Rain sensors and what they actually do

What it does

Reacts within a second or two of the first drops and drives the window shut. A rooflight typically closes in fifteen to thirty seconds, so a small amount of rain gets in during that window.

In practice that is a few drops on a worktop rather than a wet floor, which is exactly the outcome you want.

What it does not do

It does not predict. There is no forecast involved, no barometer, no cloud detection. It is a wet contact and a relay.

It also does not distinguish rain from other water. A hose, a window cleaner or heavy dew will close the window, which is harmless but occasionally puzzling.

Reopening

Most systems do not reopen automatically when the sensor dries, and that is the right decision: a window reopening unattended is worse than one staying shut. You reopen it manually.

Some systems latch closed until reset. Worth knowing which yours does at handover.

When it is worth fitting

Yes where you might open it and leave the room or the house: a kitchen, a bathroom, anything on a timer or automated.

Optional where you only open it while you are in the room and would notice rain anyway.

The cost is modest at installation and higher afterwards, because the sensor cable usually runs back to the control point.

Maintenance

Wipe it once a year. A dirty sensor either fails to detect rain, which defeats the point, or reads permanent moisture from grime and refuses to let the window open at all. The second is the more common complaint and it looks like a fault.

On a lantern

Particularly worth having. A lantern vent is at the highest point of the house and it is the one people most often forget is open.

The feature that makes a motor worth having

Most people assume the value of a motorised rooflight is the motor. It is not. It is the rain sensor, and the reason is behavioural rather than technical.

A window you have to remember to close before rain is a window you leave shut. The risk of coming home to a wet floor outweighs the benefit of ventilation, so the sash stays down and the room does not get aired.

A window that closes itself is a window you leave open. That single change is what alters how a room behaves through a summer, and it is worth more than the convenience of pressing a button.

How it works

A small detector on the frame, usually near the head of the sash, with a surface that changes electrical conductivity when it gets wet.

When it detects moisture it signals the control unit, which drives the actuator to close. That takes fifteen to thirty seconds on a mains unit and thirty to sixty on a solar one.

Most systems will not reopen automatically afterwards. The window stays shut until you open it again, which is generally the behaviour you want: you do not want a rooflight opening and closing repeatedly through a showery afternoon.

The limits of the sensor

Worth being straight about, because the expectations are frequently higher than the product.

It does not close before the first drop. It detects rain because rain has landed on it. A small amount of water gets in during the closing cycle, which is a few drops rather than a problem, and it is not zero.

It does not detect wind. A dry gale will not close the window, and a large open sash in a gust is a real load on the actuator and the stay. On an exposed or coastal site that matters, and some systems offer a wind sensor as a separate option.

It does not work without power. In a mains cut the sensor is out. A chain actuator holds its position mechanically so nothing moves, and the scenario to plan for is open when the power goes and then it rains. Solar units are unaffected.

It does not predict. A local sensor reacts. That is the behaviour you want.

Local sensor against a weather service

Some smart systems can close a window based on a forecast or a rainfall report for the nearest weather station rather than on a sensor at the window.

Take the local sensor. You want the window to close because it is raining on it, not because a service believes it might be raining three miles away, and not to stay shut on a dry afternoon because a forecast was pessimistic.

Where a system offers both, the sensor should be the primary and the forecast a supplement.

The maintenance nobody mentions

A sensor caked in grime does one of two things, and both get misdiagnosed.

It misses rain, because the dirt insulates the detecting surface. The window stays open and you get a wet floor.

It reads permanent moisture from the grime itself, and refuses to let the window open at all.

The second is the more common and it looks exactly like a motor fault. It produces a surprising number of unnecessary callouts, and the fix is a damp cloth.

Wipe it once a year when you clean the gasket. That is the whole of it.

On a group of windows sharing one sensor, a dirty sensor locks out the entire group, which looks considerably more serious than it is. If several units stop responding at once, suspect one sensor before four actuators.

Sensors in a group

On most systems a single sensor closes a whole group of windows.

That is sensible and it means one sensor position has to represent the weather at all of them. On a house where units face different directions, a sensor on a sheltered slope may not detect rain that is soaking the exposed one.

Where that matters, a second sensor or a second group is the answer, and it is worth raising at design stage rather than discovering after the first westerly.

Night purge, which is what it enables

The single most useful thing a rain sensor makes possible.

Through a summer day a building absorbs heat: through glazing, through the fabric, and from cooking and occupancy. By evening the structure is warm and will radiate that heat back into the rooms all night.

Open a rooflight late, with something open at low level, and the accumulated heat leaves the building overnight while the outside air is cool. Close it in the morning before the outside air warms.

Doing that by hand requires somebody to remember twice a day and to be confident it will not rain at four in the morning. With a rain sensor, and better still with a schedule, it simply happens.

On a south or west facing extension that is the difference between a room used in July and one avoided.

Where it matters most

Any rooflight above easy reach, where closing manually means finding a pole.

Any lantern vent, which is always high.

Any flat roof rooflight, because a horizontal opening admits considerably more water in a shower than a pitched one.

And any coastal or exposed site, where weather arrives quickly off the water and with little warning.

What we specify

A rain sensor as standard on any motorised unit rather than as an option, because the unit costs very little more with one and it is what makes the motor worth having in the first place.

A wall switch alongside it, beside the light switches at the room entrance, so the window can be operated by anybody without a remote.

And the manual override demonstrated at handover, with whatever it requires supplied rather than assumed, for the evening the power goes out with the window open.

How the sensor works

A small pad on the outside of the unit, usually on the frame near the head. Most work by conductivity: water bridging two contacts changes the resistance, the controller reads the change and signals the actuator to close.

It is a simple, robust arrangement with one consequence worth understanding. It detects water on the pad, which means water has already arrived. There is no forecasting and no anticipation.

So a small amount of rain enters before the unit shuts. On a light shower over a worktop that is a wipe. It is not a reason to leave a rooflight open with a storm visibly approaching.

What it will not detect

Wind. No mainstream domestic sensor detects it. A rooflight open in a gale is a real risk to the sash and the actuator, and nothing in the system addresses it. The answers are a wind rated unit or the habit of closing it when the forecast is bad.

Reopen. Once it has closed the unit, it stays closed. It does not restore the previous position when the rain stops, which surprises people who expected to come home to a ventilated house.

Work without power. The sensor is part of the powered system. In a mains cut it cannot close anything unless there is battery backup, and storms and power cuts arrive together.

The maintenance nobody does

A sensor accumulates dust, pollen, and in time a film of organic deposit. Two failure modes follow.

It stops responding. The dangerous one, because there is no symptom until the rain comes in.

It responds when it should not. Damp deposit bridges the contacts in humid conditions, and the rooflight closes on a dry morning. Irritating, and at least it announces itself.

A wipe with a damp cloth once a year prevents both. Put it in the same routine as clearing the sill channel and washing the ventilation filter.

Overriding it

Most systems let the sensor be disabled temporarily, which is useful on a warm evening when passing showers keep shutting the window.

The risk is forgetting. Where a system has a timed override that restores itself, use that rather than a permanent disable.

Where it is genuinely essential

Any powered rooflight in a house empty during the working day. Any rooflight over a kitchen, a wooden floor or a stairwell. Any unit out of reach.

The cost at installation is small and the cost of adding one later is a call out, so on almost any powered unit the arithmetic favours fitting it.

Positioning the sensor

Manufacturers site it where rain will reach it promptly, usually on the frame near the head of the unit.

Two things can defeat that. An overhanging tree keeps the sensor dry while rain reaches the rest of the roof at an angle. And a deep reveal or an adjacent structure can shelter it.

Worth looking at the actual position rather than assuming, particularly on a retrofitted opening kit where the sensor is added rather than designed in.

What happens after it closes

Nothing. The unit stays shut until somebody opens it again.

It does not restore the previous position when the rain stops, which surprises people who expected to come home to a ventilated house after a passing shower.

Where that matters, a scheduled opening through a gateway is the answer rather than expecting the sensor to manage it.

Testing it

Once a year, wipe the sensor and then wet it with a damp cloth while the unit is open. It should close.

If it does not, the sensor is dirty, the pairing has dropped, or the battery in a solar unit is low. All three are cheap to resolve and all three are silent failures that only reveal themselves in rain.