and 150 m², the capacity increases to around 5,000 litres; for 150 to 200 m², to around 7,500 litres. Beyond that, the benefit of increasing the tank’s capacity further diminishes: it will almost never fill up completely, as the household’s regular consumption means it is constantly being emptied.

The number of connected points carries as much weight as the roof area in this calculation. A cistern used solely for watering the garden can be relatively small. A cistern that also supplies water to the toilets and the washing machine – the two uses that consume the most non-potable water on a daily basis – warrants a larger capacity.

How much can you actually save?

With annual rainfall of between 700 and 1,400 litres per m² depending on the region, a 50 m² roof collects between 35 and 70 m³ of water per year. At the average price of mains water, around €5.50 per m³ excluding fixed charges, this represents potential savings of €150 to €200 a year for a household that connects its toilets and washing machine to the rainwater system.

The breakdown of domestic water consumption in Wallonia gives an idea of what can actually be replaced by rainwater: toilets account for around 36 per cent of a household’s water consumption, baths and showers for 32 per cent, and laundry for 13 per cent. Toilets and laundry combined therefore account for nearly half of the water bill – two uses that do not require drinking-quality water and are perfectly suited to rainwater.

In terms of investment, a complete system comprising an underground tank and a water-pressure booster unit costs around €4,000, a figure that varies depending on the tank’s capacity and the complexity of the groundworks. The payback period is generally between 6 and 14 years, though it is shorter in households that connect as many compatible water outlets as possible.

Rainwater and limescale: the link with the water softener

This is an advantage that is often overlooked in purely environmental discussions: rainwater is naturally soft, lacking the calcium and magnesium that make tap water hard in much of Belgium. A washing machine connected to the rainwater system therefore builds up limescale much more slowly, and laundry washed with rainwater naturally comes out softer, without the whitish streaks typical of hard water.

This benefit does not eliminate the need to treat mains water, which continues to supply the kitchen and bathroom, where safe drinking water remains essential. In regions where the hardness of the mains water exceeds 25 to 30°fH, such as Flemish Brabant or certain areas of Walloon Brabant, a water softener remains essential to protect the boiler, taps and fittings, and your skin on a day-to-day basis – a topic we cover in detail in our guide Why install a water softener in your home?.

The two systems complement rather than compete with one another: rainwater harvesting reduces the volume of mains water consumed, whilst the water softener improves the quality of the water remaining in the drinking water supply, particularly in the bathroom where skin and hair comfort are directly affected by water hardness, as we explain in our article on creating a wellness bathroom, why a water softener is essential. Before winter sets in, it’s worth having both these systems checked together – a maintenance routine we detail in our guide hard water: how to protect your plumbing before winter.

Installation: equipment required at Facq

The tank itself is generally part of the structural work and is installed by the contractor at the same time as the groundworks. However, it is in the technical aspects – pumping and filtration – that the choice of equipment makes all the difference to the system’s day-to-day reliability.

At Facq, the range covers the three types of pump required for a complete installation. For direct supply to consumption points, the Grundfos SBA rainwater pump and the Wilo Rain rainwater pump meet the everyday needs of a household. For more precise pressure management across the entire system, the Wilo HiMulti 3 pressure booster pump, from €1,157.96 including VAT, and the Grundfos Scala 1 pressure booster pump are suitable for systems supplying multiple water points simultaneously. For cellars prone to water ingress, the Grundfos Unilift submersible pump, from €1,105.94 including VAT, is a useful addition to the system.

Regional grants: a financial boost worth checking out before you get started

Several local authorities and regions provide financial support for the installation of a rainwater harvesting system, with the amounts varying considerably from one local authority to another.

In Brussels, the Renolution programme offers a grant of between €500 and €1,100 depending on household income, with an additional bonus of between €100 and €200 if there is extra storage capacity to cope with heavy rainfall. In Wallonia, many local authorities offer local grants, with amounts and conditions specific to each one: it is best to contact your local authority directly before signing a quote, as some local authorities make the grant conditional on specific criteria relating to volume or connection.