Why the Fiano Sliding Doors Are 10mm Toughened With a Chamfered Edge

The Fiano sliding doors are 10mm toughened glass with a chamfered edge, and it is the one specification in a veranda that people query most often. The roof is 8.8mm. The fixed side panels are 6.4mm or 6.8mm. So why are the doors, which carry no snow and hold up nothing, the thickest glass on the structure?

The answer is that they are frameless, and frameless changes what the glass is being asked to do. Everywhere else on the structure the aluminium is the structure and the glass is infill. In a frameless sliding door the glass is the panel, and the fittings simply carry it.

Nothing Is Holding the Vertical Edges

A fixed glazed panel sits in a pocket on all four sides. Any load on it is transferred into the frame along its full perimeter, and the pane is restrained from flexing in every direction. That is a comfortable situation for glass and it is why relatively modest thicknesses do the job.

A frameless sliding panel is carried at the top by its rollers and located at the bottom in the track. Its vertical edges are free. When you push it, the force goes in at one point and has to travel across the pane. When a gust catches it half open, the pressure is distributed across its face and resisted only at top and bottom. When somebody leans on it, nothing at the edge is helping.

Stiffness in a panel like that comes almost entirely from thickness, and it does so steeply: a pane's resistance to bending rises roughly with the cube of its thickness. That relationship is the whole reason for the number. Ten millimetres is not about strength in the sense of what it can survive, it is about how little it deflects while somebody is using it.

A panel that flexes noticeably feels alarming even when it is nowhere near failing, and a panel that flexes while sliding puts its rollers and its bottom guide under loads they are not meant to see repeatedly. Both problems are solved by not letting it flex in the first place.

Toughened Rather Than Laminated

Given that the roof above is laminated, choosing toughened for the doors looks inconsistent until you look at what each product does in this position.

Toughened glass gives more strength and stiffness per millimetre than a laminate of comparable thickness, which matters when the panel is doing structural work and you want it as light as it can reasonably be. It also breaks into small blunt granules rather than staying in place as a cracked sheet, and for something at chest height that a person can walk into, granulating is the safer behaviour. A laminated panel struck hard stays standing with a hole in it and sharp edges around that hole.

And a laminate has a visible interlayer line along any exposed edge. On a fixed panel that edge is buried in aluminium and nobody ever sees it. On a frameless door it is on display down the full height of every panel, and it is a surface exposed to weather and to hands for the life of the structure.

The trade-off, since there always is one, is that toughened glass carries a small but real risk of spontaneous failure from nickel sulphide inclusions, which is precisely why the roof above is not made the same way. In a vertical door, in a position where nothing falls on anybody, that risk is acceptable. Overhead it is not.

What the Chamfer Is Doing

The edge treatment is the detail most people notice last and it is the one carrying the most engineering thought.

Toughened glass gets its strength from a compressed surface skin, and that skin is thinnest and most exposed at the edge. Nearly every unexplained toughened glass failure begins at a chip or a nick in an edge, sometimes months after the damage was done. Once the compressed layer is breached, the tension in the core has a route out.

So edges on frameless panels are ground and polished to remove the microscopic chipping left by cutting, and then chamfered — the sharp ninety-degree corners taken off at an angle. That does three things at once. It removes the fragile arris that would otherwise chip against a doorframe, a ring or another panel. It means the edge you inevitably touch is smooth rather than capable of drawing blood. And it catches the light along its length, which is why a properly finished frameless panel looks like a piece of furniture rather than a piece of glazing.

It also has to happen before toughening. Nothing can be ground, cut or drilled after the pane has been through the furnace, so every dimension and every edge detail is fixed at the point the glass is ordered. That is one reason accurate sizes matter so much on this product: a frameless panel cannot be trimmed on site.

What This Means in Use

A few practical consequences follow from the specification.

The panels are heavy. Ten-millimetre glass weighs about twenty-five kilograms per square metre, so a full-height panel at the wider end of the 640mm to 1040mm range comes in somewhere around sixty kilograms. They are manageable on rollers and they are a two-person lift at least, which is worth knowing before anyone plans to move one alone.

The edges want treating with a little care during installation and afterwards. A knock against masonry while a panel is being carried is exactly the damage that causes a problem later, and it is worth inspecting edges on delivery rather than after fitting.

And the tracks want keeping clear. Grit in a bottom channel is abrasive against both the rollers and the glass edge, and it is also what blocks the drainage slots that let water out of the channel. An occasional sweep is the whole of the maintenance.

The Point of the Specification

Frameless glazing looks simple, which is the entire objective, and simplicity of that kind is generally the product of decisions rather than the absence of them. The thickness is set by how much the panel is allowed to flex. The glass type is set by how it should behave if it breaks at body height. The edge treatment is set by where toughened glass is weakest and by the fact that it is a surface people touch.

The panel width range is worth a word too. Six hundred and forty to one thousand and forty millimetres is not an arbitrary bracket. Below the lower figure the panels become numerous and the stack at the parking end grows unreasonably; above the upper one the weight and the wind area of a single panel start to work against how it feels to slide. The width you end up with is derived from your opening divided by the track count, which is another reason the track count is worth thinking about at design stage.

None of it is arbitrary, and if you are comparing quotations it is a specification worth reading closely. A frameless sliding system offered in thinner glass, or with an unpolished edge, is not the same product at a better price.