Do electric rooflights close automatically in rain?
Only if a rain sensor is fitted and wired into the control unit. A motorised rooflight with no sensor stays open in a downpour indefinitely, because nothing in the drive knows the weather has changed. Where a sensor is present, the unit closes itself without being asked, typically within ten to thirty seconds of the first drops landing on the plate.
That short delay is the part worth understanding, because it decides how much water arrives inside before the pane seats. Whether the sensor is worth specifying in the first place is answered on whether you need a rain sensor. This page is about what the automatic close actually does once it is there. The wider subject sits on electric rooflight installation.
What the sensor detects, and what it does not
A rain sensor is a small conductive plate mounted on the outside of the sash or the frame, with two tracks on it separated by a gap. Dry, the gap is an open circuit. Wet, water bridges the tracks, resistance falls, and the control unit reads the change and drives the pane shut.
It is a moisture detector, not a weather station. It responds to water already on the plate. It does not know that rain is approaching, it cannot see a forecast, and it has no idea what the wind is doing. Heavy dew and a settling mist will trigger it. Wind on a dry day will not, which matters more than people expect on an exposed flat roof.
How long the pane takes to seat
The delay is mostly travel time, not detection time. The plate reacts almost immediately. What follows is the actuator retracting its full stroke, which on a domestic drive runs at a controlled speed rather than a quick one.
A short stroke unit at ten degrees is shut in a few seconds. A long stroke unit standing at twenty degrees or more takes considerably longer, because there is more chain or spindle to bring back. Then the last part of the movement is the compression of the gasket, which is deliberately slow so the sash pulls tight rather than slamming into the frame. Twenty seconds is a fair working figure for a typical purge opener.
The water that gets in before it shuts
Some does, and it is honest to say so. A shower that begins gently gives the sensor time and almost nothing reaches the floor. A summer storm that arrives as a wall of water in a few seconds will put a measurable amount through a wide opening before the pane is home.
The volume is small in absolute terms and it lands directly below the opening. On a tiled or a timber floor it is a wipe. Over a dining table, a sofa or a run of open shelving it is an annoyance that repeats. This is a siting decision made at design rather than a fault in the electronics, and it is one of the reasons a purge opener over a kitchen is usually placed over the working area rather than over the seating.
Heated plates, and why the unit does not reopen straight away
Most sensors carry a small heating element beneath the plate. Its job is drying, not de-icing. Without it, the plate would hold a film of water long after the shower passed and the unit would be held shut for an hour on a damp afternoon.
With it, the plate dries in a few minutes and the sensor releases its override. What happens next depends on the system. Some units simply become available again and wait for a command. Others, where the close was triggered while an automation was calling for ventilation, reopen on their own once the plate is dry. Knowing which behaviour a system has is worth establishing at commissioning, because a rooflight that reopens by itself at two in the morning is a surprise if it was not expected.
What it will not react to
Three conditions pass a rain sensor without a flicker, and all three matter in Essex.
- Wind. A dry gale is invisible to a moisture plate. An open sash is a sail, and on the estuary fringe around Maldon, Burnham and Canvey that is the load worth planning for.
- Snow. Dry snow settling on a plate does not conduct until it melts, so the close can be late or absent.
- Falling temperature. A unit left open on a mild evening stays open through a cold clear night unless a thermostat or a schedule brings it down.
The answer to all three is a timed close in the evening rather than a heavier reliance on the sensor.
Where the plate sits, and what confuses it
Position is chosen so the plate sees the same weather the opening does. On a flat roof unit that usually means the top face of the sash or the frame, clear of any lip that would shelter it. On a pitched roof window it sits high on the sash where water reaches it early.
Two things upset it. Debris, because leaf litter and moss lying across the plate hold moisture and keep the unit shut long after the roof has dried. And overhanging planting, since a rooflight sheltered by a mature tree can stay dry on the plate while rain reaches the opening from the side. Clearing the plate when the glass is cleaned takes seconds and prevents a slow drift into unreliability.
Making the automatic close dependable
Three habits keep it working. Test it deliberately once a year by wetting the plate with a cup of water and watching the unit run through a full close, which also confirms the drive is still pulling the sash tight at the end of its travel.
Keep the plate clear of leaves and moss, particularly in autumn on a flat roof where debris collects against the kerb. And pair the sensor with a scheduled evening close, so the failure modes the sensor cannot see are covered by something that does not depend on weather at all. Sunspire sets the sensor up and runs the unit through several full cycles at commissioning, and the workmanship carries a ten-year guarantee.
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