The Quoting Trap in Specialty Windows

A rectangular window is quoted per square metre because its cost genuinely scales with area: the same profile, the same glass line, the same hardware, repeated. An aluminum glass specialty window does not behave that way. An arched window of the same area as a rectangular one can cost two to four times more, and a small circular window can cost more in total than a much larger rectangle.

The reason is that the dominant cost is not material. It is the setup: the bending jig or the mitre programme, the glass template, the seal run cut to a non-standard length, and the inspection that has to be done by eye because no gauge fits the shape. Those costs are incurred once per shape, not once per square metre.

This is why comparing two suppliers on $/m² for specialty windows produces a meaningless answer. One may be quoting a segmental frame built from straight mitred pieces while the other is quoting a true roll-bent curve. The two are different products with different visual results, different lead times and different failure modes, and their per-square-metre numbers are not comparable.

The rest of this article separates the fabrication routes, states the geometry limits of each, and ranks what actually moves the price.

Shape Classes and Their Fabrication Routes

The row that surprises most buyers is the triangle. It has no curve and needs no bending, so it is usually the cheapest specialty shape — but its price still sits well above a rectangle of equal area, because the glass is waterjet cut to a template, the mitres are non-standard, and the unit cannot go through the normal assembly line.

Three Ways to Make a Curve, and What Each Costs You

1. Roll bending (true curve)

The profile is passed through a three-roll bending machine and formed to a radius. This gives a genuinely smooth arc with no visible facets, and it is what an arched or circular window should look like from close range. The constraints are the minimum radius, which depends on profile depth and wall thickness, and the fact that a thermally broken profile cannot always be bent without damaging the polyamide bar. On broken profiles the bend is often done before the bar is inserted, or the curved section is left unbroken — a thermal consequence that must be accepted consciously, not discovered later.

2. Segmental construction (faceted curve)

The arc is approximated by a series of short straight pieces mitred at small angles. It has no minimum radius, works with any profile including thermally broken sections, and needs no bending equipment. The cost is visual: with few facets the shape reads as polygonal, and the number of facets needed to look smooth grows quickly with radius. It also multiplies the joint count, and every joint is a potential leak path and a place where the finish shows a seam.

3. Straight rectangular unit with an arched infill above

The practical compromise used on many projects: a normal rectangular openable window below, and a fixed arched or triangular panel above it as a separate unit. The openable part keeps standard hardware, standard seals and standard test evidence; only the fixed head is custom. This route is usually the cheapest way to get an arched appearance with an operable window, and it is the one worth asking for explicitly, because it is not always offered.

Curved Glass: The Hidden Cost Centre

The frame is usually not the expensive part of a curved window. The glass is.

• Flat glass in a curved frame is not possible. A pane cannot be bent cold into an arc without distortion and without putting permanent stress in the edges. Where the frame is curved, the glass must be cut to the curve, or formed to it.
• Cut-to-shape flat glass is the normal answer for arched heads: a flat pane waterjet cut to the arch profile. Cost rises with the complexity of the cut and with the number of panes, and the offcut waste is charged to the project because it cannot be reused.
• Heat-formed (bent) glass is required where the glazing itself must follow a curve in three dimensions, such as a true barrel vault. It carries a substantial premium, has longer lead times, and is subject to visual distortion that is normal for the process and should be agreed in writing before ordering rather than disputed on delivery.
• Curved insulated glass units are a different order of difficulty. Both panes must be bent or cut consistently, the spacer must follow the curve, and the sealing line is non-standard. Where a curved IGU is quoted, ask explicitly whether the unit is curved in both panes or has one curved outer pane with a flat inner pane, since the two differ markedly in price and in thermal behaviour.
• Safety glazing still applies. Shaped glass in a critical location must be laminated or tempered as it would be in a rectangle. Tempering a shaped pane is more expensive than tempering a rectangle because it cannot be run in a standard batch, and laminated shaped units add a further cutting step after lamination.

What Actually Moves the Price: Cost Driver Ranking

Notice what is not on the list: area. Area matters, but it ranks below all eight of these. This is the single most useful fact for a buyer comparing specialty window quotations.

MOQ, Lead Time and Why One Unit Is Disproportionately Expensive

Specialty windows rarely carry a minimum order quantity in units, because a single arched window is a normal order. What they carry is a setup cost that does not scale down. A bending jig has to be set, a glass template drawn and verified, a seal run cut to a one-off length, and the unit inspected individually.

In practice, for a 1.2 × 1.8 m arched top window in a thermally broken 6063-T5/T6 frame with cut-to-shape Low-E double glazing:

• One unit: roughly $650-1,100 FOB, with 35-50 days lead time.
• Five identical units: roughly $430-720 per unit, with 40-55 days lead time (longer overall, cheaper each).
• Twenty identical units: roughly $340-560 per unit, with 45-60 days lead time.

The lead time increases with quantity while the unit price falls. Buyers who optimise only for price per unit sometimes miss that the delivery date moves the other way, which matters more on a construction programme than the saving does.

Tolerances, Templates and Who Carries the Risk

A shaped window does not forgive an inaccurate opening. A rectangle can be shimmed square; an arch that is 8 mm too wide will not fit the reveal and cannot be adjusted on site without remaking the glass.

1. Measure after the opening is formed, not from drawings. Architectural drawings are intent, not dimension. Masonry and concrete openings vary, and the variance is usually found at the curved head where it does the most damage.
2. Take three measurements in each direction — top, middle, bottom for width; left, centre, right for height — plus both diagonals, and record the smallest and largest rather than an average.
3. For true curves, a physical template is worth its cost. A template cut on site from board or acrylic, marked with the orientation and the finished floor level, removes the interpretation step that CAD-only dimensioning leaves open.
4. Agree the tolerance band and the risk allocation in the contract. State who pays if the opening is out of tolerance against the template, and state the manufacturing tolerance of the unit itself. An unallocated tolerance risk on a shaped unit is the most common cause of a dispute and of a wasted glass panel.
5. Confirm the setting-out reference. Shaped windows are usually set out from a centre line or a springing point, not from a corner. If the reference is ambiguous, the unit will be positioned correctly in size and wrongly in place.

Structural and Performance Realities

Wind load on non-rectangular panes

Glass thickness selection for a shaped pane is not the same calculation as for a rectangle. The support condition varies around the perimeter, an arch head has a different edge restraint from a straight head, and a circular pane has no long edge at all. The governing dimension is usually the shorter span or the radius, and the calculation should be shown rather than inferred from the area. Large circular windows in particular tend to need thicker glass than their area suggests, because the whole perimeter is curved and the edge support is continuous rather than straight.

Thermal break continuity

This is the honest limitation of curved aluminium windows. A PA66-GF25 thermal break bar is a straight extruded component; it does not roll-bend well. Manufacturers handle it in different ways — bending the aluminium before inserting the bar, using a segmented bar in a segmental frame, or leaving the curved head unbroken. Each has a thermal consequence. Ask which method is used on your shape and what the resulting Uw is for the assembly as built, not for the straight section of the same system.

Certification: what a specialty window can legitimately claim

AS2047 and EN 14351-1 test evidence is generated on rectangular test specimens of a defined size. A test report for a 1.2 × 1.5 m rectangular casement does not certify an arched window of a different size and shape. What a supplier can legitimately offer is: the system's test evidence as the basis of performance, a structural calculation for the specific shape and size, glass specification to the applicable glazing standard, and where the project requires it, a project-specific assessment or calculation signed by a qualified person. A claim that a one-off shaped window "is certified to AS2047" is almost always a claim about the system, not about that unit, and the distinction should be settled in writing before the order is placed.

Procurement Checklist: Ten Questions Before You Order

1. Which fabrication route are you quoting for this shape — roll bent, segmental, or a rectangular unit with a separate arched head? Ask for it in writing, because the three are different products at different prices.
2. If segmental, how many facets, and what will the chord deviation be? That number decides whether the curve reads as smooth or as polygonal.
3. What is the minimum bend radius for this profile, and does my radius sit inside it with margin?
4. Is the glass cut to shape from flat stock, or heat-formed? If heat-formed, what distortion is normal and accepted for the process?
5. Is the insulated unit curved in both panes or curved outer with flat inner, and what is the resulting Ug?
6. How is the thermal break handled in the curved section, and what Uw applies to the unit as built?
7. Does the shape need to open? If yes, which hardware serves it, and is it SIEGENIA, ROTO or HOPO standard gear adapted, or a bespoke mechanism?
8. What is the price per unit at 1, 5 and 20 of the same shape, and what is the lead time at each? Optimising one without the other is how programmes slip.
9. Who supplies and verifies the template, what tolerance band applies, and who carries the cost if the site opening is out of tolerance?
10. What documentation ships with the unit — structural calculation for this shape and size, glass specification, and the system test report clearly identified as a system report rather than a certification of this unit?

Where Specialty Shapes Appear in a Project, and What to Do in Each

2026 Indicative FOB Prices for Specialty Window Shapes

Prices are indicative 2026 factory FOB ranges for project volumes, excluding freight and installation. Minimum per-unit area applies because setup cost does not scale down with size. Final quotation depends on shape and radius, fabrication route, glass type including any heat forming, opening requirement, thermal break handling in the curved section, finish, quantity of identical units, and the documentation package required. Contact Rogenilan Windows Doors for a shape-specific quotation with the route stated.

Specialty Window Procurement FAQ

Why is a small circular window quoted per unit while a large rectangle is quoted per square metre?

Because the circular window's cost is dominated by one-off setup — bending jig, glass template, one-off seal run, individual inspection — and none of that reduces as the circle gets smaller. Below roughly 0.8 to 1.0 m² the material content stops being the meaningful part of the price, so quoting per unit is more honest than quoting a per-square-metre figure that hides the setup.

Can an arched window open?

The arched part normally cannot. Standard hinges, friction stays and multi-point locks are made for rectangular sashes, and a non-rectangular sash needs adapted hardware with reduced load capacity and limited availability. The practical answer used on most projects is a composite: a standard rectangular openable unit below with a fixed arched head above. Ask for that arrangement explicitly, because it is not always proposed first.

What is the smallest radius you can bend?

For most aluminium window profiles the practical floor is around 300 mm radius, and it depends on profile depth, wall thickness and temper. Tighter radii are possible on shallower or softer sections but carry a higher reject rate and visible distortion risk. Where a design needs a very tight curve, segmental construction or a smaller radius achieved by faceting is often the more reliable route.

Does a thermally broken frame work with a curved head?

It works, but not by simply bending a broken profile. PA66-GF25 bars are straight extrusions and do not roll-bend well. Factories either bend the aluminium before inserting the bar, use a segmented bar in a segmental frame, or leave the curved section unbroken. Ask which method applies and what the Uw is for the unit as built — the straight section's Uw does not describe the curved head.

Can a one-off shaped window be certified to AS2047 or EN 14351-1?

Not as a unit. Those standards generate test evidence on rectangular specimens of defined size, so what exists is the system's report plus a project-specific structural calculation and glass specification for your shape. A supplier who says a one-off arched window "is AS2047 certified" is describing the system. Get the distinction confirmed in writing, along with the name and qualification of whoever signs the calculation.