Rain sensor for irrigation: legal requirement and how it works
Rain sensors are legally required in Italy on all automatic irrigation systems. Learn how they work, where to mount them, how to wire them to the controller, and which models to choose.
Legal requirement: Italian regulations
Legislative Decree 152/2006 (the Italian Environmental Code) and subsequent regional guidelines require every automatic irrigation system installed in Italy to include a rain or soil-moisture sensor. The goal is to prevent water waste: a system running in the rain is not only useless but a violation of water-saving rules. Many water utilities offer tariff discounts to users who install automatic control systems such as rain sensors.
The regulation applies to new installations but it is good practice to retrofit existing systems, especially in municipalities that apply progressive water tariffs. Beyond legal compliance, a rain sensor typically pays for itself in the first season through reduced water bills.
How a rain sensor works
A standard rain sensor contains stacked hygroscopic discs — usually compressed cork or ceramic foam. When rain wets them, they expand and open an electrical contact that interrupts the controller signal, preventing the irrigation cycle from starting. Once dried out (typically 4–24 hours depending on the model and adjustment setting), the discs contract and the contact closes: the system is operational again.
Wireless sensors work on the same mechanical principle but transmit the signal by radio to a receiver connected to the controller, eliminating the external cable run. They are easier to install but require periodic battery replacement (every 1–3 years). European wireless sensors use 868 MHz; verify compatibility with your controller before purchasing.
Where to install the sensor
Mount the sensor in a location that receives direct rainfall, not sheltered by roof overhangs or tree canopy. Ideal positions are: the roof edge on the most exposed side, a perimeter wall or fence, or a dedicated post in the garden. Recommended height is 1.5–2.5 m above ground to avoid false triggers from soil splash.
Never position the sensor under the eaves: even in moderate rain the overhang shields it and it will not trigger. A common mistake is mounting it near a sprinkler head — irrigation spray activates the sensor every cycle, blocking the system permanently.
Wiring: hard-wired and wireless
A wired sensor connects to the SENS or RAIN SENSOR terminals on the controller with standard two-conductor wire. Most controllers have a small jumper shorting the SENS terminals when no sensor is connected. Remove this jumper before wiring the sensor, otherwise the sensor has no effect. The sensor is always wired in series in the control circuit, never in the power circuit.
To set the rain threshold, most sensors have an adjustment dial ranging from 3 mm to 25 mm of rainfall. For a Mediterranean garden, 6–8 mm is the recommended value: this represents rain that meaningfully wets the soil. With lower thresholds (3 mm) the sensor trips on light showers that deliver no usable soil moisture.
Maintenance and common faults
Rain sensors need very little maintenance. Once a year, in spring, wipe the sensor body with a damp cloth and check that the hygroscopic discs are not calcified: hard water can deposit limescale that prevents correct expansion. If the sensor is always active (system permanently blocked) even when dry, the discs are probably calcified or swollen — remove and replace them (under €5).
The opposite problem — sensor that never trips — indicates worn discs or a broken wire. Check the cable from sensor to controller with a multimeter: it should show continuity when dry and open circuit when wet. If the controller ignores the sensor entirely, verify that the jumper on the SENS terminals has been removed.
Giacomo F. — Founder of SprinklerMap
Software developer and gardening enthusiast. Designed and installed residential irrigation systems in Italy before building SprinklerMap. Our story →