# Rainwater Collection

> Part of: [Greenhouse Construction](/greenhouse-construction.md) · Provider: [DutchGreenhouses](/)

Rainwater collection is built into the roof rather than added to it: the V-shaped roof drains into aluminium gutters that are also structural members, feeding a storage basin. Roof-collected rainwater is the reference irrigation source because its sodium content is near zero.

## Facts
| Property | Value |
|---|---|
| Legal minimum storage | 500 m³/ha |
| Recommended storage | 1500 m³/ha |
| Sodium in collected rainwater | 0.1–0.5 mmol/L |
| Basin capture in a dry year | 42–54 % of precipitation |
| Catchment area | The greenhouse roof footprint area |

Structure

# Rainwater collection.

    The roof is a water harvesting system.

## The roof is a water-harvesting system

    A greenhouse presents an enormous, clean catchment area to the sky, and good design puts it to work. The typical Venlo roof is a series of V-shaped valleys, and every one of those valleys drains into the aluminium gutters that also carry the structure — so the same components that hold up the glass double as a rainwater harvesting system. Collected from the roof and channelled into storage basins, rainwater is the preferred irrigation source of all: it is naturally soft, low in salts and essentially free, giving the crop the cleanest possible starting point before any treatment or fertilising.

Because of that quality, rainwater sits at the top of the irrigation hierarchy. Soft water means less pre-treatment, no need for a water softener to strip chloride, and more efficient, more predictable fertilisation because the grower is not fighting minerals already dissolved in the supply. For a substrate crop fed precisely through the dripper, starting from a near-blank slate is a genuine agronomic advantage, not just an environmental one.

The catch is that rain is intermittent, so the system stands or falls on storage. Basin sizing is a real engineering calculation that weighs local rainfall statistics — how much falls, and critically how unevenly across the year — against the crop's daily water demand, so the basin can carry the operation through dry spells without running empty. Get it right and rainwater covers most of the year's needs; undersize it and the greenhouse is forced back onto more expensive or lower-quality sources at exactly the wrong moments. In most projects rainwater is paired with a backup source and, increasingly, with drainwater recycling, so the two together close the loop toward near-zero external water use.

All of this has to be anticipated during construction, before the roof closes: the gutters, downpipes, first-flush arrangements and basin connections are part of the build sequence, not an afterthought. We size and integrate the rainwater system into the structure and the wider irrigation strategy so your greenhouse captures the water that falls on it and puts it to the best possible use. Get in touch to design rainwater collection for your project.

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## FAQ
**Why is rainwater the preferred irrigation source?**
Because of what it does not contain. Rainwater carries 0.1 to 0.5 millimoles of sodium per litre against 3 to 10 for well water in the west of the Netherlands. Sodium is not taken up by the crop, so it concentrates with every recirculation pass until discharge is forced. A low-sodium source postpones that for months.

**How large should the storage basin be?**
Dutch law requires at least 500 cubic metres per hectare, but that is a floor rather than a target. Wageningen recommends above 1,500, and its own modelling shows the legal minimum still forcing mains water and discharge in a dry year while 3,000 can eliminate discharge altogether.

**How is rainwater collected in a greenhouse?**
Through the structure itself. The V-shaped roof drains to aluminium gutters that are simultaneously structural members tying the trusses together, so the drainage system and the frame are the same components. Water is managed on site before the roof closes, which is why it appears early in the build sequence.

**Does collected rainwater still need treatment?**
Yes, though far less than other sources. Rainwater needs little beyond filtration and storage, while well and surface water carry iron, bicarbonate, sodium and pathogens that must be removed before the water reaches the fertiliser unit. Covering the basin also matters, because algae growth ruins ultraviolet transmission downstream.

## Related
[Greenhouse Construction](/greenhouse-construction.md) · [Aluminium System](/greenhouse-construction/aluminium-system.md) · [Greenhouse Glass Covering](/greenhouse-construction/greenhouse-glass-covering.md) · [Pre-treatment](/irrigation/pre-treatment.md) · [Gutter](/glossary/gutter.md) · [Rainwater Basin](/glossary/rainwater-basin.md)