Rain sensors and what they actually do
A rain sensor on an electric rooflight is a small plastic block, usually 60 to 80mm across, fixed to the outside of the sash near the top. It detects water landing on its surface and tells the control unit to close the window. That is the whole function, and understanding exactly how it senses water explains everything people find surprising about it: why it sometimes closes on a heavy dew, why it can take a few seconds to react to the first drops of a Severn Vale shower, why it will not reopen the window afterwards on most ranges, and why it is a convenience device rather than a guarantee against water getting in. This page covers the sensor itself and how it behaves. Whether a given installation warrants one is a separate question, and the wider electric specification sits on the electric rooflight installation page.
The sensing element, and how water is detected
Almost all rooflight rain sensors work by conductivity. The exposed face of the sensor carries two metal tracks laid out as an interlocking comb, with a small gap between them. Dry, the gap is an open circuit. When water bridges the tracks, rainwater carries enough dissolved mineral content to conduct a tiny current, the circuit resistance drops sharply, and the electronics behind the tracks register the change.
No moving parts, nothing to calibrate, and almost nothing to go wrong electrically. The whole assembly is a printed board, a sensing surface and a two core lead into the sash. That simplicity is the reason the part is cheap and reliable, and it is also the reason for every one of its limitations, which follow.
The heater, and why the sensor face is warm
A conductivity sensor left to itself would stay wet long after the rain stopped, and the window would remain closed for hours on a damp day. So the sensor face is gently heated, typically by a low power resistive element behind the tracks drawing around one to two watts.
The heat does two jobs. It evaporates residual water once rain stops, so the sensor resets in a few minutes rather than waiting for the weather. And it keeps the surface slightly above ambient temperature, which suppresses condensation and dew forming on the sensor when the rest of the glass is running wet on a still autumn morning in the vale. The heater runs continuously whenever the system is powered, which is a small standing consumption worth knowing about on a solar unit.
How fast it reacts, and what happens in the first seconds
Detection is close to instant. The bridging of the tracks happens with the first few drops, and the controller sees it within a second or so. What takes time is the sash.
A roof window actuator moves at perhaps ten millimetres a second, so a sash sitting at a 300mm opening takes around thirty seconds to reach the frame. During that time rain is entering the aperture. The total is normally a few seconds of proper wetting and a small amount of water on the reveal, which on a top hung unit largely runs down the outside of the sash rather than into the room. It is not nothing, and it is very much less than the window would have taken had nobody been home to shut it.
Why it does not reopen when the rain stops
On most manufactured ranges, closing on rain is a one way command. The window shuts, and it stays shut until somebody presses open again. This surprises people who expect the automation to be symmetrical.
The reasoning is sound. A window that reopened on its own would reopen during a lull in a squally afternoon, then close again, then reopen, cycling the mechanism repeatedly and letting water in each time. It would also reopen after dark, after the house was locked up, and while nobody was there to notice. A one way close is predictable and safe. Where genuinely automatic behaviour is wanted, it comes from a controller with humidity or timer logic rather than from the rain sensor, which is covered on the controls and smart home page.
What triggers it besides rain
Anything conductive that bridges the tracks will trigger a close. In practice that means a short list, and all of it turns up in Gloucestershire.
- Heavy dew, on a clear still night when the roof surface drops below the dew point. The heater suppresses most of this, but not all of it on a hard autumn morning.
- Wet mist and river fog off the Severn, common through Gloucester, Tewkesbury and the vale villages from October onwards.
- Melting snow and thawing frost running down the glass into the sensor.
- A hose or pressure washer during a window clean, which is why cleaners are told the window will shut on them.
- Bird droppings and sap drying across the tracks, which can hold a low resistance path long after the surface looks dry.
What it will not detect
The sensor only knows about water on its own small face. It has no idea what the weather is doing generally, and it cannot anticipate.
Wind driven rain arriving horizontally at a shallow angle can enter an open sash aperture and land on the reveal before it lands on a sensor sitting on the upper face of the sash. Rain approaching from directly behind a top hung sash, hitting the underside, is the awkward case. Fine drizzle, the kind that hangs over the wolds for a whole afternoon, may take longer to accumulate enough surface water to bridge the tracks than a proper shower does. And a sensor with a dried film of pollen or dust across it needs more water to reach the same conductivity than a clean one.
Where the sensor is mounted, and why the position is fixed
On a manufactured electric roof window the sensor is factory fitted to the top of the sash, immediately above the glass, on the external face. It faces upward and slightly outward.
That position is not arbitrary. It has to be somewhere rain lands early, somewhere clear of the flashing so run off from the roof above does not sluice across it constantly, somewhere sheltered enough that it is not the first thing a ladder hits, and somewhere the cable can reach the sash electronics without crossing a hinge line twice. On flat roof units the sensor usually mounts on the kerb or frame rather than the moving sash. Either way the mounting is set at manufacture and is not something to be relocated on site.
The sensitivity setting, and what adjusting it does
Some control units expose a sensitivity adjustment, either as a small potentiometer on the board or as a setting in the app. It changes the resistance threshold at which the controller decides the sensor is wet.
Turned up, the window closes on the lightest mist. Turned down, it waits for genuine rain and ignores dew, at the cost of a slightly later close in a light shower. There is no correct setting in the abstract. A bathroom rooflight in a Cheltenham loft, used for purge ventilation in short bursts with somebody in the house, can sit at low sensitivity. A stairwell unit that is routinely left open all day while the house is empty wants high sensitivity, because the consequence of a late close is water on a hardwood stair three floors down.
Rain sensors and the stack ventilation strategy
Where an opening rooflight is doing real ventilation work, it is often left open for long unattended periods, and that is exactly the pattern a rain sensor suits. The window earns its keep by being open, and the sensor removes the reason not to leave it that way.
The interaction is worth thinking through. A sensor close during the afternoon ends the airflow for the rest of the day unless somebody reopens it, so a ventilation scheme that depends entirely on one high level opening unit is more fragile than one with a second opening at low level. The airflow principles behind that are set out on the stack effect page.
Sensors on solar powered units
A solar operated rooflight carries the same sensor, but the power budget is tighter, because everything comes from a small photovoltaic panel charging a battery in the sash. The continuously running sensor heater is a meaningful load in that context.
Manufacturers deal with it by duty cycling the heater rather than running it flat out, and by reducing sensor polling in low light. The practical effect is that a solar unit may take marginally longer to reset after rain than a mains powered one on a dark December week. It closes just as quickly; it just takes longer to dry itself out afterwards. Solar specification is covered under solar powered rooflights.
Keeping the sensing face clean
The one thing that degrades sensor performance is a dirty face. Pollen, dust, lichen spores off a Cotswold stone slate roof and the fine soot that still comes off a wood burner flue all build a film across the tracks.
A film has two effects, and they pull in opposite directions. A hygroscopic film holds moisture and can make the sensor trigger on humidity alone, closing the window on a damp day with no rain. A dry insulating film does the reverse, raising the water needed to bridge the tracks and delaying the close. Wiping the sensor face with a damp cloth when the glass is cleaned deals with both. It takes ten seconds and it is the only attention the part wants.
Overriding the sensor
Every system allows the sensor to be overridden, and the method varies by range. On some, holding the open button drives the window against the sensor for a fixed period. On others there is a switch on the control unit that disables rain input entirely until it is switched back.
The legitimate uses are narrow. Clearing a smoky kitchen during a downpour. Testing at handover. Working on the unit. Leaving the override on permanently defeats the point of the sensor and, on a unit that cannot be reached, it means the window stays open in weather nobody is watching. The override is a momentary tool rather than a setting.
What a rain sensor does not replace
A closed sash keeps water out because its seals compress against the frame and the flashing takes run off around the perimeter. Those two things do the weathering. The sensor only decides when the sash is shut.
Water appearing at an electric rooflight almost never has anything to do with the sensor. It has to do with an upstand that is too low, a flashing lap running the wrong way, a covering that terminates short, or falls that pond water against the kerb. Those are the four details that decide whether a rooflight stays dry, and they are the reason the workmanship carries a ten year guarantee. A sensor on a badly detailed opening protects nothing.
Testing the sensor at handover
A rain sensor is tested before the scaffold comes down, and it is a genuinely useful two minute exercise for the householder to watch.
- Open the window fully from the wall switch or remote.
- Wet the sensor face with a small amount of water from a bottle. Tap water works; distilled water conducts poorly and can give a false result.
- Confirm the sash begins to close within a second or two and runs to a full seal.
- Dry the face and confirm the window will reopen on command, showing the sensor has reset.
- Confirm the override behaves as expected and cancels itself.
Doing it on the day means the sensor is known good, and it means the householder has seen the behaviour rather than reading about it. It is the same principle applied to the rest of the unit at handover, and it is described alongside the other commissioning checks on the roof window installation page.
Rain sensors in Gloucestershire conditions
The county gives a rain sensor a busy life. Prevailing weather comes in from the south west off the Bristol Channel, which means most rain arrives at the exposed face of a great many roof slopes rather than gently from above.
Along the vale, still winter nights over the river produce saturated air and heavy dew, and sensors there trigger on moisture more often than sensors on the wolds. In the Stroud and Chalford valleys, cloud sits low in the combes and drizzle can persist for hours at a time. On the higher ground around Painswick and the Cotswold edge, showers are heavier and shorter, which is the pattern a rain sensor handles best: a sharp close, a quick dry, and a reopen when somebody notices the sun is back. None of it changes the specification, but it changes how often the householder sees the window shut itself, and knowing that in advance stops it reading as a fault.
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