Fixed glazing: how large a single pane can go

Ask for the largest pane available and the answer depends on who is answering. A glass trader quotes the float sheet. A tempering plant quotes the furnace. A facade engineer quotes the frame. A forwarder quotes the container door. All four answers are true, and none of them is the size a project can actually build.

The same sequence governs every elevation: cut, temper, carry, ship. A pane that fails at the last step has already been tempered, which is the most expensive place to find out.

Float glass is drawn as a ribbon and cut into stock sheets. The Guardian clear float data sheet lists the standard sheet at 2,438 by 3,658 mm for the common thicknesses and the jumbo sheet at 3,210 by 6,000 mm, in thicknesses from 2 mm to 15 mm. Other producers work to the same two sizes because the tin bath sets them, not the brand.

A 4,000 by 3,000 mm pane fits inside the jumbo with room to spare, so the cut plan is where the money sits. Two panes nested into one jumbo cost less than two panes cut from two sheets, and the remainder either becomes a smaller pane on the same order or becomes waste that the price carries. A cut plan drawn before the order is placed is worth more than a discount argued after it.

Tempering is a heat and quench cycle, and the furnace sets a limit in both directions: the largest sheet it accepts and the smallest. A jumbo line takes 3,300 by 6,000 mm. A 2,440 by 3,660 mm furnace is the standard size in many plants, and on that line a 3,000 mm wide pane is not possible in tempered glass whatever the sheet allows. Thickness follows the machine as well, typically 4 mm to 25 mm on the jumbo lines.

The harder limit is timing. Once a pane is tempered it cannot be cut, drilled, notched or edged. Every hole, every corner radius and every polished edge is finished before the furnace. A drawing change after tempering is a new pane, a new cycle and a new lead time.

Tempered glass carries a small risk of spontaneous breakage from nickel sulphide inclusions. The probability scales with the area of glass in the building, and the consequence scales with what sits underneath.

The heat soak test, defined in EN 14179-1, holds the tempered pane at about 260 degrees Celsius, within 10 degrees, for at least two hours after a peak near 290 degrees. The cycle drives the inclusion through its phase change inside the oven instead of on the facade. It does not change the glass. It removes the panes that would have failed later.

Glass weighs 2.5 kg per square metre for every millimetre of thickness. A 12 mm pane at 4,000 by 3,000 mm is 12 square metres and 360 kg before the frame, and it arrives on four vacuum cups with two operatives and a slab that has to be ready for the load. Handling is a design input, not a site improvisation.

In service the panel is a plate. Dead load runs down through the frame to the slab, wind load goes sideways into the same frame, and the glass carries the pressure between the two. Frame depth and fixing centres come out of that calculation, which is why a slim face is an outcome of the engineering and not a starting point for it.

North American practice sizes glass with ASTM E1300, in its current 2024 edition. The practice covers vertical and sloped glazing under a uniform lateral load up to 15 kPa, uses a breakage probability of 0.008 for its non-factored load charts, and assumes the edge support is stiff enough to hold edge deflection to 1/175 of the span. It states plainly that it does not address the appearance of a deflected pane, and that it does not consider the effect of deflection on the seal of an insulating unit.

That last sentence saves arguments. A pane can pass the load calculation and still bow enough for a client to reject it, and an insulating unit can pass the same calculation and still lose its seal early. Both need a separate acceptance line on the drawing, written before the glass is ordered.

A frameless corner is not a mullion deleted. It is a structural sealant joint, and the guidance for it is ASTM C1401, which defines structural sealant glazing as the case where the sealant provides the primary attachment of the glass to the frame. The guide is written for walls within 15 degrees of vertical, and it describes a chain of adhesion instead of a single bond: finish to aluminium, sealant to finish, sealant to coating, coating to glass. Every link in that chain is a test.

The joint bite is sized for dead and wind load together, and the two panes at a corner carry each other, so the node has the same engineering behind it as a steel connection. Site conditions count as much as the drawing: substrate cleaning, primer, tooling, and a cure that the programme cannot hurry.

PF-35 builds that corner as a 45 degree joint with no visible post, on panes up to 4,000 by 3,000 mm. The joint is why a fixed glazing elevation reads as one sheet.

This is where large panes are lost. A standard dry container has a door aperture 2,340 mm wide and 2,280 mm high, with an internal height of 2,393 mm. A 40 foot high cube raises the door to 2,585 mm and the internal height to 2,698 mm. A 3,000 mm high pane fits through neither door and stands inside neither box.

Three routes remain. A flat rack carries the crate without a roof and loads from the side. An open top loads from above. Break-bulk on an A-frame trailer is the third, and for a single elevation it can be the cheapest. Glass travels on edge, in a crate built for the route, with edge protection doing the work that the crate corner only appears to do.

Weight arrives before volume. A 40 foot box takes roughly 28,700 kg of payload, and one pane at 4,000 by 3,000 mm in 12 mm weighs 360 kg, so eighty panes reach the limit before the crates are counted. The crate plan and the container count are one calculation, not two.

• Fix the opening and the sightline on the elevation. Nothing downstream corrects a sightline.
• Take the frame depth from the wind load and the fixing geometry, not from a catalogue.
• Set the glass build: thickness from ASTM E1300, lamination and coating from the brief.
• Break the elevation into panes against the sheet and the furnace, then check the weight.
• Check every pane against the container door before the order is released.
• Set the crate, the route and the lifting plan, and put all three into the submittal.