Roof window blinds and when to specify them

A blind orders against the window code, not against a measurement

Manufactured roof windows are made in a fixed set of sizes, each with a letter and number code, and every blind in that manufacturer’s range is made to fit those codes exactly. You order a blind for a 780 by 1400mm unit by quoting the code, and it arrives with brackets that clip into pre drilled points in the frame.

Two things follow. Blinds do not cross between brands, because the frame fixing points and the channel profile differ. And a blind for a discontinued frame profile may not exist any more, which is one of the practical arguments for changing the window rather than keeping a twenty five year old one going. The coding system is set out on the roof window sizes page.

Glare is an internal problem and heat is an external one

This is the distinction that decides most of the specification, and it gets missed constantly.

An internal blind sits behind the glass. Sunlight has already come through the glazing and turned into heat inside the room by the time the fabric intercepts it. The blind stops you being dazzled, and it stops light landing on a screen or a pillow, but the energy is already in. A dark internal blind in full sun can get genuinely hot to touch, because it is absorbing that energy and radiating it back into the room.

An external blind sits in front of the glass and stops the sun before it reaches the glazing. That is the only arrangement that meaningfully reduces solar gain. On a south or west facing loft slope in July, the difference between the two is not marginal.

Where the plaster has to stop, and the space nobody leaves

The single most common reason a blind cannot be fitted is that the plasterboard reveal has been brought right up to the frame and skimmed over the channel.

Every manufacturer publishes a clearance: the lining should stop short of the inner edge of the frame so the blind’s side channels, its top box and its bottom bar are all clear of the plaster. Depending on the range that is typically somewhere between 15 and 40mm at the head and the sides. Build the reveal tight and the blind either will not go in at all, or it goes in and then jams against the plaster halfway down.

The other half of it is the head. A roller blind needs a cassette or at least a barrel across the top of the frame, and a pleated blind needs its top rail. If the head lining has been formed hard against the frame, that space has gone. On a job where a blind is even a possibility, the reveal is set out with the clearance built in, and the cost of doing so is nothing.

Roller blinds, and the side channels that decide how well they work

A roller blind on a roof window is not a vertical roller turned sideways. Because the window is sloping, gravity does not hold the fabric flat, so the bottom bar runs in aluminium side channels that keep tension across the width and hold the blind wherever it is left.

Those channels are the part that matters. A blind with proper full length channels sits flat against the glass, blocks light at the edges, and does not flap when the window is open on the vent. A cheap retrofit with no channels sags away from the glass, lets a bright band up each side, and rattles.

Roller fabrics come in translucent, dimming and blackout weights. A translucent roller is the everyday choice for a study or a landing: it kills glare, keeps daylight, and stops anybody seeing a lit room from a neighbouring garden.

Pleated blinds, and the two rail arrangement

A pleated blind is a concertina of fabric running between two rails that both slide in the side channels. Because both rails move, you can hold it anywhere: closed at the top to shade the head of the window while leaving the bottom clear for the view, or closed at the bottom for privacy while keeping the light coming in above.

That flexibility is the reason to specify one. On a room where the roof window looks out at a neighbouring elevation lower down the slope, a bottom up pleated blind gives privacy exactly where it is needed and takes no daylight away. Pleated fabrics are usually translucent rather than fully opaque, so they are a light quality choice rather than a darkening one.

Venetian blinds in a sloping frame, and the rooms that suit them

An aluminium venetian in a roof window runs the same way, with the slat stack held in side channels and a tilt control at the bottom bar. It gives directional control that no fabric blind can: slats angled to cut a low western sun while still letting light bounce off the ceiling.

It is the right answer in a bathroom, an en suite or a utility, because the slats do not absorb moisture, they wipe clean, and steam does not mark them. It is a poor answer in a bedroom, because the slats never close tightly enough to darken a room, and it is a noisy answer under a window left open on the vent.

Blackout as a specification rather than a colour

A dark fabric is not a blackout blind. Blackout means an opaque coated fabric, running in channels that overlap the frame at the sides, with a top box that closes off the head. Get any one of those three wrong and you have a dark grey room in June at four in the morning.

Even a properly specified blackout blind leaves a faint outline of light at the perimeter, because the channels have to have running clearance. That is normal and it is invisible once you are asleep. The full treatment of darkening a room in the roof, including the interaction with the reveal, is on the bedroom and blackout page.

Energy blinds, honeycomb cells and what the numbers mean

An energy blind is a pleated blind with a honeycomb cell structure and a reflective backing. The cells trap a layer of still air against the glass, and the foil face reflects long wave heat back into the room in winter and short wave heat outward in summer.

The claimed improvement in the U value of the window and blind together is real but modest, and it only applies while the blind is closed, which is at night. Think of it as a night time insulation layer on a large area of glass rather than as a glazing upgrade. On a loft bedroom with two large windows in a cold gable end room, closing them at dusk makes a difference you can feel. The glazing side of the same question is on the energy efficient rooflights page.

External awning blinds, and the honest limit of what they do

An awning blind is a black mesh screen that unrolls down the outside of the glass and clips at the bottom. The mesh is open, so you can still see out through it and daylight still comes in, but it intercepts most of the solar energy before it reaches the glass.

They are the highest value item on this page for a west facing loft conversion. A room that hits the high twenties by four in the afternoon in July, with the sun full on the glass, comes down several degrees with an awning across it. The mesh does not darken the room and it does not stop you opening the window underneath.

The limits are worth stating. The fabric is exposed to weather, so it has a shorter life than an internal blind. It is rated to a wind speed, above which it should be rolled up, and a solar or mains powered one with a wind sensor does that on its own. And it does nothing for glare on a screen directly under the window, because it is a shade rather than a curtain.

External roller shutters, and the loft rooms that need one

A roller shutter is a slatted aluminium shutter that runs down the outside of the glass in rigid guides and closes completely. It blocks essentially all light, cuts solar gain to almost nothing, adds an insulating air layer, and reduces rain noise and outside noise noticeably.

It is the specification for a shift worker’s bedroom, a nursery, a home cinema in the roof, or a top floor room on a busy road. It is powered, since nobody is winding a shutter by hand from inside, and it needs either mains or a solar module. It is also the most visible item from outside, since the headbox sits above the window on the roof slope, which makes it a poor candidate on a conservation area elevation.

Solar blinds on a north facing slope, and the charging problem

A solar blind needs light on its cell to keep its battery up. On a south or west facing slope in Gloucestershire that is never an issue. On a north facing slope in a Stroud valley, overshadowed by the hillside behind and by mature trees, the cell can spend most of the winter in shade and the battery gradually runs down.

The units can be charged from a lead in that situation, which nobody enjoys doing on a window two metres above a landing. If a powered blind is wanted on a north facing or heavily overshadowed slope, running a mains spur while the reveal is open is the better decision, and it costs very little at that stage. Whether daylight from a north slope is worth having at all is discussed on the north facing slope page.

Insect screens, and the channels they share

An insect screen for a roof window is a pleated mesh that pulls down from a cassette at the head and runs in the same side channels as a blind. It stays in place with the window open, so a bedroom can be ventilated on a summer night without everything in the Severn Vale coming in with the air.

Where both a blind and a screen are wanted, they can share a window, but the channel arrangement has to be planned, since two sets of running gear need two sets of clearance. Specifying them together at order stage is straightforward. Adding a screen to a window that already has a blackout blind fitted tight to the frame is often not.

Bathrooms, kitchens and the fabric that survives the room

Fabric in a wet or greasy room has a harder life than fabric in a bedroom. In a bathroom or shower room the fabric is repeatedly wetted by steam and then dried, and an untreated cotton blend will mark and eventually grow spotting along the bottom bar where water collects.

The specification for those rooms is either an aluminium venetian, which wipes down, or a coated polyester roller with a moisture resistant finish. Over a hob, the same logic plus grease: anything with a texture holds it. It is also worth remembering that the blind is above head height and will not be cleaned often, so choosing something that tolerates neglect is more useful than choosing something that cleans beautifully.

Retrofitting a blind to a window that is already in

Adding a blind later is entirely possible, and it is worth knowing what changes. The frame has to be identified, which means finding the data plate on the sash, usually visible when a centre pivot is rotated. That gives the range and the size code, and the blind is ordered from that.

Then the reveal gets checked. If the plaster has been taken hard to the frame, the lining has to be cut back to open up the clearance, which means a plastering and decorating visit as well as the blind. And on frames from before the current profiles, the blind range may simply have moved on, in which case a universal blind with surface mounted channels is the fallback, and it never looks as neat as a factory one.

A west facing conversion through a Gloucestershire summer

Put the pieces together for the room that struggles most, which is a west facing loft conversion under a dark tile roof anywhere across Gloucester, Cheltenham or the vale. The sun is on that glass from mid afternoon until it sets, at the point in the day when the room has already absorbed heat through the roof.

An internal blackout blind will darken it and make it hotter. A solar control coating on the outer pane cuts the incoming energy by roughly half before anything else happens. An external awning blind takes out most of the rest. Opening the window at the top of the room in the evening lets the accumulated heat out by stack effect, and an insect screen makes that tolerable with the window open overnight.

That combination, specified together at order stage, costs a fraction of what it costs assembled piece by piece over three summers. It is the conversation worth having at survey, and it is the reason the blind question gets asked before the window is ordered rather than after the plaster is dry. The costs page explains how a specification turns into a figure.