Electric rooflight rain sensors, and what they treat as rain

A sensor can only react to rain that reaches it. Placed badly, it lets the first drops land on the open sash and the floor below before anything moves. Placed well, it sees weather a few seconds before the room does.

There are two common positions. Some electric opening rooflights carry a sensor built into the frame, on the outer edge of the glazed unit, so it shares the exact weather the glass is getting. Others use a separate sensor on its own short cable, fixed to the roof deck, the kerb or a nearby wall. A built-in sensor is the simplest to live with, because it leaves the roof outline clean and cannot be knocked out of place by anyone working up there. A separate sensor gives freedom to choose the spot, which matters when the rooflight sits in a sheltered position.

Shelter is the trap. A sensor tucked under an overhanging eave, behind a parapet or in the lee of a chimney can stay dry while rain slants onto the open glass. The test is simple. Stand where the sensor will be and ask whether a slanting shower would hit it before it hit the sash. If the answer is no, it moves, or we choose a different product.

Height and angle count too. A sensing plate laid flat collects water from above and holds beads of it. One fixed on a steep face may shed drops almost as quickly as they land, and a light drizzle can slide past it unnoticed. Most units are designed to be mounted at a slight tilt on the same plane as the glazing, and the maker's fixing guide sets that out. We follow it, and we note the position on the quote.

A rain sensor does not know it is raining. It knows the surface of a small plate has changed. Understanding that one fact explains nearly every odd behaviour people report.

The typical sensing plate carries a pattern of fine conductive tracks. Dry, the tracks are separate and nothing flows between them. A drop of water bridges two tracks and lets a tiny current through. The control reads that as wet and sends the close command. The plate is often gently warmed so that moisture dries off again after a shower, which lets the control know the rain has stopped. Without that warmth, a plate could stay wet through a foggy morning and keep the sash shut.

So the sensor reacts to anything conductive that bridges the tracks. Rain does. So do heavy dew, sea-fret style mist on a cold morning, a hose, the spray from a pressure washer, meltwater, and the water that runs off a roof above when a neighbour empties a bucket. The table sets out how each usually plays out.

None of that is a fault. It is the sensor doing the job it was given. What it cannot do is tell the difference between a storm and a spray of water from a hose, so the system leaves a human override in place, which we come back to below.

People talk about a rain sensor as a way to protect a floor. It does that. It also protects the quiet of the room, and that part is easy to overlook.

An open sash changes how a room sounds. Glass that is shut is part of a sealed envelope. Glass that is open is a gap straight to the sky. Rain falling on the sash makes a different sound when it hits an open frame, the kerb and the gap beside it, and the room hears all of it. The moment of closing matters as well, because a sash that shuts while a downpour is already under way shuts into a wall of noise, and the seal closes onto a wet gasket.

The sensor moves the closing to the start of the weather, not the middle. The sash is already shut, seals compressed, by the time the rain is heavy enough to drum on the glazing. What you then hear is the laminated acoustic pane doing its work, which is a far softer sound than rain on an open vent. For a loft bedroom or a kitchen extension under a lantern, that difference is the point. Our page on whether electric rooflights are noisy when they open and close deals with the motor itself, and the quiet spec describes the glazing build for the closed position.

This is why we treat the sensor as part of the quiet specification, not an accessory. The written spec that comes with each quote states the glazing, the seals and the upstand. For an electric opening unit it also states the sensor type and where it is fixed.

A rain sensor rarely works alone. It sits in a small network with the motor, the control unit and the switches you use, and the order of priority between them is set in the control.

In most systems the sensor has priority over your normal open command while it reads wet. You press open, the control refuses, or opens and closes again. That is deliberate. A sash that opened in rain on your instruction would defeat the sensor entirely. A good control also has a clear way of overriding the sensor for a short time, for example when you are cleaning the glass from a ladder and want the vent open despite the wet plate. Our guide to controls for electric rooflights covers the switches, remotes and keypads, and which of them can override the sensor.

Wind is a second input in some systems. A separate wind detector can close the sash in a gust, and the sensors are often sold together. Not every house needs one. A rooflight over a sheltered rear extension sees different wind from one on an exposed ridge, and we talk it through at the survey.

Timers and thermostats are a third. A control set to open the rooflight at a set temperature, for night cooling, sits beneath the sensor in priority. The sensor can always close the sash, and the timer can only open it when the plate is dry. That arrangement is what makes unattended night ventilation reasonable. The account of night ventilation without the noise explains how the pieces combine.

A sensor needs power to be useful, and it needs a dependable route back to the control. Both have to be designed in, because neither is pleasant to add once the ceiling is plastered.

The simplest arrangement has the sensor built into the frame and powered through the same supply as the motor. Nothing extra to route. A separate sensor adds a small low-voltage cable, which has to run from the roof to the control without being trapped, crushed or chewed by something in the loft. That run is planned with the wiring, and the wiring has its own page: wiring and powering an electric rooflight. Where a mains feed is needed, the question of who does what is answered in our note on whether an electric rooflight needs an electrician.

Solar powered opening rooflights change the picture slightly. The sensor, the motor and a small battery all live on the unit, and the panel keeps the battery topped up. The sensor still closes the sash, and it does so even when the battery is low, because closing is the priority movement. The solar powered opening rooflights guide explains how the pieces are sized.

A cut to the house supply raises the obvious question. If the sash is open when the power goes, what then? Some systems carry a battery backup that completes a close, others stay where they stopped until the supply returns. The page on what happens to an electric rooflight in a power cut sets out the choices, and they affect which sensor and control we suggest.

A sensing plate is exposed to everything the roof sees. Over time that shows, and the way it shows is predictable.

A film of pollen, dust or bird droppings on the plate can hold moisture and keep it reading wet. The sash then stays shut on a fine day, and the room gets no ventilation. A leaf lying across the tracks does the same. Salt-laden air is less of a concern inland than on the coast, but a sheltered plate under a tree can collect a greasy film from sap and honeydew. Keeping the sensor clear is part of routine care, and looking after an electric rooflight describes how, with a soft cloth and plain water, and no abrasives that might scratch the tracks.

Trees deserve a thought at the planning stage. A rooflight under an overhanging branch drips after the rain has stopped, because water collects in the leaves and falls in big slow drops. A plate under those drops keeps reading wet, and the sash may close and open again repeatedly. Where a tree is close, we either move the sensor out from under it or choose a position where drips do not fall on the plate.

The opposite failure is a plate that stays dry in genuine rain because the water runs off it. A smooth, glossy, steeply tilted plate can shed drops from a shower before they bridge the tracks. That is why the maker's mounting angle is worth following to the letter.

The job is the same on every electric unit, but the sensor's natural home differs, and so does the noise question.

A hinged electric roof window on a pitch tends to have the sensor on the frame or the head of the unit, beside the glass. The slope does the work of shedding water, and the window is usually small enough for a single sensor to cover it. On a loft bedroom, the hinged sash closes against a seal at the head and sides, and an early close keeps rain off the ceiling reveal.

A flat-roof opening lantern or sliding rooflight is different. The glazed area is large, water lies on the roof around it, and a sliding unit has tracks that hold water in a way a hinged sash does not. The sensor on a flat roof has to be raised enough to read real rain, not standing water on the deck, yet low enough not to be a tripping hazard for anyone maintaining the roof. On a terrace access unit, the point is covered in sliding rooflights for roof terrace access, and the watertightness of the closed unit is handled in whether a sliding rooflight is watertight when it is closed.

A stairwell or landing rooflight is often out of sight from the rest of the house. You cannot see it raining on it. A sensor is especially useful there, because nobody will notice the open sash until the carpet is wet. The stairwell case gets its own treatment in electric rooflights for stairwells and landings.

Crawley sits inland, with the Weald to the south and the Downs beyond, and the rain that reaches it arrives on the prevailing wind from the south-west. That matters for the sensor in a small way. A plate on the south-western face of a roof gets wet first, and a plate on the lee side can lag behind. Where a rooflight is open on the side that faces the weather, we position the sensor on that same side.

The town's homes are mostly New Town stock from the 1950s to the 1970s, with loft conversions and flat-roofed extensions added since. Those flat roofs sit low, often close to fences and trees, and the rain on them is noisier than on a pitch. A lantern over a kitchen extension can sound like a tin roof in a downpour, which is exactly when you want the sash down and the glazing doing its work. The page on electric rooflights over a kitchen extension follows that room through a wet evening.

Not everyone needs a rain sensor. It helps to know when it earns its place.

If the rooflight is reachable, you are home most days, and you are happy to close it by hand, a manual or simple switched unit works. The sensor starts to matter when the rooflight is out of reach, out of sight, or opened while nobody is at home. Anyone who opens a loft window on the way to work and does not think about it again is exactly who the sensor is for. Stairwells, high kitchen lanterns and night ventilation are the same. The question of value is taken on in whether an electric rooflight is worth it over a manual one, and the wider picture is in the hub for electric and opening rooflight installation.

A motorised unit that is open a lot also needs the sensor more, and it raises the small matter of how an open unit sounds and feels. We come back to that in the account of how electric opening rooflights work.

The sensor is a small part of the job, and it is easy to forget until it fails to behave. Writing it down avoids that. For each electric opening unit, the quote and the quiet spec name the following:

1. The sensor type: built into the frame or separate.
2. Where it is fixed, in words and on the sketch, with the reason for the position.
3. How it is powered and how it connects to the control.
4. What the override does and which control has it.
5. Whether a wind sensor is included, and what it is set to do.
6. How the sensor is kept clean.

On the day, the work is installed to current Building Regulations, and where it is notifiable we handle the Building Control notification. The sensor test with a cup of water is part of handover, and the sash is left closed with the control set as agreed. The work carries our 10-year workmanship guarantee, described in our guarantee.

If you already have a manual opening rooflight and would like to add a motor and a sensor, read whether a manual opening rooflight can be motorised first. When you are ready to talk about your own roof, get a quote and tell us which room the rooflight serves.

These are the points that come up most often once a sensor has been chosen.

Will the rooflight close if I am out?

Yes, provided the control has power and the sensor is wired in. The sensor works without anyone present. That is its main purpose. If you want to be sure, the day-of-handover test shows the sash closing on its own.

Can I stop it closing while I clean the glass?

Most controls have a temporary override for exactly that. You open the sash by hand command, and it stays open for a set time or until you cancel. The sensor returns to full control afterwards. The override is described on the quote so you know which button does it.

Why did it close on a dry morning?

Usually dew, mist, or a film on the plate. A dirty or damp plate reads wet. Wiping it with a soft cloth and plain water clears it. If it keeps happening, the position or the tilt may need adjusting, which we look at together.

Does a rain sensor work with smart home controls?

The sensor stays in charge whatever you use to open the unit. A voice or app command can ask the sash to open, and the sensor can still refuse or close it. The page on whether an electric skylight works with Alexa or Google Home explains the arrangement, and the page on whether electric rooflights close by themselves when it rains covers the same ground from the short-answer side.