The short answer
Some do. A rain sensor belongs to motorised roof windows: a small moisture-detecting panel outside signals the motor to close the sash when drops land on it. A manual window has nothing to drive it shut, so it has no sensor. Some makers let you add one later. A sensor suits a window left ajar, and is no reason to leave a roof open when you are out.
How a rain sensor works
The sensor is a small plate fitted on the outside of the window frame or glass, exposed to the weather. It detects moisture on its surface. When a few drops land on it, it sends a signal to the control unit, which tells the motor to close the sash. The window shuts itself, and in many systems it stays shut until you open it again by hand or remote.
The idea is simple and works well for the everyday case: you open the window in the morning, go to work, and a shower arrives at lunchtime. Without a sensor, the rain comes in onto the floor, the lining and perhaps the ceiling below. With one, the window closes in seconds.
Sensors are normally sold as part of an electric or solar-powered window, or as an add-on for models that support them. Details change between makers and ranges, so the data sheet for your exact model is the place to check what is available.
Our answer on electric roof windows covers the motor side, and when the extra cost is worth paying.
What a sensor will and will not do
A sensor reacts to wetness on its own surface. It does not forecast rain, and it cannot see a storm approaching. If wind drives rain sideways under the open sash before the sensor has wet, a little water can land on the sill. If the panel is dirty or blocked with leaves, it may react late or not at all.
Power is the other limit. The motor runs from the mains or from a battery charged by a solar cell, and a flat battery or a switched-off supply means the window stays where it is. A sensor with no power is a plastic plate. Ask what the system does when the supply fails, and whether the sash can be closed by hand.
For those reasons, treat the sensor as a safeguard for a window you intend to watch, rather than for one you will be away from for days. A roof opening left ajar in an empty house is a risk that no accessory makes small. Close it when you leave, and let the sensor catch the unexpected shower while you are at home or out for the afternoon.
Adding a sensor to a window you already have
The route depends on what you have. If your window is already electric, the maker may offer a sensor that connects to the control unit, and fitting it is often a matter of attaching the panel outside and wiring or pairing it. If your window is manual, you cannot simply clip one on. The sash has no motor, so there is nothing for the sensor to command.
Some makers offer conversion kits that add a motor to certain manual models. Whether one exists for yours depends on its age and range, so we begin by identifying the window from the label on the frame. If no kit exists, fitting a new electric window is the route, and that is a roof job as well as an electrical one, as our answer on the cost of fitting a new roof window explains.
The outside panel means a visit to the roof, or a reach from a tower, because it fixes to the frame or the glass beside the flashing. Cabling inside has to be routed from the control unit to the window, and that is a job for an electrician working with the roofer. Plan it together so the ceiling is opened once.
Looking after the sensor, and what to ask when buying
A sensor needs very little attention, but the outside panel should be kept clean. Leaves, bird droppings and moss can cover it, and a covered panel reacts slowly. A wipe with a soft cloth when you clear the gutters is enough, and it is worth testing it by dabbing the panel with a damp cloth and watching the sash close.
When you buy, ask four things. First, whether the sensor is included or sold separately. Second, whether the window has a manual release if power fails. Third, how the sash behaves when it has closed in rain, and whether it reopens by itself afterwards. Fourth, whether the sensor can be overridden for when you want fresh air in a light shower.
Check the written details for the Part L value too. Roof windows replacing old ones are held to 1.4 or Window Energy Rating Band B, and motors and sensors do not change that.
We cover the full range of work under roof windows and skylights, and we begin each job with a look at the frame, the flashing and the route a cable would take.
