Guide · Electric and Opening Rooflights
Electric blinds fitted inside the rooflight
A blind on a rooflight has a harder job than a blind on a window. Gravity is working across it rather than down it, so the fabric wants to sag away from the glass, and there is no windowsill to stop at. The answer the industry settled on is a fabric held under tension in side channels, wound on a sprung tube in a cassette at the head, with a small motor inside the tube driving it.
This page covers what is inside that assembly, what it needs from the frame, how it is powered, and what it does to the temperature and the condensation behaviour of the glass above it. It is the blind side of the wider subject set out on the electric and opening rooflights hub.
The motor that lives inside the tube
The drive is a tubular motor, a slim cylinder containing a direct current motor, a planetary gearbox and a limit encoder, all sitting inside the roller the fabric is wound on. On a rooflight blind that tube is small, often around twenty five millimetres in diameter, because the cassette has to fit within the depth of the sash head.
The whole assembly turns as one. The motor body is held against rotation at one end while the tube turns around it, which is why a rooflight blind is quiet: there is no external gearbox and no exposed drive, and the fabric itself damps what little noise there is. Torque requirements are modest, since the blind is lifting fabric rather than glass, and the load rises only slightly as the roll builds up on the tube.
Side channels, and the clearance they need
The channels are the reason the system works at all. Each side of the fabric carries a welded or sewn bead, and that bead runs in a groove in the channel, so the fabric is held to the glass line along its whole length regardless of the angle of the roof.
The tolerances are tight. Too little clearance and the bead binds, the motor stalls at the same point every time and the fabric wears at the edge. Too much and light spills down both sides and the blind rattles in a draught. Manufacturer blinds are cut to the window code and drop straight into channels that are already part of the frame, which is why a factory blind is almost always the better answer than a generic one adapted to fit.
The bottom bar, and the tension it carries
At the free edge there is a weighted bar, sometimes with a spring cassette at each end pulling against the roller. That opposing tension keeps the fabric flat on a horizontal or shallow plane, where gravity would otherwise let it belly away from the glass by fifty millimetres in the middle of a long drop.
It also gives the blind a defined stopping position. On a well set unit the bar sits neatly against the bottom of the reveal with no gap of daylight beneath it. Where a blind has been supplied for the wrong size of unit, this is where it shows: either the bar stops short of the sill line, or it reaches the end of its travel before the fabric is taut.
Fabric weight, and the cassette depth it forces
Fabric choice drives the physical size of the head. A light pleated fabric folds into a very small stack. A blackout fabric with an acrylic backing and an aluminium coating is thicker, rolls into a larger diameter and needs a deeper cassette to hide it.
On a factory motorised roof window that depth is already accounted for in the sash head, and the blind disappears completely when open. Where a blind is being added into an existing reveal, the cassette has to go somewhere, and a deep cassette across the head of the opening in a shallow reveal reads as a box. That is worth seeing in the reveal before it is ordered rather than after, and it argues for deciding the fabric at the same time as the unit.
Blackout, dim out and the difference
Blackout fabric has a coated back that passes no light through the weave at all. Dim out reduces light substantially without eliminating it. The distinction matters in a bedroom under an east facing slope, where a June sunrise at twenty to five puts direct sun onto the glass.
Even with true blackout fabric, a small amount of light comes down the side channels and around the bottom bar. Good channels with a deep groove reduce it to a thin outline rather than a glow. That is as good as it gets on a roof: a blind on a rooflight is a very dark room, not a photographic darkroom, and anybody who needs the second should know that before choosing.
A blackout blind on a roof window makes the room dark enough to sleep in, with a fine outline of light at the edges.
Solar reflective fabrics, and where they sit relative to the glass
An internal blind stops light after it has already passed through the glass, so the solar energy is inside the room and the blind is re radiating some of it back. A metallised backing helps considerably. It sends a good share of that energy back out through the glass before it becomes heat in the room.
An external awning does better, because it stops the energy before it reaches the glass at all, and on a south or west facing rooflight it is the more effective of the two by a clear margin. The internal blind is doing several other jobs though: privacy, glare control, blackout and the look of the room from inside. Households with a serious summer gain problem often end up with solar control glass and an internal blind rather than choosing between them, a balance covered on the energy efficient rooflights page.
The supply a blind wants, and where it comes from
A tubular motor for a rooflight blind runs at extra low voltage, typically twelve or twenty four volts, and draws well under an amp. On a wired installation that is a separate two core pair to the head of the unit, run alongside the actuator supply at first fix.
The two are separate circuits even where they share a route and a transformer, because the controller has to drive them independently. A blind that opened whenever the pane opened would be useless in summer, when the point of the arrangement is a pane open for air and a blind down for shade. Running the pair while the ceiling is open costs almost nothing, and it is the single most useful piece of provision on the whole job.
Solar powered blinds, and battery driven tubes
Where no cable exists, blinds follow the same route as units. A small photovoltaic strip on the outside of the sash head charges a cell that drives the tubular motor, with a radio receiver taking commands from a handset.
The energy budget suits it well. Raising or lowering a blind takes a few seconds against thirty for a rooflight pane, and the load is a fraction. The pattern of use is heavier though, because a blind may be operated twice a day every day of the year while a rooflight is opened seasonally. On a shaded north facing slope a solar blind that is being cycled twice daily is working the battery harder than a solar vent on the same roof.
Setting the limits, and the positions it remembers
Commissioning sets two end points into the motor’s memory: fully open, with the fabric wound in and the bar tucked into the cassette, and fully closed, with the bar at the bottom of the channel and the fabric taut. Everything else is calculated between those.
Most controllers also hold at least one intermediate position, and it is more useful than it sounds. A blind at two thirds down cuts the direct sun off a kitchen worktop while leaving the top third of the glass giving daylight to the room. Setting that position at handover, and putting it on a button, is the difference between a blind that gets used and one that lives permanently at one end or the other.
Blinds on a motorised pane, and the order they move in
Where the rooflight itself opens, the blind and the pane have to be aware of one another, since a blind hanging in front of a sash that is about to hinge inwards will be caught by it. On a factory motorised unit the controller manages this and the blind clears the way before the pane starts to move.
On top hung and outward opening units the interaction is much simpler, since the sash swings away from the fabric. Roof windows that pivot at the centre are the case needing care, because the lower half of the sash comes into the room, and that is one of several reasons a blind is best ordered with the unit rather than assembled from parts later.
Condensation behind a closed blind
A blind held close to the glass creates a still air layer between fabric and pane. That layer is a genuine thermal benefit at night, which is why a closed blind measurably reduces heat loss through a rooflight. It also means the inner face of the glass runs colder than it otherwise would.
In a bedroom, with two people breathing overnight, that colder pane can collect surface condensation behind the fabric which is then discovered in the morning. The answer is ventilation rather than a different blind: a trickle vent left open, a ventilation flap on the unit, or the blind raised a hand’s width overnight to let the air move. In a bathroom the same physics applies with more moisture behind it, and there the extract fan is doing most of the work.
Blinds on a lantern, slope by slope
A lantern is not one plane, it is four or more, and a blind system has to answer each of them. The usual arrangement is an individual blind in each glazed panel, running in channels fixed to the rafter bars, so that four or six small blinds are grouped onto one channel of a handset and move together.
The alternative is a single blind across the underside of the whole structure at ceiling level, which is simpler and quite different in effect. It shuts off the lantern as a feature and leaves the volume above it dark, whereas panel blinds keep the shape of the roof visible. On a kitchen where the lantern is the architectural point of the room, panel blinds are usually what is wanted. Details of the structure itself sit on the roof lantern installation page.
What each blind type does electrically
Four fabric types cover almost all domestic work, and they differ in what they ask of the motor as much as in how they look.
| Type | Roll or stack | Cassette depth | Does best at |
|---|---|---|---|
| Roller, dim out | Rolls on the tube | Shallow | Glare control in a living space |
| Roller, blackout | Rolls, larger diameter | Deeper | Bedrooms, east and south slopes |
| Pleated | Stacks in folds | Very shallow | Awkward shapes and small panes |
| Honeycomb | Stacks, cellular | Medium | Insulating value at night |
Cleaning fabric that lives above your head
Rooflight blinds collect dust on the upper face, which is the side you cannot see, and in a kitchen they collect a fine film of cooking grease along with it. Left long enough that film becomes visible as a discoloured band across the fabric.
The routine is undemanding. Run the blind fully down and vacuum the exposed face with a soft brush attachment once or twice a year, and wipe the side channels out with a dry cloth so nothing collects in the groove. Coated blackout fabrics take a damp cloth. Woven fabrics do not enjoy water and are better brushed. Doing this at the same visit as the glass keeps it from being forgotten, and the wider list of what a motorised unit wants is on maintaining the mechanism.
Fitting a blind into a frame that is already up
Whether this is straightforward depends entirely on one thing: whether the unit was made with the channels and the cassette recess in it. Factory motorised roof windows generally are, and the blind clips in from below in twenty minutes with no tools beyond a screwdriver.
A unit without that provision needs the channels fixing to the reveal or to the frame face, and a cassette mounted across the head. It works, it looks like an added component rather than part of the window, and it needs a power route that was never planned for. That is the argument for putting the provision in at order stage even when the blind itself is going to wait, and it is why the specification conversation covers blinds before anything is ordered.
Essex rooms where the blind decides the specification
Three situations put the blind at the front of the conversation rather than the end. Loft bedrooms in the interwar semis across the commuter towns, where a roof window in an east facing slope gets full sun before five in the morning and blackout is the whole reason the window is bearable. Kitchen extensions with a south or west facing lantern, at Great Baddow, Billericay and across the newer estates, where the afternoon gain in July decides whether the room is used at all.
And bathrooms and shower rooms, where the fabric has to tolerate humidity, the electrics have to respect the zones around the enclosure, and privacy from a neighbouring upper window is a real consideration in a tight Chelmsford terrace. In each of those the blind is not an accessory bolted on at the end. It is part of what the rooflight is for, and it belongs on the drawing with the glass specification.
Keep reading
More on electric and opening rooflights
Rain sensors and what they actually do
Read the guide →Wiring an electric rooflight at first fix
Read the guide →Solar powered rooflights without a mains run
Read the guide →Remote controls, wall switches and smart home links
Read the guide →Opening rooflights for the stack effect
Read the guide →
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