Quick answer
A rain sensor for irrigation is a small device wired to a sprinkler controller that stops scheduled watering after a set amount of rain has fallen, then lets watering resume once the sensor dries out.
It reads rain at one spot, not the water in your soil. UF/IFAS says the most popular type uses expanding disks, and that it works best with a low set point, clear sky exposure and a yearly test.
Quick facts
The key facts about Rain Sensor for Irrigation, with sources:
- Three common types
- Water weight, electrical conductivity and expanding disks (hygroscopic) 1
- Most popular
- Expanding disks, for low cost, simple installation and low maintenance 1
- Set point
- Use the smallest set point possible; most devices go down to 1/8 inch; UF suggests not exceeding 1/4 inch 1
- Dry-out time
- About 19 hours on average after rain stops, regardless of brand, in UF tests 1
- Test
- At least once a year with a hose; disk types respond within a few minutes 1
- Disk life
- Replace expanding disks at least every two years 1
- Wireless range
- Up to 300 feet from the receiver 1
Key takeaways
- A rain sensor is a switch. It interrupts a schedule after enough rain and does not decide how long to water.
- It measures rain at one spot, so it cannot tell if your soil is wet, dry or soaked from the sprinklers.
- Placement decides most failures: open sky, out of the spray, away from roof runoff.
- Test it every year and replace expanding disks at least every two years, per UF/IFAS.
- A soil moisture sensor and a rain sensor answer different questions. Some yards use both.
What a rain sensor does, in two sentences and a catch
A rain sensor collects or absorbs rain. When enough has fallen, it opens a switch in the sprinkler controller’s wiring. The controller then skips its scheduled runs until the sensor dries out.
UF/IFAS describes these devices as ones that interrupt the cycle of an automatic irrigation system controller when a specific amount of rainfall has occurred 1. The WaterSense specification uses the term rainfall device for a device that senses or measures rainfall to reduce or interrupt irrigation in response to rain events 5.
The catch is what it cannot see. It does not read soil moisture. It also does not adjust how long each zone runs. A soil sensor and a weather-based controller work differently, and the comparison below shows where.
This page owns the device itself. For the controller it plugs into, see irrigation controllers. For controllers that adjust on their own, see smart irrigation controllers.
Three sensor types and what UF/IFAS says about each
| Type | How it senses rain | Strength | Weakness |
|---|---|---|---|
| Expanding disks (hygroscopic) | Disks absorb water and expand in proportion to rainfall. That presses a pressure switch, which stays open while the disks are swollen and closes as they dry 1 | The most popular type: low cost, simple installation and low maintenance 1 | Replace disks at least every two years 1 |
| Water weight | A small cup collects rain. At a preset weight, the connection to the valve is interrupted until the cup empties or the water evaporates 1 | Fast shutoff, almost immediately in a test 1 | Debris, birds or frogs can trigger it, and it needs more upkeep 1 |
| Electrical conductivity | Electrodes detect the water level in a small collection dish. Electrode spacing can be adjusted 1 | Fast shutoff, almost immediately in a test 1 | Debris and animals can set it off, and it needs more upkeep 1 |
Wired or wireless
Expanding-disk sensors come wired or wireless. UF/IFAS says wireless units work up to 300 feet from the receiver, and that the receiver connection and the battery are what to check when one stops working 1.
Rain sensor vs. soil moisture sensor: what each one measures
| Feature | Rain sensor | Soil moisture sensor | Where it is covered |
|---|---|---|---|
| What it reads | Rainfall that lands on the device 1 | Water content in the soil around a buried probe 2 | This page and soil moisture sensors |
| How it interrupts | Opens a switch after a set rainfall amount 1 | Compares the soil reading to a threshold. Wetter than the setpoint bypasses the run 2 | UF/IFAS AE221, AE437 |
| When it lets watering resume | When the device dries out, about 19 hours on average in UF tests 1 | When the soil reading falls below the threshold 2 | UF/IFAS AE221, AE437 |
| Where it sits | In the open, near the roofline or a post, away from spray 1 | Buried in the root zone, centered about three inches deep for turf 2 | UF/IFAS AE221, AE437 |
| Does it know the soil is wet from sprinklers? | No. It senses rain on the device only | Yes. It reads the soil, whatever put the water there 2 | Inference from how each device works |
| Counts as a smart controller by itself? | No 5 | No, not as the sole method; it interrupts or modifies a schedule 5 | WaterSense spec, section 1.0 |
Which one answers which problem
A rain sensor answers “did it rain enough to skip a run?” A soil moisture sensor answers “is the root zone wet enough to skip a run?” Neither one sets run times. For that, see the smart controller page.
UF/IFAS also says Florida’s rain shutoff rule can be met with a rain sensor or another shutoff device, such as a soil moisture sensor or an ET controller 1.
Where to put it and how it is wired
UF/IFAS gives these placement rules for the sensor 1. They matter more than the brand.
- Give it open sky. It needs unobstructed rainfall. Keep trees, overhangs and awnings from blocking rain onto the device.
- Keep it out of the spray. Do not mount it in the path of sprinkler spray, or the system can trip its own sensor.
- Keep it off roof runoff. Do not mount it where water runs directly off the roof.
- Mount it near the controller. It is typically mounted near the roofline. A closer sensor means a shorter wire and fewer wire breaks. If vandalism is not a concern, a fence post or deck railing can work.
- Wire it to the sensor terminals. If a sensor does not work with the controller, it can be hard-wired in series with the common wire.
- Turn the sensor switch to active. Many controllers have a sensor switch that must be set to Active for the sensor to count 1.
- Rain fallsOn a sensor with open sky
- Set point reachedDisks expand, cup fills or electrodes see water
- Switch opensThe controller skips scheduled runs
- Sensor dries outAbout 19 hours on average in UF tests
- Watering resumesOn the next scheduled start
Sequence follows UF/IFAS EDIS AE221. The dry-out time is UF’s average across brands, not a promise for your sensor. Source: [1]
Rules that can require one: the Florida statute and local codes
A state statute in Florida. UF/IFAS says Florida Statute 373.62, in its 2010 version, requires anyone operating an automatic landscape irrigation system to properly install, maintain and operate technology that inhibits or interrupts operation of the system.
The same publication says Florida is one of only a few states with such a statute, and that the rule applies to all automatic landscape irrigation systems 1. Statute text changes, so check the current version before you rely on it.
It need not be a rain sensor. UF/IFAS says a soil moisture sensor or an ET controller can meet the requirement too 1.
Other states and cities. We did not verify a statute for other states in the sources we loaded. Some cities and water suppliers set their own rules for new systems. Check with yours, and see the tracker linked in the local section for the rules we have verified.
A product label for smart controllers. The WaterSense weather-based spec requires a labeled controller to be able to connect to a rainfall device or soil moisture sensor 5. That is a design rule for the controller, not a rule for your yard.
Example: reading a yearly hose test of three sensors
UF/IFAS gives the test: make sure the controller’s sensor switch is on Active, turn on a zone, and spray the sensor with a hose.
Weight and electrode sensors should shut off the system almost immediately. Expanding-disk sensors should shut it off within a few minutes 1.
The three yards below are invented to show how to read the result.
| Sensor (example) | What happened in the hose test | What the UF/IFAS troubleshooting list points to |
|---|---|---|
| A: expanding disks, 3 years old | Zone kept running after five minutes of spray | Disks may be worn: replace at least every two years. Also check the sensor switch and the wires 1 |
| B: water-weight cup, under a tree | Shut off at once, but also trips on windy days | Clean it and move it to a spot with open sky and no debris 1 |
| C: wireless, 220 feet from the receiver | No response | Check the receiver connection and the battery. Range tops out at 300 feet 1 |
None of this tells you how much rain your yard needs. It only proves the switch works. To check run times after a wet spell, use the sprinkler runtime calculator.
Where you meet the term: installs, bids and old systems
On a new irrigation install. A contractor may list a rain sensor as a line item or bundle it with the controller. Ask for the type and the set point. Our irrigation service guide shows what a full system includes.
On an older system. Many yards have a sensor that was installed once and never tested. A disk type that is more than two years old is a candidate for replacement, per UF/IFAS 1.
On a controller screen. Look for a “sensor” port, a rain delay button or a sensor switch labeled Active or Bypass. The port is where the sensor wire lands, per UF/IFAS’s programming guide 4.
In an HOA or builder spec. New builds sometimes include one to meet a local rule. Check your documents, and the HOA landscaping research for how rules can overlap.
What a rain sensor adds, and what it does not
What it can do
- Stops scheduled runs after enough rain, which keeps sprinklers from watering a wet lawn on a timer 1.
- Low cost and simple installation for the expanding-disk type, per UF/IFAS 1.
- Works with a basic timer. It needs only a sensor port or a spliced common wire 1.
- Meets a shutoff rule where one applies, as with Florida’s statute 1.
What it does not do
- UF/IFAS says water savings have not been found significant in practice 1.
- It cannot tell if soil is already wet from watering or other sources. It reads rain on the device only.
- It does not adjust run times. A weather-based or soil moisture controller does 5.
- It can fail quietly. A worn disk or a broken wire may leave the system watering in a storm 1.
Limits and open points
The best published numbers here come from one university program in a wet, warm climate. UF/IFAS tested dry-out times and set points, and its guidance fits Florida conditions. Our readers may see slow dry-out in humid shade and fast dry-out in dry wind. We did not find a national dry-out figure.
UF/IFAS says the 19-hour dry-out time held across brands in its tests 1. That does not mean every unit resets in 19 hours. Mounting spot, ventilation and the disk’s age change it.
The same publication says water savings have not been found significant 1. That is not a case against the device. A rain sensor mostly prevents the obvious waste of watering during a storm, and many rules ask for it for that reason.
We found no extension source that compares rain sensors against soil moisture sensors in head-to-head residential trials, so the comparison table above rests on how each device works, not on a study.
What drives the price and how long a sensor lasts
We do not publish prices. A line for a rain sensor moves with the type (disks, weight or electrode), whether it is wired or wireless, how far the sensor sits from the controller and whether the wire run is easy.
UF/IFAS notes that a closer sensor means a shorter wire and fewer wire breaks 1.
The service life depends on the type. UF/IFAS says to replace expanding disks at least every two years, and notes that wireless units need battery checks 1. It does not give a life for the housing or the switch, and we give none.
The labor is usually small compared with the rest of an irrigation job. For what shapes a bid, read the cost hub, and to compare scope lines use the landscaping estimates guide.
Who installs it, and how to set the set point
A homeowner with a basic timer can often mount a sensor. An irrigation contractor can do it as part of a repair or a new install. The set point is the one setting to get right.
- Pick a spot with open sky, away from spray and roof runoff 1.
- Mount it and run the wire to the controller’s sensor terminals, or splice it in series with the common wire if needed 1.
- Set the point as low as the device allows. UF/IFAS says to use the smallest set point possible, 1/8 inch on most devices, and not to exceed 1/4 inch 1.
- Switch the controller’s sensor setting to Active 1.
- Run a hose test and note how long it takes the zone to stop 1.
- Write the install date on the housing so you know when to replace disks.
Low-voltage wiring only, and call 811 before you dig
Sensor wires carry low voltage from the timer. Do not touch mains power inside the controller; a licensed electrician handles that. If you bury wire or trench to move the sensor, call 811 first. Keep ladder work near the roofline safe, and do not work near power lines.
Upkeep: the short list from UF/IFAS
Upkeep tasks are listed here without fixed dates. UF/IFAS gives the frequency where it has one 1.
- Test at least once a year with a hose and a running zone.
- Replace expanding disks at least every two years.
- Clean the sensor if it trips on debris or stays off.
- Relocate it if trees, roof runoff or spray keep affecting it.
- Check the wires for breaks, and the controller’s sensor switch.
- For wireless units, check the receiver connection and replace the battery.
Warning signs a sensor is not working
- Sprinklers run in a downpour. The switch is open on the wrong side, the wire is broken or the sensor is set to bypass.
- The system stays off for days after rain. The sensor may be in a damp, shaded spot. UF/IFAS measured an average dry-out of about 19 hours 1.
- Random shutoffs on dry days. Water-weight and electrode types can be triggered by debris, birds or frogs 1.
- No change in the hose test. Replace or repair the sensor.
- The system trips itself. The sensor sits in sprinkler spray. Move it 1.
If a sensor test keeps failing after you clean and rewire it, a licensed irrigation contractor can look at the controller. Landscaping licensing by state shows who is licensed where.
Common rain sensor myths
- Myth A rain sensor makes my system smart.
- Reality It only interrupts a schedule. The WaterSense spec says rainfall devices and soil moisture sensors do not modify ET, so alone they do not meet its on-site weather sensor requirement 5.
- Myth A bigger set point saves more water.
- Reality UF/IFAS says to use the smallest set point possible and not to exceed 1/4 inch 1.
- Myth Once installed, it works forever.
- Reality UF/IFAS says to test at least yearly and replace expanding disks at least every two years 1.
- Myth Any sunny spot is fine.
- Reality It needs open sky but must stay out of spray and away from roof runoff 1.
- Myth It tells the system the soil is wet.
- Reality It senses rain at the device, not water in the soil. A soil moisture sensor reads the soil 2.
The regional variable: state and local shutoff rules
The one rule that varies by place is whether a shutoff device is required at all.
UF/IFAS says Florida Statute 373.62 requires technology that inhibits or interrupts operation of an automatic landscape irrigation system, and that Florida is one of only a few states with such a statute 1. Other places may rely on city codes or water supplier rules.
Check yours, and keep in mind that sensors do not replace watering days or bans. The tracker lists what we have verified.
When a rain sensor is worth adding, and your next step
Add one if your timer has no rain feedback at all, and your rules or your habits call for one. It is a small part of a bigger fix, not the fix itself.
If you want the system to read the ground, consider a soil moisture sensor or a controller that adjusts run times.
UF/IFAS describes ET controllers as ones that schedule watering from weather data, site and plant details, aiming to replace water lost since the last run 3.
The soil moisture sensor entry covers that route, and it can sit beside a rain sensor.
Your next step is to find your sensor, if you have one, and run the hose test. Then compare your yearly water use with the lawn water cost calculator.
Questions about Rain Sensor for Irrigation
What does a rain sensor do on a sprinkler system?
It stops scheduled watering after a set amount of rain. UF/IFAS describes rain sensors as devices that interrupt the cycle of an automatic irrigation controller when a specific rainfall has occurred. When the device dries out, watering can resume on the next scheduled start. It does not read soil moisture.
How does a sprinkler rain sensor work?
The most popular type uses expanding disks that absorb rain and swell in proportion to rainfall. The swelling presses a switch that stays open while the disks are wet and closes as they dry. Weight and electrode types collect rain and cut the connection to the valve at a set level.
Where should a rain sensor be installed?
In the open, with unobstructed rainfall. UF/IFAS says to keep it out of sprinkler spray, away from direct roof runoff and clear of trees, overhangs and awnings. It is typically mounted near the roofline, because a shorter wire means fewer wire breaks.
What is the best set point for a rain sensor?
Use the smallest set point possible. UF/IFAS notes that most devices go down to 1/8 inch and recommends not exceeding 1/4 inch. A higher set point lets the system keep watering after rain that was enough to skip a run.
How do I test my rain sensor?
Set the controller’s sensor switch to Active, turn on a zone and spray the sensor with a hose. UF/IFAS says weight and electrode sensors should shut the system off almost immediately, and expanding-disk sensors within a few minutes. Test at least once a year.
How long does a rain sensor last?
UF/IFAS says to replace expanding disks at least every two years. It gives no life for the rest of the device, so check the maker’s warranty. Wireless units also need battery checks. Test yearly so a worn sensor does not go unnoticed.
Why will my sprinklers not turn back on after rain?
The sensor may still be wet. UF/IFAS measured an average dry-out of about 19 hours after rain stopped, regardless of brand. A shaded, damp spot can be slower. Check the mounting spot and the sensor switch, and clean or move the sensor if it stays tripped.
Is a rain sensor the same as a soil moisture sensor?
No. A rain sensor senses rain landing on the device and interrupts a schedule. A soil moisture sensor reads water in the root zone and bypasses runs when soil is wet enough. They can work together. Neither one sets run times on its own.
Are rain sensors required by law?
In some places. UF/IFAS says Florida Statute 373.62 requires technology that interrupts an automatic landscape irrigation system, and Florida is one of only a few states with such a statute. A rain sensor is one way to comply. Other places vary, so check your city or water supplier.
Do rain sensors really save water?
UF/IFAS says savings have not been found significant in practice. A rain sensor mostly prevents watering during or right after a storm. Bigger savings come from adjusting run times, which weather-based and soil moisture-based controllers do.
Sources
- UF/IFAS Extension, Residential Irrigation System Rainfall Shutoff Devices, or Rain Sensors (EDIS ABE325/AE221), retrieved .
- UF/IFAS Extension, Smart Irrigation Controllers: How Do Soil Moisture Sensor (SMS) Systems Work? (EDIS AE437), retrieved .
- UF/IFAS Extension, Smart Irrigation Controllers: What Makes an Irrigation Controller Smart? (EDIS AE442), retrieved .
- UF/IFAS Extension, Smart Irrigation Controllers: Programming Guidelines for Evapotranspiration-Based Irrigation Controllers (EDIS AE445), retrieved .
- U.S. EPA WaterSense, Specification for Weather-Based Irrigation Controllers, Version 1.1 (September 2, 2021), PDF read as text, retrieved .
Expert review
Written by the HyreYard Research Desk. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Types, placement, set points, dry-out time, test method, disk life, wireless range and the Florida statute reference come from UF/IFAS EDIS ABE325 (AE221), which shows no revision date on the page.
The soil moisture comparison uses UF/IFAS AE437 and the WaterSense Version 1.1 specification, read as text. The statute number is as UF/IFAS prints it; check current law. The sensor test examples are invented and labeled. No prices or schedules are given.
Suggest a correction. We fix errors and say what changed.