A glazed roof has one job and a dozen ways of failing at it. The glass itself is almost never the problem. Water gets in at the joints, the junctions and the drainage, which is to say at the places where two different materials meet and someone had to make a decision about how.
Knowing the usual suspects in order of likelihood is most of what a building owner needs, because it tells you what a contractor should be looking at and whether the one you’ve got is looking at it.
Perimeter seals, first on the list
Where the glazed roof meets the building is the busiest interface on the structure. It has to accommodate thermal movement in two directions, keep water out, and stay flexible for decades while sitting in full sun.
Silicone and gaskets don’t last decades. They harden, they shrink, they lose adhesion at the corners where movement is greatest. The glass can be perfect and the perimeter can be letting water in along a metre of joint.
This is also the failure that shows up furthest from its source, because water that gets past the perimeter runs onto structural steel and travels. A stain two floors down and twenty metres along is a perfectly ordinary symptom of a perimeter seal, which is why chasing the stain is the wrong instinct.
Drainage, which fails more often than anything gets blamed for
Atrium roofs sit flat or at a shallow pitch, and that means they rely on gutters, channels and outlets rather than on water simply running off.
Leaf litter blocks them every autumn. So does moss, silt, bird nesting and, on sites near industry, whatever is in the air. A blocked outlet turns a gutter into a reservoir, and water sitting above a joint designed to shed it will eventually find a way through.
Undersized outlets are the version of this you can’t clean your way out of. Plenty of older atria were drained for the rainfall patterns of their day, and a short intense downpour overwhelms them. The result looks exactly like a seal failure and isn’t.
Failed sealed units
A double-glazed unit with a broken edge seal lets moisture into the cavity, which is misting rather than leaking. The two get confused constantly, because both present as water where it shouldn’t be.
Misting between the panes is a failed unit and needs replacing; nothing can be done to a sealed cavity from outside. Water on the room-side surface of the inner pane, forming on cold mornings and clearing by lunchtime, is condensation and a building issue rather than a roof defect.
The confusion costs money either way, which is why tracing a glazing leak properly before committing to a repair is the step worth paying for.
Fixings, gaskets and glazing bars
Pressure plates loosen. Gaskets shrink back out of the rebate. Capping comes away at the ends. Fixings corrode, and on coastal buildings they corrode fast.
Any of those can let a sound pane move slightly in its frame, and a joint designed to be static doesn’t stay watertight once something is working in it.
Cracked glass and impact
Less common than the rest, and more obvious. Thermal stress cracks start at an edge and run; impact damage from falling debris or hail usually shows a point of origin. Either way a cracked pane is a replacement rather than a repair, and overhead it’s a safety question before it’s a water one.
How it actually gets fixed
The honest answer is that the diagnosis is the job and the repair is the easy part.
A competent leak investigation means close access to the glass, not binoculars from a lobby. It usually means a water test, run methodically from the lowest suspected point upwards, one area at a time, with someone inside watching where it appears. Starting at the top and hosing the whole roof tells you nothing except that it leaks somewhere.
From there the repair depends on what was found. Seal replacement is cutting out the old material properly and remaking the joint, not running fresh silicone over hardened silicone, which is the commonest bad repair on an atrium and buys a season at most. Gasket replacement needs the right profile for the system. Unit replacement is a glazing job with an access problem attached. Drainage is often the cheapest fix of the lot and the one that gets missed because it isn’t glamorous.
Standards for how a glazed envelope should perform and be tested come from the Centre for Window and Cladding Technology, and the access side carries its own duties under the Work at Height Regulations.
The thing to get right in the brief
Ask for a diagnosis before you ask for a price on a repair. A contractor who quotes a sealant job off a photograph of a stain hasn’t found the fault; they’ve found the symptom.
Specialists who handle both the access and the glazing, https://skyaccessglazing.co.uk/commercial-glazing-services/glazing-leak-repair among them, can do the investigation and the remedial work under one scope, which matters because a water test and a repair otherwise mean paying for access twice. Buildings with other glazed elements get looked at on the same visit, and the range of architectural glazing products on a modern facade is wider than most asset registers record.
A repair that doesn’t start with a water test is a guess with an invoice attached.
