Rain sensors and automatic closing on rooflights

Most sensors work on a simple principle. A small plate sits on the outside of the frame or the glass, with two sets of fine contacts or a coated surface that changes electrically when it gets wet. Dry, the surface carries almost no current. A film of water bridges the contacts, the reading changes, and the control unit takes that as rain.

Many plates are warmed slightly by a tiny heating element. That is deliberate. It dries the plate quickly once the rain has passed and stops morning dew or frost from being read as a downpour. A plate that dries in minutes rather than hours is the difference between a rooflight that behaves sensibly and one that stays shut all afternoon.

The sensor itself does not move anything. It sends a signal to the control unit, which sends power to the motor, and the motor winds the sash shut. That is why the sensor belongs to an electric or solar rooflight, and why a manual window with a pole and a handle does not have one. The cheapest way to think about it is as a switch with a very specific job: it listens for water and says one word, close.

The sequence is short and worth knowing, because it explains the odd behaviour people sometimes report.

1. Rain lands on the plate and wets the contacts.
2. The control unit reads the change and, after a very short delay, tells the motor to close.
3. The sash shuts fully against its seals. On most systems it shuts even if you opened it from the wall switch a minute ago.
4. The plate dries, but the window normally stays closed, because reopening by itself would put the room back at risk if the shower returns.

Some systems then let you override from the wall switch or the remote, which is useful on a warm, drizzly day when you want air and accept a little damp. Others lock the window shut while the plate is wet and only release it once it has dried. Which behaviour you get depends on the maker and the control unit, so it is something to ask about before you choose rather than after.

One practical point is the delay. A good sensor reacts to the first few drops, not to a steady downpour. A shower arriving sideways on a west wind, which is not unusual on the Exe estuary or the Devon coast, can wet the room through a gap of a few centimetres before a sluggish sensor has acted. The more sensitive the plate and the quicker the motor, the less water gets in.

A sensor only protects the window it can feel. On a single roof window it is built into the top of the frame, or fixed beside it, on the same pitch as the glass. That is where the rain arrives, so it detects water as soon as the window does.

The positioning is more of a question on a wider installation. If you have a flat rooflight opening on a kerb, or a lantern with an opening vent, the sensor needs to sit where it catches rain without being shielded by the roof edge, a parapet or an overhanging eave. A plate tucked under a deep overhang can stay dry while the glass is soaked.

It also matters where the sensor sits relative to the window it protects. On a run of several rooflights wired together, one sensor can close the lot, which is simpler but means a shaded rooflight on the lee side of the roof will close because of rain falling on the weather side. That is the safe way round. The risky arrangement is one sensor guarding a window that faces a different way, because the two can be wetted at different moments.

We settle the sensor position during the survey, when we can see the roof, the overhang and the direction the weather comes from, rather than leaving it to the box.

There are two routes to a rooflight that closes itself in the rain.

Built-in

Several makers supply electric and solar roof windows with the sensor already part of the unit, wired and tested in the factory. The VELUX INTEGRA range works this way, with the rain sensor part of the electric and solar roof windows. This is the tidiest option, because there is no extra plate to site and no extra cable to run.

Add-on

A separate sensor kit can be added to some electric opening rooflights and lantern vents, wired into the same control unit as the motor. This suits a rooflight chosen for its size or style where the sensor was not part of the standard unit. The sensor then has to be positioned, fixed and weathered on the roof, which is a small extra piece of roof work and wants thinking about at the same time as the rest.

In both cases, compatibility is the point to check. The sensor, the control unit and the motor have to be designed to work together, and a sensor from one system will not necessarily talk to a motor from another. If you already have an electric rooflight and want to add a sensor, our answer on whether an existing rooflight can be made electric covers what usually can and cannot be added later.

A rain sensor is dependable at one thing, and it is better to know the edges of that thing than to trust it for more.

• It cannot act without power. If the supply is off, a mains unit will not close until you close it by hand, and a solar or battery unit depends on its charge.
• It cannot see weather coming. It reacts when water arrives, not before, so a room with a rooflight open in a forecast of wind-driven rain is better closed by you.
• It cannot close a window that is blocked. A curtain, a plant or a draught excluder caught in the sash will stop the motor.
• It cannot protect against water that arrives some other way, such as condensation, a flashing problem or a gutter overflowing past the frame.

Power is the one people ask about most. We cover it separately in what happens to an electric rooflight in a power cut, including the manual release and the battery back-up some systems carry.

The simplest habit is also the best one. Close the rooflight yourself when you leave the house for the day or go to bed in unsettled weather, and treat the sensor as the second line of defence for the shower you did not expect.

A sensor that closes too readily is almost as irritating as one that closes too slowly.

Dew and mist are the usual cause. In a damp valley, on the moorland edge or near the estuary, a cold morning can leave a film of moisture on the plate. A heated plate copes with most of it. An unheated one can read dew as rain and shut the window just when you wanted the fresh air.

Spray is another. A rooflight close to a gutter, a downpipe or a neighbour's wall can be wetted by splash, and so can one directly below the line where snow or meltwater slides off a higher roof. If a sensor keeps shutting on a dry day, the first question is what is spraying it.

Many control units let you adjust how readily the sensor responds, or switch it off for a set period. That is not the same as leaving the window to look after itself. If you turn the sensor off to air a room on a muggy afternoon, you are back to depending on yourself to close it.

Snow is a special case. Settled snow on the glass can keep the plate wet as it melts, so the window may stay closed for a while after the sky clears. That is the sensor being cautious, not faulty, and it clears once the plate dries.

Whatever the setting, test it. Wet the plate gently with a damp cloth and watch the window close. It takes a minute and tells you the whole system is talking to itself.

A rain sensor is a roof feature, but the consequences land in the room, which is where our work is judged. Think about what is under the window and what a shower could reach.

The lining and plaster around an opening are not designed to take regular wetting. A sudden opening in a flat ceiling over a hallway or landing shows water on paintwork quickly, and a lath and plaster ceiling in a Georgian terrace in St Leonards or Southernhay can take on damp that shows as a stain months later. Lime plaster tolerates moisture better than gypsum, but a repeated soaking is no good for either.

For that reason we think about the opening position in the same conversation as the finish. A window that opens over a polished floor, a good rug or a staircase has more to lose from a missed shower than one over bare tile. A deep splayed lining throws rain onto its own sill rather than the floor, because the angled face catches and drains it. The sill itself should be formed and sealed so any water that does reach it can run to a safe place.

These points go into the finish schedule that comes with every quote, alongside the lining shape and the finish. The detail is on our page about how a rooflight is finished inside.

A few habits help the system do what it was bought for.

Keep the plate clean. Leaves, pollen, bird droppings and a film of dust change how it reads. A gentle wipe with a damp cloth, from a ladder or a safe access point, restores it. Do not use abrasives, which scratch the contacts.

Check the motor travel. If the window does not seat fully against its seals, the sensor has done its job but the room is not protected. The sash should close firmly and evenly along the whole frame.

Think about the whole day. A rooflight can be opened for ventilation in the morning and left open as the weather turns while you are at work. A sensor will close it, and a wall switch will let you reopen it on return. If you want that pattern, choose a system whose overrides suit you.

Use the sensor with the ventilation the window is meant to provide. Our guide to opening rooflights for ventilation and cooling covers where a rooflight does the most good, and how a sensor lets you use it more often.

Whether a sensor comes built in or as an add-on, the questions we go through at survey are the same: where the weather hits the roof, what sits below the opening, whether the room is lived in all day, and how you want to control the window.

A sensor is part of the electrical side of the job, which is covered in our answer on whether electric rooflights need an electrician, and part of the choice between powered and manual units, set out in electric or manual opening rooflights. Both belong to the broader picture on our electric and opening rooflight installation page.

New opening rooflights are building work, so the installation has to meet current Building Regulations, and where the job is notifiable we make the Building Control notification. The work carries a 10-year workmanship guarantee. The quote comes in writing with a finish schedule, which means the sensor position, the control and the finish of the opening are all settled before the roof is opened. When you are ready, ask for a quote and tell us which room the rooflight is going in.