A summer without rain, the watering can becomes your constant companion, and the water meter keeps merrily spinning. Yet the best garden helper falls literally from the sky and, for most people, runs unused into the drain: rainwater. Catch it, and you water more softly, with less lime, and for free. The only real question is how much your roof yields and whether a barrel is enough or a cistern is worth it. Let us work through it calmly.
Auf einen Blick
- Rule of thumb: 1 mm of rainfall brings 1 litre of water per square metre of roof. With 700 mm of annual rainfall and a 50 m² roof, that is roughly 28,000 litres a year.
- Factor in the runoff coefficient: tile and metal roofs deliver around 80 to 90 percent, green roofs only 30 to 50 percent.
- A rain barrel (200 to 500 litres) is the cheap way in for balconies, raised beds and small plots. For 40 to 80 euros you are set.
- A cistern (2,000 to 10,000 litres) bridges real dry spells and pays for itself over the years, but costs 1,500 to 5,000 euros installed.
- A rain collector on the downpipe, an overflow and a lid against mosquitoes are essential. Empty the barrel in winter, or the frost will crack it.
How much water does your roof really give?
The maths is pleasingly simple, because nature hands us a round number: one millimetre of rainfall equals exactly one litre of water per square metre. So if 700 mm of rain falls over the year, a typical figure for much of Central Europe, then every square metre of your roof receives around 700 litres annually.
What matters is not the sloping roof surface but the footprint, meaning the area your roof covers as seen from above. A gable roof with an 8 by 5 metre footprint counts as 40 m², no matter how steep it is. Rain falls vertically, and a steeper roof does not catch any more of it.
Part of the water does evaporate, though, or stays caught in the gutter. The runoff coefficient captures this loss, a factor between 0 and 1. So the full formula reads:
Annual yield in litres = roof area (m²) times annual rainfall (mm) times runoff coefficient.
One for the garden notebook
Calculate your yield in three steps
Determine the roof area
Measure the footprint your roof covers, not the pitched roof surface. For a gable roof: length times half the building depth per side, or simply length times depth for the whole building. For the rain barrel, only the area draining to the one downpipe where you collect counts. Often that is a quarter or half of the roof.
Look up your region's rainfall
You will find regional annual rainfall from the national weather service. Rough reference points: Berlin and Brandenburg around 560 mm, the Rhineland and northern Germany around 750 mm, the Black Forest and Alpine foothills over 1,200 mm. When in doubt, use 650 mm, which keeps your estimate on the cautious side.
Multiply by the runoff coefficient
Tile, slate and metal: factor 0.8. Bitumen and gravel flat roof: 0.5 to 0.6. Extensive green roof: 0.3 to 0.5, because the substrate layer stores a lot of water. Multiply all three numbers and you have your rough annual yield.
A concrete example: you collect at a downpipe draining one roof half of 25 m². Your location gets 700 mm of rain, and the roof is covered with clay tiles (factor 0.8). The maths: 25 times 700 times 0.8 gives 14,000 litres a year. That is 14 cubic metres that would otherwise vanish down the drain. At around 4 euros per cubic metre for supply and wastewater, you could catch about 56 euros a year, and the plants reward you with better growth on top.
The yield is not spread evenly across the year. In wet November every barrel overflows within hours, in dry July it is empty after two watering rounds. That is exactly why harvesting is not about the annual yield but about storage volume for the drought weeks in between. That is the heart of the barrel versus cistern decision.
The runoff coefficient by roof type
- Tile, slate, sheet metal: 0.8 to 0.9
Hard, smooth surfaces with a proper pitch. Almost everything ends up in the gutter here. Metal roofs sit at the top end because they barely soak up water.
- Flat roof with gravel or bitumen: 0.5 to 0.6
The shallow pitch and rough surface hold water back, and some of it evaporates straight away. Still a usable collector.
- Extensive green roof: 0.3 to 0.5
The substrate and planting layer is a sponge. Wonderful for rain retention in a changing climate, but weak for collecting in the barrel. What does arrive is already pre-filtered, though.
- Greenhouse and shed roofs count too
Do not forget the small roofs. A 3 by 4 metre shed roof delivers around 6,700 litres a year at 700 mm, often right where you need it.
From the store to the bed, water travels most economically via targeted drip irrigation, which doses straight to the root instead of evaporating across a wide area.
Barrel or cistern: the honest comparison
The rain barrel
The rain barrel stands visibly at the downpipe, usually holds 200 to 500 litres and is connected in half an hour. You need no digger, no permit and no big budget: 40 to 80 euros for a solid barrel, plus a rain collector for 20 to 30 euros. The drawback is the volume. A 300-litre barrel is empty after two or three thorough watering rounds and useless in a dry week. If you want more, couple two or three barrels into a cascade with a connector set. The best way in for balconies, terraces, allotments and small to medium beds.
The cistern
The cistern sits in the ground, frost-safe and shielded from light, and holds 2,000 to 10,000 litres depending on the model. With it you bridge genuine dry spells of several weeks and can also supply a large vegetable garden, the lawn or an automatic irrigation system. The price is steep: 1,500 to 3,000 euros for the tank, plus excavation, pump and filter, so an installed setup quickly adds up to 3,000 to 5,000 euros. In return, such a tank lasts decades and pays off over the years where water demand is high. Plan the pit and the downpipe connection in from the start, ideally alongside a terrace project or a new build.
Which size suits your garden?
As a rough guide: in peak season a vegetable bed needs around 20 to 30 litres per square metre per week. A 20 m² bed therefore draws 400 to 600 litres weekly. Work out how many days of dry spell you want to bridge and you have your target volume. On the watering side you save the most by watering straight to the root, as described in the article on water-wise watering in summer.
- Balcony and raised bed: 200 to 300 litres
A single barrel is enough. It refills after every shower, and on a small footprint you rarely need more reserve.
- Allotment and townhouse garden: 500 to 1,000 litres
Two or three coupled barrels or a larger buried tank. That carries you through most summer weeks when a little rain falls in between.
- Large productive garden with lawn: from 3,000 litres
Now the cistern pays off. Anyone supplying a lawn, fruit trees and a large vegetable plot is well served with 5,000 litres and a pump. As a rule of thumb, plan about 5 percent of your annual yield as storage volume.
The right connection to the downpipe
Whether barrel or cistern, without the right hardware at the downpipe it will never last. Three components belong to it:
Fit a rain collector
The rain collector (also called a downpipe filter) is cut into the downpipe and diverts water into the barrel until it is full. After that the surplus automatically runs on down the pipe. It also filters out coarse leaves. Install it just below the desired water level of the barrel.
Divert the first flush
The first rain after a dry spell washes dust, pollen and bird droppings off the roof. A pre-filter or a small diversion of this first flush keeps the water cleaner. On cisterns a built-in filter basket handles this.
Route the overflow safely
When the barrel overflows, the water must drain away in a controlled way, or it pools against the house wall. Route the overflow back into the downpipe, into a bed or into a soakaway. On the cistern an overflow with a trap and a backflow flap is standard.
Why rainwater is the better watering water
Rainwater is soft, meaning low in lime, and slightly acidic. That is exactly what many plants love, the ones that react sensitively to hard tap water with lime, thriving on rainwater instead.
Hydrangea macrophylla
- Standort
- partial shade, sheltered
- Saison
- flowering June to September
- Wasserbedarf
- high, water lime-free
- Frosthärte
- down to about minus 18 degrees, depending on variety
The bigleaf hydrangea is the prime example: with lime-free rainwater, blue varieties such as Hydrangea 'Endless Summer' keep their colour true, while lime tips them towards pink. Rhododendron, azalea, blueberry and potted citrus also thank you for the soft water.
For watering in the garden, collected rainwater is perfectly fine. It is not drinking water, and for lettuces and herbs eaten raw, water at the root rather than over the leaves. As a rule of thumb: rainwater may go to the root anywhere, but for the harvest-ready leaf, prefer tap water.
Remember the frost. A full plastic barrel can burst in winter as the ice expands. Empty barrels in late autumn, from around mid-November · CW 46, turn the rain collector to its winter setting or leave the lid open. Cisterns in the ground sit below the frost line and stay in service.
Open water containers are a serious danger to small children: a toddler can drown in just a few centimetres of water. Secure every barrel with a firm, child-proof lid and cover cistern shafts with a bolted, load-bearing lid. This is not a comfort feature but a duty.
The same part fights both mosquitoes and algae: a tight-fitting, opaque lid. It keeps female mosquitoes from laying eggs and lets no light inside, so no algae either. If larvae do appear, a biological product based on the agent from Bacillus thuringiensis israelensis targets them specifically and spares plants, hedgehogs and birds.
Common questions about harvesting rainwater
Häufige Fragen
How much rainwater can I collect from my roof?
The rule-of-thumb formula is: roof footprint in square metres times annual rainfall in millimetres times the runoff coefficient. With a 50 m² roof, 700 mm of rainfall and a tile roof (factor 0.8), that comes to around 28,000 litres a year. What counts is the footprint seen from above, not the pitched roof surface, plus the runoff coefficient of your roof type: tile and metal deliver 0.8 to 0.9, green roofs only 0.3 to 0.5.
Is a cistern worth it or is a rain barrel enough?
It depends on your water demand and your area. For a balcony, raised bed and small beds, a barrel of 200 to 500 litres for 40 to 80 euros is plenty. As soon as you supply a large vegetable garden, a lawn or an automatic irrigation system and want to bridge dry spells of several weeks, the cistern with 2,000 to 10,000 litres pays off. It costs 3,000 to 5,000 euros installed but lasts decades and pays for itself through the saved water costs.
Is rainwater good for all plants?
For watering, rainwater is ideal because it is soft and low in lime. Lime-sensitive plants such as hydrangea, rhododendron, azalea, blueberry and potted citrus even clearly prefer it over hard tap water. In the garden you can use it everywhere without concern. For raw-eaten leafy vegetables such as lettuce, water at the root rather than over the leaves. Collected rainwater is not drinking water.
How do I keep mosquitoes out of the rain barrel?
A tight-fitting lid is the most effective measure, because it prevents female mosquitoes from laying eggs. A dark, opaque barrel also stays free of algae because no light gets in. If larvae do appear, a biological product with Bacillus thuringiensis israelensis helps, as it only hits mosquito larvae and is harmless to plants and animals. A few water fleas also keep the surface in check.
Do I have to empty the rain barrel in winter?
Yes. A full plastic barrel can burst in hard frost because freezing water expands. Empty it in late autumn, from around mid-November · CW 46, and set the rain collector to its winter position or leave the lid open so no water collects. Buried cisterns sit below the frost line and stay in service all year.
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