The short answer
In light rain you can leave a roof window on its ventilation setting, where the sash sits only a little open and the sloping glass sheds most of the water. In steady or wind-driven rain, close it. Nothing is damaged by a shower, but wet sills and floors follow quickly. An electric window with a rain sensor closes itself, which is the tidy answer if nobody is home.
What light rain does to a window that is slightly open
A roof window on its ventilation setting is open by a small margin, not thrown wide. That small margin is what makes a shower tolerable.
Most roof windows offer more than one way to open. The sash can swing out to a full opening, or it can be held in a restricted position that leaves a narrow gap along the top edge. On many models there is also a ventilation flap built into the frame, a small shaded slot that lets air move while the sash stays shut. The restricted position and the flap exist for exactly this reason: air can move while the glass itself remains the main barrier.
The pitch helps. Rain lands on a sloping pane and runs downhill, away from the gap at the head. Drops fall at an angle to the slope, not at right angles to it, so on a steeper roof the glass gives the opening more cover. On a shallow pitch, the opening sees more of the sky and takes more of the shower. Our guide on roof pitch and roof windows explains how the angle changes what the window can do.
So a fine, vertical drizzle on a steep roof with the window on its ventilation setting does no harm. A few drops may reach the head of the frame. That is what the flap and the drainage channels around the sash are designed to handle.
When the window should be shut
The trouble starts when the rain has a direction or a weight. Both change how much water reaches the opening.
- Wind from the front of the slope. Rain blown up the roof enters the gap from below, where the sash edge has no cover at all. Windward slopes are the ones to watch, and in the Weald that is often the south-west face.
- Heavy or prolonged rain. A gap that copes with a five-minute shower lets in a steady trickle when the rain lasts an hour. Water can reach the lining and the sill inside before anyone notices.
- A window set low on the roof. It is reachable, which is good, but it sits where water sheeting off the tiles above arrives at its full flow.
- A sash opened fully. On a full opening, the pane may be partly flat to the rain and the room is exposed. A full opening is for warm dry days.
The simplest habit is to close the window before you leave the house, and before you go to bed if the forecast has turned. The answer on which way a roof window should open is useful here, because the hinge position decides where the gap appears and which edge takes the weather.
A quick guide by kind of weather
This is a general rule of thumb, not a guarantee. Your roof, your window and your exposure will shift it.
| Weather | Window position | Why |
|---|---|---|
| Dry and calm | Open as wide as you like | Nothing for the gap to catch |
| Fine, still drizzle | Ventilation setting only | Sloped glass sheds most of it |
| Steady rain | Shut | Water reaches the sill and lining |
| Windy rain | Shut | Rain is driven into the gap from below |
How a rain sensor takes the worry away
If you cannot rely on remembering, let the window remember for you. An electric or solar-powered roof window can be fitted with a rain sensor that shuts the sash when the first drops land.
The sensor is a small panel on the outside of the window frame or the glass. When its surface gets wet, it sends a signal to the motor and the sash closes. Some models then return to their earlier position once the surface is dry again, depending on the control system. You choose the behaviour when the system is set up.
It is worth knowing what that does and does not cover. The sensor reacts to water on its own surface, so a sudden squall can lay a few drops on the carpet before the sash finishes closing. It also depends on power. A mains-fed window needs its wiring completed by a qualified electrician where the work is notifiable, and a solar-powered one needs enough light to keep its battery topped up. Our page on electric and opening rooflight installation covers the options, and VELUX window installation covers those controls in that brand's range.
A sensor suits a few rooms best. A loft bedroom used daily is one, a spare room or a hallway that is often empty is another, and a window over a stairwell that cannot be reached without a pole is the clearest case of all.
Rain, sound and the room below
Whether the window is open or shut, rain on a roof window is a sound you will hear from the bedroom. What you hear depends on the glazing build, not on the weather alone.
A shut window on a thin glazing build drums. A laminated acoustic inner pane damps the tapping, and gasketed seals stop the small rattles that come with gusts. Open on its ventilation setting, the window lets through the outside sound as well, and no glazing build changes that. If a room is for sleep, the ventilation setting suits a quiet night, and the shut position suits a wet one. The quiet spec we write for each quote records the glazing build we chose for that room and the noise we listened for. You can read how it works on the quiet spec page, or look at whether roof windows are noisy in rain for the detail on drumming.
Wet-weather questions about a roof window left open
People tend to ask these once it has already rained.
Will rain through the ventilation flap ruin the ceiling?
Not in ordinary weather. The flap is designed to admit air and keep out water, and any drops that get past it drain through channels in the frame. Persistent rain is a different matter, so shut the window before a long wet spell.
Can I fit a rain sensor to a window I already own?
It depends on the model. Some manual windows cannot take a motor at all, and then the answer is an electric replacement sash or a new window. We assess that at the survey, and you can send photos through the quote page.
Does the same advice hold for a window on a bathroom roof?
A bathroom window is often left open to clear steam, which makes the sensor more useful. See the separate answer on fitting a roof window in a bathroom.
Where do I start if I want an automatic window?
Start with the room and how it is used. Tell us that through the roof window installation hub or by WhatsApp, and we will suggest the control that suits it.