Guide · Electric and Opening Rooflights
Rain sensors and what they actually do
A rain sensor is a small pad, usually somewhere around a hundred millimetres across, wired or paired to the same controller that runs the motor. It has one job. Notice that the roof is getting wet, and send a close command before the room below finds out. Everything else about the device, the mounting position, the threshold, the little heater underneath it, the delay before it acts, exists to make that single decision reliable.
It is also the accessory people ask about most and understand least. This page covers what sits behind the plastic lid, how wet the pad has to get before anything happens, how many seconds pass between the first drop and a closed pane, and where on an Essex roof a sensor stops being a convenience and starts being part of the specification. The wider subject of electric and opening rooflights is covered on the hub page.
The conductive grid behind the plastic lid
Lift the cover off a standard sensor and there is a printed circuit board with two metal tracks on its upper face, interleaved like the teeth of two combs pushed together but never touching. A small gap separates them, typically under a millimetre. Dry air across that gap is an insulator, so no current crosses.
Water is not an insulator, particularly rainwater carrying dissolved salts and whatever it collected on the way down. A film bridging the tracks lets a tiny current flow, the resistance between the combs collapses from effectively infinite to a few tens of kilohms, and the board reads that fall as wet. The tracks are gold plated or stainless on any decent unit, because bare copper sitting in water with a voltage across it corrodes within a season.
How wet the pad has to get before it acts
One drop landing squarely between two tracks can be enough on a sensitive setting. In practice the board wants to see the resistance fall and stay fallen for a short interval, often a second or so, because a single splash from a gutter or a bird landing on the housing should not close a rooflight in July.
Most controllers offer a sensitivity adjustment, either a small potentiometer under the lid or a setting in the handset. Turned up, the unit reacts to the first serious drops of a shower. Turned down, it wants the pad properly wet before it commits. The right setting is a property of the position rather than a preference, and it is worth revisiting after the first wet autumn.
Heating elements, and why the pad dries itself
Under the sensing board there is usually a small resistive heater holding the surface a couple of degrees above ambient. It is drawing very little, in the order of a watt, and it is doing two useful things.
First, it clears the pad quickly once rain stops, so the sensor resets in a minute or two rather than sitting wet for an hour while dew hangs on it. Second, it keeps morning condensation off the tracks. Without a heater, a clear cold night followed by a still dawn puts a film of dew on every upward facing surface in the garden, and a sensor cannot tell that film from drizzle. The cost of the heater is a small standing current, which matters if the whole system is running on stored charge.
The seconds between the first drop and a closed pane
Detection is fast. The board sees the resistance fall almost immediately and the close command is issued within a second or two. What follows is the slow part, because the actuator travels at ten to twenty millimetres per second and a full stroke takes between fifteen and forty seconds.
So the honest figure for the whole sequence is somewhere between twenty and forty five seconds from the first drops to a pane sitting on its gasket. In a shower that arrives gradually, almost nothing gets in. In a summer downpour that starts at full intensity, a small amount of water reaches the reveal, which is why the sensible position for an open vent is over a hard floor rather than over a sofa.
A sensor buys you the difference between a damp reveal and a wet room, not between wet and perfectly dry.
Where the sensor is mounted, and what that decides
Position matters more than the model. The pad wants clear sky above it, mounted on the frame of the unit itself or on a short bracket close by, tilted very slightly so water runs off rather than pools.
The failures are all forms of shelter. A sensor tucked under the ridge of a lantern sits in the rain shadow of the glass above it. One mounted on the sheltered side of a tall extension stays dry while a south westerly drives rain into the opening from the other side. One under the edge of a mature tree gets a delayed and unreliable version of what is actually happening. On any roof where wind consistently comes from one quarter, and across most of central Essex that means the south west, the pad belongs on the windward side of the opening.
Drizzle, mist and the weather that never trips it
Fine drizzle is the sensor’s weak point. Droplets that small can land, sit as separate beads and never join into a film across the gap, so the resistance barely moves and nothing happens. The same is true of the sea fret that comes up the Blackwater and the Crouch on a still spring morning.
Turning the sensitivity up helps and brings its own problem, which is nuisance closing. This is the genuine limit of the device, and it is worth knowing before you rely on it. A rain sensor is very good at heavy rain, decent at ordinary rain, and unreliable in the fine wet air that leaves a car windscreen beaded but never running.
Optical sensors, and the units that use them
A second approach borrows from the car industry. An infra-red emitter shines into the underside of a clear cover at a shallow angle, and a receiver a few millimetres away collects the light that reflects back off the outer surface. Dry glass reflects almost all of it internally. A droplet sitting on the outside changes the refractive boundary and lets some of that light escape, so the received signal drops.
Optical sensing has no exposed conductors, so nothing corrodes and salt spray does not create a false wet reading. Against that, it responds to anything that sits on the cover, including a leaf, a smear of pollen or a bird dropping. On rooflights the conductive grid is still the common fitment, with optical appearing on some of the higher specification control systems.
What happens once the shower passes
Nearly all systems close on rain and leave the pane closed. That is a deliberate choice, not an omission. Reopening a rooflight automatically means a pane moving in a house where nobody has asked it to, and the argument for keeping that under human control is a strong one.
Some controllers do offer an automatic reopen after a set dry period, typically fifteen or thirty minutes, usually paired with a temperature or humidity rule so the unit only reopens if the room still wants ventilation. Where the rooflight is being used seriously for summer cooling through the stack effect, that rule earns its place. Where it is opened occasionally, manual reopening is simpler and less surprising.
Wiring the sensor into the control loop
The sensor runs at extra low voltage and connects to the controller, not to the motor. On a wired installation that means a two or four core cable from the sensor position back to the control box, which is another run to think about at first fix alongside the motor supply and the switch drop.
Wireless sensors avoid the cable and pay for it with a battery, usually a lithium cell lasting two or three years, and with a radio link that has to reach the controller through whatever is in the way. On a single storey extension the controller is often a metre from the sensor and range is a non issue. On a three storey conversion with the controller in the hall cupboard, a wired sensor is the quieter engineering.
Setting a threshold that suits the position
Two Essex situations pull the setting in opposite directions. On an exposed estuary roof at Canvey or out along the Blackwater, salt carried on the wind settles on the pad and slightly lowers its dry resistance, so a sensitive setting can produce closures on a dry blustery day. The pad needs a wipe more often and the threshold set a little lower.
In a sheltered urban position, a Chelmsford terrace surrounded by other roofs, salt is not the issue and neither is nuisance triggering. There the setting can go up, so a shower is caught early. Getting this right is a five minute job at commissioning and a fifteen second job afterwards, but it needs someone to have thought about it rather than accepting the factory default.
Frost, snow and readings that mislead
Dry snow does not conduct. It can settle on a sensor pad for hours without changing the resistance across the tracks at all, which means a rooflight left open in a snowfall may stay open until the snow on the pad begins to melt. The heater helps here, since it turns the first flakes into water and gives the board something to read.
Ice is the other winter case, and it belongs to the mechanism rather than the sensor. A pane frozen to its gasket resists the actuator, the current rises, and a well set control board stops the drive rather than tearing the seal. Waiting an hour usually solves it. Forcing it never does.
Wind sensors, and the input people confuse with rain
Many controllers accept a second input from an anemometer, a small three cup rotor that closes the unit above a set wind speed. It is a different device answering a different risk. Rain protects the room. Wind protects the hinges, the actuator bracket and the pane itself, all of which take a real load once the sash is open and acting as a sail.
On sheltered sites this is an unnecessary refinement. On a bungalow at Canvey with nothing between the roof and the estuary, or on a tall rear extension at Beaulieu Park catching the wind across open ground, it is the input that saves the mechanism. Combined sensors carrying both a rain pad and a small wind rotor in one housing exist and simplify the wiring considerably.
Dirt, pollen and the wipe the pad wants
The one maintenance item on a sensor is keeping its face clean. A film of dust, pollen and organic debris is mildly conductive once damp, and a badly soiled pad can sit permanently in the wet state. The symptom is unmistakable: the rooflight closes and will not open, or it closes again within seconds of being opened, in perfectly dry weather.
A damp cloth across the pad twice a year settles it, ideally at the same visit as the glass. May is the month it matters most in Essex, when oilseed rape and tree pollen coat everything horizontal for a fortnight. The rest of the mechanism asks for little, and what it does ask for is set out under keeping an electric rooflight working.
Sensor types compared
Four fitments cover almost everything on domestic work, and they differ in what they need from the building rather than in what they detect.
| Type | How it senses | Needs | Best suited to |
|---|---|---|---|
| Wired conductive pad | Resistance across interleaved tracks | Cable back to the controller | New build and first fix work |
| Wireless conductive pad | Same, with a radio link | Battery change every few years | Retrofit and finished ceilings |
| Optical | Infra-red reflection off a clear cover | A clean cover surface | Coastal and salt exposed roofs |
| Combined rain and wind | Pad plus cup anemometer | One cable, a clear mounting mast | Exposed and high level openings |
Behaviour during a power cut
A sensor with no supply cannot close anything, and this catches people out because the weather that brings a cut is often the weather that brings the rain. Some control systems carry a small battery reserve for exactly this reason, and what each arrangement does is set out under backup power and what happens in a cut.
The practical point at specification stage is simply to know which behaviour your system has, because the two are very different in a February gale and neither is obvious from a brochure.
Essex roofs where a sensor stops being optional
Three situations move it from useful to necessary. A unit that is beyond comfortable reach, high in a converted loft or over a stairwell, because nobody is going to fetch a ladder when the sky darkens. A household that is out all day, which describes most of the commuter towns along the Great Eastern line, where the unit is opened at eight and nobody is home at two when the shower arrives. And a holiday or second property along the coast at Mersea or Bradwell, where the gap between visits is measured in weeks.
Against that, a kitchen vent over a hard floor in a house with someone in it most days genuinely manages without one. The specification follows the pattern of occupation and the exposure of the roof, which is one of the things a survey settles before anything is ordered.
Keep reading
More on electric and opening rooflights
Wiring an electric rooflight at first fix
Read the guide →Solar powered rooflights without a mains run
Read the guide →Remote controls, wall switches and smart home links
Read the guide →Electric blinds fitted inside the rooflight
Read the guide →Opening rooflights for the stack effect
Read the guide →
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