A sealed unit has a finite life and everyone in the trade knows it. The edge seal that keeps moisture out of the cavity degrades, the desiccant in the spacer eventually saturates, and the unit mists. In a window that’s a mild irritation. Overhead, in a roof twenty metres up over a public floor, it’s a project.

How you know a unit has gone, and why counting matters

The clear sign is condensation inside the cavity. It often appears in cold weather and clears as the day warms, which is why it gets explained away as weather for a season or two before anyone takes it seriously.

Permanent haze or a tide mark between the panes is the later stage. By then the cavity has been wet for a while, and the gas fill, if there was one, has gone.

What’s worth doing early is counting. Every unit on a given roof was almost certainly made at the same time by the same manufacturer with the same sealant, and they age together. Two failures on a roof with two hundred units is a repair. Twenty is the leading edge of a replacement programme, and pricing it one pane at a time over the next six years will cost considerably more than doing it in planned phases.

The system decides what the job is

This is the question that sets the cost, and it gets answered by the framing rather than the glass.

Capped systems, where a pressure plate and a snap-on cap hold each unit against gaskets, are the kind ones. Remove the cap, remove the plate, lift out the unit, reverse the process with the new one. Contained work, one bay at a time, with the rest of the roof untouched.

Structurally bonded and silicone-jointed roofs are harder. The glass is held by the adhesive joint rather than a mechanical clamp, so removal means cutting the unit out, cleaning the frame back to sound substrate, and remaking the bond. Silicone cures within a temperature and humidity window, which puts the weather in charge of the programme.

Then there are older bespoke assemblies with no drawings, no system name and no spares. On those, one unit can only come out if a section of roof comes apart around it. A survey tells you which of the three you’ve got, and on an unfamiliar roof that survey is the first thing to buy.

Specification, and the trap of like-for-like

The replacement is not simply a copy of the original.

Overhead glazing above occupied space wants laminated glass in the lower leaf, so a broken pane holds together in its frame rather than emptying onto the floor. Plenty of older atria don’t have that, and putting the original specification back in is the wrong answer even though it fits.

Coatings are the other trap. Solar control and low-emissivity coatings have changed over the decades, and a unit matched on performance may read differently from the floor: a slightly different tint, a different reflection. In a roof where every pane is in view of every pane, that shows. Sometimes the right answer is replacing a whole visible area rather than the three that failed.

Weight and thickness follow from the specification, and the framework has to take them. A heavier build-up in the same opening needs checking against what the structure and the glazing bars were designed for, which is a calculation rather than an assumption.

Bespoke shapes stretch the programme. Triangular, curved and tapered units on a feature roof are made to order, and the measuring has to be exact because there’s no trimming a sealed unit on site.

Getting there, and keeping the building open

Access is most of the cost. A roof with tested anchor points or a maintenance unit is straightforward. One with nothing is priced as rope access, a machine on the floor if the slab can carry it, or a scaffold built inside the space.

Floor loading is the thing to settle first. Lobby slabs over car parks and plant rooms frequently can’t take a lift at full extension, and the answer comes from a structural engineer rather than from the lift hire company.

Underneath, the work has to be sequenced around whatever the building does. Exclusion zones, protection decks and out-of-hours shifts are all cost, and all of it scales with how little disruption the occupier will accept. A roof opened up has to be weathertight again before anyone goes home, so the day’s work gets planned backwards from that.

The Health and Safety Executive’s work at height guidance sets the hierarchy the access plan should follow, and the Centre for Window and Cladding Technology publishes the performance guidance a replacement specification ought to be written against.

The decisions here rest on the same logic as choosing between full-height glass and solid construction anywhere else in a building, which this comparison of glass partitions and walls sets out for commercial interiors.

Whatever happens, the condition record is what makes the next decision cheap. A unit-by-unit misting count, written down and dated, means the count is already there when the programme question arrives rather than starting from zero.

Contractors specialising in atrium glazing systems will usually want to open a unit before pricing a batch, because the condition of the gasket and the frame behind the glass decides whether the rest of the roof is a swap or a rebuild. Ask for that before you ask for a number.