Butt-joint glass partitions: the joint, in detail
A butt joint is 4mm of silicone doing the work of a post. Whether the screen looks like one sheet of glass or like several panels in a row is decided almost entirely by how that joint is made.
What butt-joint glass partitions are, and why the joint disappears
In butt-joint glass partitions the panels stand side by side with nothing between them but a thin bead of clear silicone, usually 3mm to 6mm wide. There is no vertical post, no mullion, no frame. The only metal is the shallow channel at floor and head. Stand back three metres from a well made butt-jointed screen and you will struggle to find the joints at all, which is the entire point of building one this way.
The joint disappears because of what is not there. A post, even a slim one, casts a shadow and reads as a line from any angle. A silicone joint between two polished glass edges has almost the same refractive behaviour as the glass either side of it, so there is nothing for the eye to catch. Get the panels truly in plane and the joint becomes a faint vertical thread. Get them 2mm out of plane and the joint catches light down one side and becomes the first thing anybody notices.
That is the honest summary of this whole page: the material cost of a butt joint is a tube of silicone, and the quality of it is entirely labour and setting out. Anyone can sell you a butt-jointed screen. Not everyone can make the joint vanish.
Tolerance: a straight panel in a room that is not straight
Toughened glass is cut, edged, drilled and notched before it goes into the furnace, and after toughening nothing can be altered. So every panel in a butt-jointed run is made to a dimension that was decided at survey, and the room has to be measured as it is, not as the drawing imagines it. In a London house that means the two flanking walls are not parallel, the floor falls across the opening, and the ceiling dips in the middle.
The way a butt joint handles this is by putting the tolerance where it does not show. The perimeter of the screen, against the walls and inside the channels, can absorb several millimetres in a sealant line that nobody looks at. The joints between panels cannot: a joint that varies from 3mm at the bottom to 7mm at the top is visible the length of the screen. So the panels are sized to leave generous perimeter clearance and consistent inter-panel gaps, which is the opposite of how a joiner would approach the same opening.
This is why we template on anything awkward rather than working from dimensions alone. A pattern taken off the real opening goes back to the workshop at 6 Atlas Road in Wembley and the glass is cut under it. On a three panel screen in a square room, dimensions are fine. On a screen meeting a bay, a chimney breast return or a staircase, a template is the difference between a clean job and a visible fudge.
The detail almost nobody mentions: roller wave
Toughened glass passes through the furnace on rollers, and the heat and the rollers leave a very slight repeating ripple in the surface called roller wave. It is invisible on its own. It becomes visible where two panels meet and their ripples run out of step, because reflections down the joint then disagree. The fix is simple and free if it is thought about: panels for one screen are toughened in the same batch and oriented the same way through the furnace, so the wave matches across the joint. If you are comparing quotes for butt-joint glass partitions, asking whether the panels will be run in the same orientation is a quick way to find out who actually processes their own glass.
Getting panels into the same plane
Two adjacent panels can be perfectly spaced and still look wrong if one sits a millimetre or two forward of the other. The eye is far better at spotting a step than a gap. Getting panels coplanar is a matter of the channel being straight and consistently packed, and of the glass being set on resilient blocks at the right heights rather than dropped in and hoped for.
The channel is where this is won or lost. A length of aluminium U channel fixed to a floor that falls, or to a wall of lath and plaster that bows, will take the shape of what it is fixed to. It has to be packed and shimmed to a laser line first and fixed second. On a long run we will often set the channel, leave it, and come back to glaze, because getting the channel right is not a job to rush on the morning the glass arrives.
Panel height matters here too. A butt-jointed screen with panels of visibly different heights in the same run reads as a mistake even when the ceiling is the thing that is wrong. Where a ceiling dips significantly across a long screen, the honest choices are to keep all panel tops level and vary the sealant line above, or to step the panels deliberately and make the step obvious enough to read as intentional. The bad outcome is a gentle unintended drift.
The silicone itself: grade, colour and width
Not all clear silicone is the same material, and the difference shows up in year three rather than week one. Cheap general purpose sealant is usually acetoxy cure, which is the one that smells strongly of vinegar while it sets. It cures hard, it is prone to going cloudy, and in a kitchen or bathroom it grows mould readily. What a glass joint wants is a neutral cure, low modulus silicone: it stays flexible, holds its clarity far better, and bonds properly to glass without the acid.
Colour is simpler than people expect. Genuinely clear silicone is the right choice in a butt joint, because it lets the joint read as glass. Translucent or milky sealant, which is what some trade tubes actually contain, gives you a visible white thread down every joint. On the perimeter, against a painted wall, a colour-matched sealant is often better than clear, because clear silicone against plaster shows every imperfection behind it.
Width should be as narrow as the tolerance allows and no narrower. Below about 3mm there is not enough material for the joint to flex, and silicone that cannot move will eventually split away from one edge. Above about 6mm the joint stops being a thread and starts being a feature. Within that range, consistent beats narrow every time. A dead straight 5mm joint looks better than a 3mm joint that wanders.
The edges under the silicone matter as much as the silicone. A flat polished edge on both panels gives a clean, optically consistent joint. An arrised or ground edge, which is cheaper, leaves a slightly matt surface and a visible shadow in the joint. We polish the edges of butt-jointed panels as standard for exactly that reason, and if a quote is noticeably cheap, edgework is one of the places the saving usually comes from.
Butt joint, slim post or full frame?
A butt joint is the right answer when the look is the priority and the screen is a plain run of fixed glass. It is the wrong answer in three situations, and it is worth being clear about them rather than selling you the joint regardless.
If sound matters, a butt joint is not your friend. A silicone joint is far from airtight across its length, and in combination with the perimeter gaps a butt-jointed screen leaks noise. If you need genuine separation, a framed system with gasketed panels will outperform it, and a stud wall with a glazed panel in it will outperform both. If sound does not matter, none of that is worth paying for.
If a door has to go into the run, something has to carry the hinges and provide a stop for the leaf to close against. That is a glass-to-glass hinge on an adequately wide fixed panel, or a slim post or a jamb. A door hung off a narrow butt-jointed panel will flex every time it swings, and you will feel it. And if the screen has to turn a corner, a butt-jointed internal corner in clear silicone works and looks excellent, but it needs both panels to be exactly on their line, so it is the one detail where a slim corner post is sometimes the sensible economy.
What a butt joint looks like in five years
A properly specified neutral cure silicone joint in a dry internal room ages very slowly. It stays flexible, it does not shrink back noticeably, and the clarity holds up. The thing that actually degrades a joint is cleaning. Spray glass cleaner collects along the top edge of the silicone and dries there, which over a couple of years leaves a faint line on either side of the joint. The answer is to spray the cloth and not the glass, which also keeps cleaner out of the floor channel.
In a kitchen screen the joint picks up cooking residue and in a bathroom it picks up soap. Both wipe off with warm water and a little washing up liquid. What should not be used on silicone is anything abrasive, anything with bleach in it used repeatedly, or a blade. Bleach will not clean a joint that has gone mouldy through from the inside, and a blade will cut the bead and start the failure you were trying to avoid.
If a joint does eventually fail, and after many years one can, it is re-pointed rather than replaced. The old bead is cut out, the edges are cleaned back with solvent, and a fresh bead is run. It is an hour’s work on a domestic screen and it does not involve taking the glass out. That is worth knowing, because it means a butt joint is not a permanent commitment in the way a bonded structural assembly is.
We cut, polish and toughen the glass for butt-jointed screens in our own workshop, which is what lets us keep a run in one batch and one orientation. Survey is free anywhere in London, the quotation is fixed and written, and it will say what grade of silicone is going into the joints. 020 4634 6045.
FAQ
Questions about butt-joint glass partitions
How wide is the gap between butt-jointed glass panels?
Normally 3mm to 6mm, filled with clear silicone. Narrower than about 3mm leaves too little material for the joint to flex, and silicone that cannot move eventually tears away from one edge. Wider than about 6mm and the joint stops reading as a thin line and becomes a visible feature. The width chosen depends on how much tolerance the opening demands, because the joints are one of the places variation can be absorbed. Consistency matters more than narrowness: a dead straight 5mm joint looks far better than a 3mm joint that varies down its length, so do not push an installer to a figure the room cannot hold.
Do butt-joint glass partitions let sound through?
Yes, more than people expect. The silicone joints themselves are not the main path. The bigger leaks are the clearance gaps round the perimeter of the screen, the gap at the head if the screen stops short of the ceiling, the void between floor joists running beneath the glass, and any door gap. A butt-jointed screen will take the edge off noise and make a television in the next zone feel like background, which is often all that was wanted. It will not give you speech privacy. If you need a room you can take calls in, a framed gasketed system or a stud wall with a glazed panel is the honest specification.
Does the silicone in a butt joint go yellow over time?
Good quality neutral cure silicone in an internal room holds its clarity for many years. The products that discolour are cheap acetoxy cure sealants, the type that smell strongly of vinegar while curing, which tend to cloud and to grow mould in damp rooms. Ultraviolet light accelerates ageing, so a joint in direct sun through a south facing window will age faster than one in a hallway. Most apparent yellowing in older screens is actually dried cleaning product and grease sitting along the edge of the bead rather than the silicone itself. If a joint genuinely fails, it is cut out and re-pointed without removing the glass.
Get a price for your glass screen
Tell us the opening and what you are trying to separate. We survey across London and quote in writing with the glass and hardware stated.