Once a roller blind gets past about 2400mm wide it needs a heavier tube, and past about 3000mm it is usually better made as two blinds than one. Width is the real limit on a roller blind, not the drop.
Width is really a tube question
Roller tubes come in 32mm, 38mm and 45mm. The wider the blind, the more the tube wants to bow under the weight of the fabric, and a bowed tube does two things you will not like. The middle of the blind sags, and the fabric starts walking sideways as it rolls.
So a wide window goes on 45mm, and with a heavy blockout on a long drop that is not really optional. A three metre wide blockout on a 32mm tube will start disappointing you inside a season.
Where we split a wide opening
Two or three blinds across one opening, sharing a headbox or a pelmet, is the normal answer for a big span. The join sits at a mullion if there is one, or at the centre of a sliding door.
You end up with a gap of a few millimetres where the blinds meet. In a living area that is barely visible. In a bedroom it shows, so we plan the split at the measure, run a pelmet across the top, or overlap the fabrics slightly to close it up.
Splitting also makes the blind usable. Two 1800mm blinds are easy to run. One 3600mm blind on a chain is a two handed job every time you touch it.
Fabric roll widths and why joins show
Fabric comes off a roll, and most ranges top out between 2000mm and 3000mm wide. Beyond that the fabric itself has to be joined, and a join in a plain blockout is obvious in daylight. Splitting into separate blinds avoids the join completely, which is the main reason we lean that way on big openings.
Patterned and textured fabrics hide a join better than a flat colour, but nothing hides it entirely once the sun is behind the blind. It tends to be the first thing people notice in a photo of a wide window.
Do not forget the roll diameter
Long drops build a fat roll. Three metres of blockout wound onto a 45mm tube finishes around 90mm across, sometimes more with a heavier fabric. That bundle has to clear the reveal, the architrave, any curtain track, and the top of a sliding door.
It also means a face fixed blind on a tall window sits further off the wall than people expect. Allow at least 100mm of height above the opening for the fixing and the roll, and check the roll will not foul a door handle when the blind is right up.
Big blinds and why motors make sense
Wide blinds are where a motor stops being a luxury. A tubular motor inside the tube lifts the weight evenly, runs the blind at the same speed every time, and takes away a chain that is genuinely hard work on a heavy blockout.
Running several wide blinds across one wall on a single remote channel is the other reason. They all start and stop together, which looks far better than four blinds sitting at four different heights across a living room wall.
Sliders, stacked glass and raked heads
Sliding doors need the bottom rail clear of the track and the handle, so we check the handle position before setting the drop. Stacked windows, where a fixed pane sits above an opening one, are usually better as one blind covering both than two small ones with a rail across the middle.
Raked or angled heads cannot be covered by a square roller blind, and pretending otherwise leads to a triangle of light at one end. On those windows a shutter or a curtain often does a better job, and we will tell you that at the measure rather than sell you a blind that will annoy you.
Bifold doors are worth a mention as well. The blind has to clear the door leaves when they fold back, so the projection gets set to suit the stack rather than the glass. That is much easier to work out standing in front of the opening than over the phone.
Big windows are worth getting measured properly, because the tube, the split and the roll all have to be worked out together. We do that as part of the free measure and quote. Start with the roller blind range, look at motorised roller blinds for wide spans, or see how a cassette tidies up a big headbox.
Common questions
How wide can one roller blind be made?
It depends on the fabric and the drop, but past roughly 3000mm we would rather split the opening into two blinds under one pelmet. Wide single blinds sag in the middle, track to one side as they roll, and are hard work on a chain.
Will the join between two blinds let light through?
There is a gap of a few millimetres where the two blinds meet. In a living area you will not notice it. In a bedroom it shows, so we plan the split at a mullion, run a pelmet across the top, or overlap the fabrics slightly.
Do large roller blinds need extra support?
Wide blinds either get an intermediate bracket holding the tube up, or they get split into separate blinds with a shared bracket at the join. Either way the point is the same, stopping a long tube bowing under the weight of the fabric on it.