Do you need a rain sensor?
The test is simple: if the rooflight can be left open when nobody is watching it, specify a sensor. If it sits at head height in a kitchen where somebody shuts it on the way out, you have a genuine choice. Everything else is detail around those two cases. A sensor is a small, inexpensive device that adds no maintenance beyond wiping a sensing face once a year, and the situation it prevents, a stairwell window standing open through an afternoon shower, is the single most common way water reaches a floor through a perfectly sound rooflight. What the device is actually measuring is set out on the rain sensors page, and the automatic closing behaviour itself is covered under automatic closing in rain. This page is about whether to have one.
The question is really about who is in the room
A rooflight is only exposed while it is open, and it is only left open when the person who opened it has gone somewhere else. That is the whole risk. A window in a kitchen or a living space is under more or less constant supervision. A window at the top of a stairwell, in a spare bedroom, in a bathroom or over a landing is not.
Add to that the units nobody can reach in a hurry. If closing a window requires fetching a hook pole from a cupboard and standing on the half landing, then the practical response time to a shower is minutes rather than seconds, and by then the carpet is wet. That is where the sensor earns its keep rather than merely being convenient.
Where the answer is almost always yes
Some positions make the decision themselves.
- Above a stairwell or a double height space. Nothing under it can be moved out of the way, and the unit is far from anyone.
- Loft bedrooms run on a night purge cycle. A unit that opens itself at two in the morning must be able to close itself as well.
- Holiday lets and second homes around the Forest of Dean and the Cotswold villages, where a changeover day can leave a window open with the house empty for hours.
- Any unit over a hard floor with furniture beneath it, where a shower does more damage than a wet mat.
- Bathrooms, which get opened to clear steam and then left, and which nobody revisits until the evening.
In each of those the sensor is not an upgrade. It is the thing that makes an opening unit sensible in that position at all.
Where it is a genuine choice
A kitchen rooflight in a single storey rear extension, reachable from the floor and in a room somebody is standing in, is the classic case for going without. So is a ground floor flat roof unit over a utility, opened for twenty minutes while the washing goes round.
Even there, two arguments tend to swing it. The first is that the sensor costs very little compared with the surrounding work, and adding it afterwards means a return visit and a cable. The second is habit: people ventilate more freely when they know the window will look after itself, and a ventilation feature that gets used is worth more than one that gets hedged.
What a sensor is not deciding for you
It is worth being clear about the boundaries, because expectations get set high. A rain sensor detects moisture arriving on its own surface. It is not a weather forecast, it does not know the wind is about to get up, it does not know the temperature has dropped, and it does not know a neighbour’s bonfire smoke is coming your way.
Nor does it see the first drops of a shower that starts as a fine drizzle blown sideways under an eaves overhang, which is why sensor position matters as much as having one. And it makes no judgement about wind load on an open sash, which is a separate function handled by the actuator’s own obstruction detection and, on more capable systems, by a wind sensor as an additional device.
Bundled with the unit, or added as a device
How a sensor arrives depends on the system. On several manufactured roof window ranges the sensor is integrated into the top of the sash and comes as part of the electric variant, with nothing separate to mount or wire. On others it is a discrete puck that fixes to the frame or the adjacent covering and connects back to the control board.
On a made-to-size flat roof rooflight or a lantern vent, it is nearly always a separate device chosen alongside the control system. Where several units share a closing group, one sensor serves all of them, so the cost of covering a whole loft is the cost of covering one window. Which arrangement applies to a given specification is recorded at survey with the window code.
Where the sensor goes, and the things that shade it
Position decides how well it works. The sensing surface needs open sky above it, clear of a chimney stack, a taller roof plane, a parapet or a tree canopy, and it should sit where rain arriving on the prevailing wind reaches it early rather than late. In Gloucestershire the prevailing weather comes from the south west, so a sensor tucked behind a stack on the north east corner of a roof will be the last thing on the house to know it is raining.
It also has to be somewhere the sensing face can be wiped, because a film of dust, pollen and moss spores builds on a horizontal surface within a year and changes what the sensor reads. On a Cotswold stone slate roof, where moss growth is heavy on the shaded slopes, that annual wipe matters more than it does on a modern estate. It is a two minute job, and it is described under maintenance.
A Gloucestershire year, and how often it would act
The county gets rain spread across the whole year rather than concentrated in a season, and the west of it more than the east. The Forest of Dean and the western slopes take considerably more than the Severn Vale, and the wolds catch showers that pass over Gloucester without dropping anything. What that means for a sensor is that it will act in every month of the year, not just the wet ones.
The pattern that matters most is the summer convective shower: a clear bright morning, windows opened wide, and a heavy short burst in the middle of the afternoon. That is precisely the day on which every window in the house is open and nobody is home. A device that costs very little and acts perhaps a hundred times a year, silently, is doing steady work.
Living without one, and the habits that replace it
Going without is a perfectly reasonable decision in the right position, and it works if the habits are in place. Close reachable units when leaving the house. Use the frame’s ventilation flap rather than the sash for background air, since the flap is weathered and can stay open in rain. Keep an eye on the forecast in a week of showers.
The one thing worth doing either way is making sure the wiring allows the decision to change. A control board that will accept a sensor input, and a spare core in the cable to the unit, cost nothing at first fix and mean the device can be added later without opening a ceiling. That is the arrangement recommended by default, whether or not the sensor goes in on day one.
What to settle at order stage
Three questions. Which units can be left open unattended, which is the list that needs covering. Whether the sensor is integrated in the sash or a separate device on this particular product, which affects where it can be positioned. And whether the closing group covers every unit on every slope, or whether a second sensor is needed for a roof plane the first one does not see.
Those answers go into the specification alongside the actuator stroke and the control type. It is a small part of the paperwork, and it is the part that decides whether an opening rooflight is a feature you use or one you worry about. The rest of the operating side is on the electric rooflight installation hub.
Next on this subject.
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