How the rain sensor works
An electric rooflight is a skylight with a motor on the sash. Opening it is a button press. Closing it in rain is a separate job, and it is the rain sensor that does it.
A rain sensor is a small unit with a surface that reacts to wet. When drops land on it, it signals the control, and the control tells the motor to close the sash. Some sensors sit on the rooflight frame itself, and others are fixed on the roof close by. Either way, the idea is the same: the thing that notices the rain is on the outside, where the rain is.
Most systems close the window and then hold it shut until the sensor has dried and the control allows it to open again. Some only close and leave you to open it by hand later. The behaviour depends on the product, so ask how the one you are choosing behaves once the rain has stopped. If the answer matters to you, for example because the rooflight is over a bedroom and you want it to stay shut when you are asleep, say so at the survey.
Our guide to electric rooflight wiring, controls and rain sensors covers the parts in more depth.
What the sensor needs to work
A sensor is only useful if the whole chain behind it is working. Four things have to be in place.
- Power to the motor. A rooflight that has lost its supply cannot close itself. A cut in the mains, a tripped circuit or a disconnected plug stops the motor, however good the sensor is. The sensor cannot close a sash that has no power to move it. Some systems include a battery back-up. If yours matters in a power cut, ask whether the unit has one.
- A sensor in a sensible place. It has to be where the first drops land, and not sheltered by an overhang or a chimney.
- A clean surface. Leaves, moss and dirt on a sensor stop it reading properly. It wants an occasional wipe, just as the glass does.
- A control set to automatic. Many controls have a mode that switches the sensor off, for example so you can ventilate a steamy bathroom in light drizzle. If someone has set it to manual, the rooflight stays open.
These are all easy to arrange at installation, and easy to forget afterwards. A fixed sensor at the wrong spot is the commonest slip, and a wipe with a damp cloth sorts most dirty ones. The installer should show you how the control works and what each mode does before they leave.
Rain is not the only weather
A rain sensor reacts to wet. It does not know about wind, and it cannot tell that the weather is turning before it starts to spit. Two things follow from that.
First, a rooflight left open for a long time on a day with showers forecast is a rooflight that closes after the first drops. A few drops may reach the sill or the floor below before the motor completes the close, because the sash takes a few seconds to travel. For a sash that is only partly open, the delay is short.
Second, strong wind is a different problem. A wide-open sash acts as a sail, and a hard gust puts strain on the hinges and the motor. Some systems have a wind sensor as well as a rain sensor. If your roof is exposed, ask whether one is available for the unit you want. In any case, close the window in a gale.
For bedrooms, the simplest habit is, as ever, to shut the rooflight when you go out and when you go to bed, and let the sensor be the back-up.
What to ask at the survey
If a rain sensor matters to you, and for a skylight over a bedroom, a kitchen island or a stairwell it probably does, there are five questions worth asking before you agree to a quote:
- Is a rain sensor included, or an extra?
- Where will it be fixed, and how is it wired or powered?
- What happens in a power cut?
- Can the sensor be switched off, and how do you turn it back on?
- Does the unit have a wind sensor as well, or can it be added?
We answer these at the free survey, when we can see the roof, the position of the unit and where the power comes from. The written quote that follows is fixed, and it names the sensor and controls included. A qualified electrician makes the mains connection. Planning and Building Regulations points for rooflights are gathered in one guide.
Where to go next
Rain does not announce itself, and a shower can arrive in the time it takes to put the kettle on. That is the whole case for a sensor over a room you leave empty for hours: the house stays dry while you are at work or out for the day. It also explains why we ask where the unit sits and who is home. A rooflight over a spare room needs less from its controls than one over a kitchen you cook in every evening, and the survey is where we settle which kind you have.
The parent page, electric opening rooflights, covers the whole job from survey to sign-off. A sibling read, opening rooflights for ventilation and purge air, shows where an opening unit does most good.
When you are ready, ask for a free survey. You can also message us on WhatsApp (07880 284133) with a photo of the room and the roof. A skylight that closes itself in the rain is a good thing to have over a room you use every day, as long as the sensor, the power and the control all work together, and that is what the survey is for.