Do you need a rain sensor?

Often it arrives fitted whether you asked or not

On many factory-built electric roof windows the sensor is not an option box. It is part of the unit, wired internally at the works, with the pad set into the top of the outer frame. In that case the question is not whether to buy one but whether to enable it, and enabling it is generally the right call.

The genuine decision points are elsewhere: on a surface-mounted actuator fitted to a bespoke unit or a lantern, on a manual unit being converted to electric, and on any system where the sensor is a separate accessory with its own wiring back to the control unit. Those are the cases worth thinking about properly.

Where a sensor plainly earns its place

Four situations, and they are all about somebody not being there.

A rooflight over a stairwell or a double-height space, where the sash is out of reach and nobody would climb to it in a downpour. A house that is empty on weekdays, where a vent left open at eight in the morning meets a shower at eleven. A holiday let or a second property, where the person who opened it is not the person who will be there when the weather turns. And any unit in a room you enter rarely: a landing, a hallway, a utility, a home office used two days a week.

In all four the sensor is not really protecting the room from water. It is giving you permission to leave the vent open, which is the whole point of having an opening rooflight.

Where you can reasonably do without

A rooflight in a low loft slope in a bedroom, within arm’s reach, opened and closed by hand every day, does not need one. Nor does a unit in a kitchen where somebody is always standing under it. Nor a bathroom vent that gets opened for twenty minutes after a shower and shut on the way out.

There is a second argument for leaving it off. A sensor takes control away from you, and in rooms where you are present and attentive that is an irritation rather than a benefit. A cook who wants the vent open while it drizzles cannot have it, because the pad is wet. That is worth understanding before you specify one for every unit in the house out of habit.

The override that annoys people in April

This is the single commonest complaint we hear about sensors, and it is not a fault. While the pad is wet, most systems refuse an open command. Light drizzle, a damp morning, dew that has not burnt off by nine: the switch does nothing and the homeowner concludes the unit has failed.

It has not. The pad is often heated on better units precisely to dry itself quickly and shorten that window, but the behaviour is deliberate. Most systems allow the sensor to be temporarily overridden from the controller or remote for a set period, and knowing where that override lives is part of handover. We show it at commissioning and leave the instructions on site.

Sensitivity, and the difference between two pads

Not all sensors behave alike, and the difference shows in a wet Oxfordshire spring. A basic conductive pad bridges a circuit when water lands across it and reacts to the first few drops, which means it closes early and can also be triggered by heavy dew or by mist rolling off the Thames valley on a still morning.

A better sensor is heated and tuned, so it discriminates between condensation and actual rainfall, and dries itself in minutes rather than hours once the shower passes. If you are choosing between units and a sensor matters to you, the heated pad is the specification worth paying for. It is the difference between a vent that reopens at noon and one that stays shut until the afternoon.

What it costs a solar unit in battery terms

On a solar-powered rooflight the sensor draws from the same battery as the motor. That is a modest but continuous load, and on a shaded slope through December it competes with the charge coming in from the panel.

The practical effect is that a solar unit on a poorly lit slope with an active sensor may run its battery lower over winter than one without. It is not usually a reason to go without the sensor, since a solar unit is often chosen precisely because it is somewhere unreachable. It is a reason to think hard about which slope the panel sits on, and to accept slower travel in midwinter as normal rather than as a symptom.

Retrofitting a sensor to a unit that has none

It depends on what the control unit will talk to. On a modern radio system a compatible sensor can often be added and paired later, which makes it a genuine second chance. On an older hard-wired controller, adding a sensor usually means a cable from the sensor position back to the controller, and if the ceiling is finished that is the same problem as any other retrofit cable run.

Where the unit is a sealed factory item with no sensor input at all, the answer is that it cannot be added, and the choice becomes living without or replacing the unit. That is worth knowing before you decide to leave the sensor off to save money at order stage.

What a sensor is not, and never has been

It does not forecast. It reacts to water that has already landed, and a comfort vent takes some seconds to close, so a little rain comes in every time. It does not see rain that misses the pad, so wind-driven rain arriving at a sharp angle can enter while the pad stays dry. It does not reliably detect snow. And it is not a security device: it closes a sash, it does not lock one.

Treat it as a device that limits an oversight, not one that removes the need to think. We install and replace rooflights across Oxford and up to twenty five miles by road. The electrical work is carried out by a qualified electrician, we have fifteen years in the trade, and the workmanship carries a ten-year guarantee.