Do you need a rain sensor?
If the rooflight opens by itself on a schedule or a temperature setpoint, yes, treat it as part of the specification rather than an accessory. If the unit only ever opens because a person pressed a button and that person is in the room, the case is much weaker. The honest test is not how often it rains in Essex. It is how often the unit will be open with nobody paying attention, and what is directly underneath it when that happens.
People tend to decide this the wrong way round, weighing the sensor against the weather. The weather is a constant. What varies between two identical houses is the floor finish under the opening and the habits of the household. How the sensors themselves behave is covered on the automatic closing page, and the wider control options on the electric and opening rooflight page.
What sits directly underneath the opening
Start here, because it decides more than anything else. An opener over a tiled kitchen floor with a run of units either side is a low consequence position. Water that comes in gets wiped up and nothing is worse for it.
An opener over an engineered oak floor, a dining table, a piano, a stairwell with a carpet at the bottom, or a run of open shelving is a different proposition. Water on a hard floor is an inconvenience. Water on a timber floor that then cups along the boards is a repeated cost. Where a purge opener is going over anything that will not shrug off a wetting, a sensor stops being a preference.
How often the unit will actually be left open
Second question, and it is about people rather than products. Some households open a rooflight while cooking and shut it when they finish, in the same room, every time. Others open one on the first warm morning in May and close it in September.
The second pattern is far more common than anyone predicts at the point of ordering, particularly with a unit at three metres in a vaulted extension where opening it is a small production and closing it feels optional. If the unit is going to sit open for weeks at a stretch, it will be open during rain at some point in that period. That is not a maybe.
Where a sensor is close to essential
Four situations tip it decisively. Any unit driven by automation, because a controller opening on internal temperature at two in the afternoon has to be able to shut when the weather turns at three. Any unit in a room that is regularly empty, which includes most kitchen extensions during a working day.
Any opener above a floor finish or furniture that a wetting would damage. And any household where somebody works away, because the unit that gets left open is the one nobody has walked past since Tuesday. Where two or more of those apply, fit the sensor and stop thinking about it.
Where it adds very little
Over a shower room or a utility with a tiled floor and a gully, in a household that opens the unit deliberately and closes it in the same hour, the sensor spends its life doing nothing. A small unit at low level in a loft room where the handle is within reach is the same case: the person who opened it is standing under it.
There is a second situation worth naming. On a sheltered slope with a modest stroke, an opening angle of eleven degrees on a pitched roof window presents very little to falling rain, and the amount that finds its way in during a normal shower is measured in drops rather than in puddles. That is a real difference from a flat roof unit lifted twenty three degrees into an open sky.
Flat roof units against pitched slope windows
The geometry is not remotely comparable and it deserves stating plainly. A top hung unit open on a flat roof is a rectangular funnel with the sky above it. Rain falls into it. A centre pivot window on a forty degree slope, open at a modest angle, has the sash itself acting as a small canopy over most of the opening.
On flat roof openers the default answer is yes. On pitched slope roof windows the answer depends on the stroke, the exposure and what is below. This is also why the same household can sensibly fit a sensor to the kitchen opener and skip it on the two loft windows.
Fitted at order against added afterwards
A sensor is one of the more retrofittable parts of an electric rooflight, which takes some pressure off the decision. The plate mounts on the outside of the frame or on the kerb and reports to the control unit, and on a wired system that means a thin cable back to the controller.
The catch is the same as everywhere else on this subject: cable. If the controller is in a cupboard and the route to the unit is still open, adding a sensor later is a small job. If the ceiling has closed, it becomes the same problem as any other retrofit run, and the answer is usually a wireless sensor with its own cell. Ordering the sensor with the unit costs a little; ordering the cable route with the unit costs nothing.
One sensor across several openings
A sensor reports to the controller, not to an individual actuator, so a single plate can close every unit on a channel. In a room with three rooflights that is one sensor, not three.
Siting it well matters more than the count. Put it on the most exposed unit, the one that wets first, rather than the one nearest a convenient cable. A sensor on a sheltered pane behind a parapet will read late and the group closes a few seconds after it should. Where openings face genuinely different directions across a large roof, a second sensor on its own channel is reasonable.
What a sensor does not cover
It reads water on a plate, and that is all it reads. It does not detect wind, so a unit left open on a rising forecast stays open until somebody or a schedule closes it, and an open sash is a sail with a serious lever on its hinges. On an exposed site the wind case is the more expensive one.
It also cannot beat physics on timing. From first drops to a closed pane is typically ten to thirty seconds depending on the stroke, so a squall arriving as a wall of water will still put some inside. And it reads condensation and heavy dew as rain, which produces the occasional unit that shuts itself on a clear autumn morning. None of that argues against fitting one. It argues against treating it as a guarantee.
The alternatives, and where they do the same work
A schedule that closes every unit at a fixed time each evening removes the overnight risk on its own, and costs nothing beyond a controller that offers timing. App control with a notification lets you close a unit from work when the forecast turns, which covers the empty house case for anyone who will actually look at their phone.
A shorter stroke reduces the volume of water an open unit can admit, and siting a purge opener away from directly above a table changes the consequence rather than the probability. In practice most well specified extensions end up with a sensor and a schedule, because they cover different failures. Sunspire settles this at survey, since it depends on the room, the floor and the household rather than on the product.
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