Who this guide is for
- Clients commissioning a facade of large precast or unitised elements
- Owners planning maintenance on an existing panel or curtain wall facade
- Anyone investigating water appearing inside a unitised facade
- Readers comparing facade proposals that describe their joints differently
- Anyone responsible for a building whose facade joints are reaching an age
A single sealed line, and what it depends on
A sealed joint puts all of the duty on one material at one plane. While the seal is sound the arrangement works, and it is simpler to form, inspect and describe than the alternative. The record's warning is about what happens afterwards: a joint seal is not a permanent condition, and a facade relying on a single line has no second defence once that line ages.
That turns joint renewal into a scheduled operation rather than a repair. It also makes access a design decision, because joints are the maintained element in this kind of facade and they are reached from outside at height. Whether an access arrangement exists determines whether renewal is planned or waits until water appears indoors.
A baffle with a chamber behind it
A two-stage joint accepts that the outer line will one day admit water and gives that water a route back out. Behind the baffle is a chamber where water is collected and returned outside at intervals, and the horizontal and vertical joints have to work together for that to happen, because water in a vertical joint has to be intercepted at a crossing.
It is the same logic as a drained facade, moved into the joint. The outer line is allowed to leak, which is what makes it tolerable that it ages, and the chamber behind is what makes that tolerance real rather than theoretical.
The curtain wall version of the same choice
The least intuitive part of curtain walling, the record says, is that the glazing rebate is not sealed. Glass sits on setting blocks in a rebate that is vented to the outside and drained, so that water and vapour reaching it are returned rather than trapped. The assembly is designed to be wet in defined places.
That has a practical consequence for anyone maintaining such a facade: sealing a drainage slot with the intention of improving the facade is one of the most reliable ways to damage it. Filling a rebate with sealant, or breaking the downward path at a transom, converts a zone designed to be wet into a closed one that holds water against a glazing unit's edge seal, which is where it does the most harm.
Where the water appears is not where it entered
The mullion-to-transom junction record explains why a drained joint reports faults in the wrong place. Water passing the outer gaskets runs down the glass into the horizontal channel of the transom and has to be handed sideways into the vertical channel of the mullion to get back out, so the intersection is a corner in a drainage route rather than merely a joint between two sections.
Because the route is inside the sections, water appears internally at the bottom of a mullion run, several panels and often a storey away from where it entered. Chasing a curtain wall leak at the point it shows is usually chasing the wrong junction. A break in the inner air seal line reads differently again: draughts and whistling concentrated at the grid intersections, and condensation forming on the inside faces of the framing rather than on the glass.
Joints do more than weather
On both facades the joint is also where movement is taken up. A precast panel is rigid and the structure behind it shortens and deflects, so if adjustment was not designed into the fixings the deviation moves into the joint, where it becomes visible from the street and the weathering detail starts to vary panel by panel.
At the slab edge of a glazed facade, the slot between the floor and the back of the spandrel carries several planes that are not the same plane: the separation between storeys, the insulation line, the air barrier and the acoustic separation. The junction record notes that closing one of them does nothing for the other three, and that a stop placed for fire is not an air seal.
- The joint weathers the building
- The joint absorbs tolerance the fixings did not
- The joint takes up structural movement the panel cannot follow
- At floor level, several separate planes cross the same line
Choosing with the second half of the building's life in view
The records frame the decision as a question about ageing rather than about installation. Which joint philosophy is present determines what an aged outer seal actually means, and whether the facade has a second line behind the first is the whole of the moisture conversation on a panel facade.
Two further questions follow from it. How the joints will be inspected and renewed later, and from what access equipment. And what record will exist of panel positions, joint details and drainage arrangements, because a facade of near-identical bays is difficult to describe after the fact and a later repair otherwise begins by identifying what is already there.
Questions about a unitised facade joint
- 1Ask whether the joints are a single sealed line or a baffle with a drained chamber
- 2Ask what happens at each joint when the outer line ages
- 3Ask how water in a vertical joint is intercepted where joints cross
- 4On a glazed facade, trace where the drained rebate discharges at each level
- 5Ask which openings in the frame are drainage and must never be sealed
- 6Ask how much adjustment is built into the fixings and what happens when it runs out
- 7Ask which fixings support and which restrain, and whether the restraints can slide
- 8Ask which planes are carried across the slot at each floor edge, and by what
- 9Ask what access is assumed for joint inspection and renewal on each elevation
- 10Ask how a leak would be traced when it appears away from its point of entry
- 11Ask what record of unit positions, joint details and drainage will exist at handover
- 12Where sealant work is planned, establish first what each opening is for
Common mistakes to avoid
- Assuming a glazing rebate is sealed rather than deliberately vented and drained
- Applying sealant over a drainage slot in the belief that it improves the facade
- Tracing a leak at the point where water shows rather than at the point of entry
- Treating a single sealed line as a permanent condition
- Reading uneven joints as workmanship rather than as tolerance arriving where adjustment ran out
- Closing one plane across a floor-edge slot and assuming the others are closed with it
- Leaving joint access and renewal to be worked out after handover
When to involve a professional
- Ask whether the joints are designed as one sealed line or as a baffled joint with a drained chamber
- Ask how the drained route is continuous through each joint intersection
- Ask where the air seal line runs and whether it is a separate plane from the drainage
- Ask what fire and acoustic design has determined for the zone at each floor edge
- Ask how the joints will be inspected and renewed, and what access that assumes
- Ask what will be recorded at handover about unit positions, gaskets and drainage details
Frequently asked questions
Questions readers ask about this topic
What is a two-stage joint?
It is a joint with an outer baffle and a drained chamber behind it, so that water passing the outer line is collected and returned outside rather than continuing inward. The horizontal and vertical joints have to work together, because water in a vertical joint has to be intercepted where the joints cross.
Why should I not seal a gap in a curtain wall frame?
Because some of those openings are the drainage route. Water passing the outer gaskets is meant to reach a channel, run down, cross to the outside and leave, and sealing over a vent slot converts a zone designed to be wet into a closed one that holds water against the edge seal of a glazing unit.
Why does a facade leak appear a storey below the fault?
Because on a drained facade the route is inside the framing. Water collected at a transom is handed into the mullion channel and runs down it, so it can appear internally at the bottom of a mullion run several panels and often a storey away from where it actually entered the facade.
Is a sealed joint a poor choice?
The records do not say so. They describe a sealed line as simple and dependent on the seal remaining sound, and note that a facade relying on a single line has no second defence once that line ages. The consequence is about maintenance planning and access rather than about the joint being wrong.
What should be recorded about a facade's joints?
Unit positions, fixing types, gasket references and joint details. A facade of near-identical bays is difficult to describe after the fact, and without that record a later repair begins by identifying what is already there before any work can be specified.
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