Rain Sensors on Electric Rooflights

A rain sensor reacts to rain arriving, and makers do this in different ways. VELUX states that the sensor on its solar roof windows reacts to the vibration of raindrops, while on its electric model moisture triggers the closing. Other makers use a small surface whose electrical behaviour changes when wet, and some warm that surface slightly so that dew or frost dries off and does not keep the sash shut for hours. Whether a given sensor is warmed, and how sensitive it is, differs by maker and model, and the data sheet states it.

The sensor reports one thing: there is water on me now. It does not forecast, it does not measure how hard it is raining and it does not know whether you want the rooflight open for a reason. It is a switch that responds to wet.

Once the surface dries, most sensors allow the control to accept an open command again. Some systems reopen the sash on their own when the rain stops, and others stay shut until a person opens them. The choice is made when the system is set up, and it is worth settling before it is fitted, since a sash that reopens by itself in the dark may not be what the household wants.

Position is the first thing to get right with a separate sensor. It has to be wetted by the same rain that would wet the open rooflight, and it must not be wetted by anything else.

On electric roof windows the sensor is commonly built into the window itself; VELUX describes an integrated rain sensor on its solar and electric models, so it sits on the slope with the window and gets the same rain. On flat rooflights the sensor is commonly a separate small unit set on the roof or on the kerb, with a cable back to the control. The right place depends on the model, and the maker's fixing instructions are followed.

Placement checks made on the roof at the survey include:

• Whether anything above the sensor, such as an overhanging eave, a chimney or a parapet, would shelter it while rain still reaches the open sash
• Whether water from a gutter, a downpipe or a roof above could drip onto it when no rain is falling on the sash
• Which way the wind drives rain on this slope, since rain that falls at an angle can wet the sensor and miss a sheltered rooflight, or the other way round
• Where the cable can run, and whether it can pass the flashing or the kerb without being pinched
• Whether the sensor can be reached later for cleaning

A sensor tucked under a tile lip or sheltered by a neighbouring dormer reads dry in a shower that soaks the open sash. A sensor under a drip from a tile edge reads wet after the rain has stopped and holds the sash shut. Both faults come from position, and both are avoided by looking at the roof.

The sensor does not move the sash. It sends a signal to the controller, and the controller drives the motor. The common arrangements differ in how that signal travels.

In every arrangement the rain signal has priority over a command to open. When the sensor reads wet, an open command from the wall switch or remote is normally refused, or the sash closes again at once. How the system behaves is a function of the controller, which is why a sensor is ordered with a controller that supports it. Whether a sensor can be added later depends on the controller and on what the maker lists for it.

The signal also tells the motor which direction to go and where to stop. The travel limits on the actuator are set when the unit is fitted, so the sash shuts fully and seals against its gasket and does not stop a few millimetres short. Closing a sash into its seal is the end of the motor's travel, and it is tested at handover.

The pair of choices, wired against radio and mains against battery, is covered with the other controls in our comparison of electric and manual opening rooflights.

A sensor that is dead is the same as no sensor. It must have power at the moment it rains, which may be weeks after anyone last touched it.

A wired sensor is usually fed from the controller, which draws on the same supply as the motor. A radio sensor needs its own power: a battery, a small solar cell, or both. Battery life is stated by the maker and varies with how often the sensor transmits. A sensor with a battery will eventually need the battery replaced, and the sensor should be placed where a person can get to it for that.

The supply to the motor has the same duty. If the mains is switched off, or the transformer is unplugged, or a solar panel is shaded for a week, a motor may not close the sash when asked. That is the weak point of any system that relies on a sensor, and it is why a power failure and a weather change at the same time are worth thinking about.

Some actuators can be closed by hand or by a spare battery when the supply fails. Others cannot. Ask which applies to the model you are quoted. How a motor is wired and who connects it is covered in our answer about electricians and electric rooflights, and the solar route is set out in the answer on running an electric rooflight on solar.

A rain sensor is a convenience. It makes it less likely that a sash is left open in a shower. It is not a promise that the sash will never be open when it rains.

• It cannot act before the first drops land, so some rain can enter while the sash travels shut.
• It cannot cope with wind-driven rain that skirts the sensor, or with rain that falls straight down on a sheltered sensor but blows into the open sash from the side.
• It cannot work with a flat battery, a switched-off supply or a cable that has been cut.
• It cannot tell the difference between rain and water from a hose, a cleaner's bucket or snow melting off a nearby roof.
• It cannot close a sash that is blocked, for instance by a curtain, a plant or an item on the sill.

The practical answer is the one a household gives itself: close the sash by hand or by remote before leaving the house for the day if the sky looks unsettled. A sensor then deals with the surprise shower, and a person deals with the forecast.

Closing against an obstruction is a point of safety as well as damage. Actuators have limits on the force they apply, and the maker's instructions cover trapping risk. Keep the path of the sash clear of anything that could be caught.

The sensor is part of the order from the start, because the controller and the motor have to be chosen to work with it. The survey sheet records the sensor type and where it will sit.

On the roof the order of work is the frame and flashing first, then the sensor in its position with its cable routed, and then the motor and control inside. A cable that has to go through a flashing is sealed where it passes, because a sensor cable is a path for water as well as a signal.

Inside, the controller is wired or paired to the sensor, the travel limits on the motor are set, and the sash is run through its full travel. If a remote is part of the order, it is paired to the controller at this stage, and the household is shown which command overrides which.

A new opening in the roof needs Building Regulations approval, and replacing a whole unit is notifiable. The sensor does not change either position. The planning side is set out in the rooflight planning guide.

A sensor that has never seen water has not been tested. A sound handover opens the sash and wets the sensor with a small amount of clean water, applied the way rain would land on it. The sash should start to close within moments and finish fully shut. The test is then repeated once the sensor has dried, to confirm that the control accepts an open command again, or stays shut, whichever was agreed.

1. Open the sash to a wide position from the wall switch or remote.
2. Wet the sensing surface with a few drops of water.
3. Watch the sash close and seat against its gasket.
4. Try an open command while the sensor is still wet, and check that it is refused or that the sash closes again.
5. Let the sensor dry and confirm the behaviour you asked for.

Write down the result with the sensor type, its position and what powers it, and keep it with the unit's paperwork: it tells whoever looks at the system later where the sensor is and how it should behave.

A check worth repeating each spring

Run the same wet test once a year. It is quick, and it confirms the battery, the cable and the motor are all still doing their jobs. If the sash does not respond, check the supply first and the maker's instructions second.

Where the sash will be left open for long periods, the opening position matters as well. Our guide to how far an opening rooflight opens explains the travel, so a sash left on a partial vent position can be judged against the rain you expect.