Most people looking at a glass tower see the glass and the frame. Few think about the narrow gap behind it, running along the edge of every floor. That gap is easy to forget, hard to reach and covered over as soon as the finishes go in. Yet it is one of the details on which the safety of a building depends. This article explains what the gap is, why it must be closed, and how the trades involved can avoid leaving it open.
Where the gap comes from
A curtain wall is a lightweight skin hung on the outside of the building structure. It is attached to the edge of each floor slab through anchors, and it does not sit tight against the concrete. A space is left between the back of the facade and the slab edge, partly to absorb the tolerances of the concrete and partly so that both can move.
That space is small, but it runs along the full perimeter of every floor and joins the floors above and below into one continuous vertical path. Without a barrier, the facade would act as a chimney. The perimeter gap is therefore not a flaw. It is a normal and necessary feature that has to be handled on purpose.
Why it must be sealed
The floor slab is a barrier between storeys. If smoke or flame can pass around the edge of it, through the gap behind the facade, the barrier is bypassed. The purpose of closing the gap is to keep each floor separate for the time and in the manner that the project's fire strategy requires. This article describes the idea in general terms only. The performance level, the materials and the detail come from the fire engineer and the applicable code.
The gap also has to be closed in a way that allows for movement. Because the facade and the slab can move relative to one another, the seal cannot be rigid. A good detail stays in place while the movement joint opens and closes. A seal that cracks away from one side after the first season of movement has stopped doing its job.
Sealed and supported are two different things
A common mistake is to treat the job as only a matter of filling the gap. A fill that has nothing holding it up can sag, slump or be pushed out, and the gap opens again. The seal normally needs a support behind or beneath it, fixed to the structure or the facade, so that it stays in its position under its own weight, under air pressure and under movement.
This is why drawings should show both parts of the detail. The first is the sealing material that closes the gap. The second is the support that carries it. If a drawing only shows a hatched area labelled as the seal, ask who designs the support, what it is fixed to, and how it is held when the facade moves. A detail that cannot be explained is likely to be built differently by each crew.
Tolerance is part of the picture. The concrete edge is never perfectly straight, and its position can vary along a floor. The gap may therefore be wider in one bay and narrower in the next. A detail that only works when the gap is exactly the drawn width will fail in the real building. Ask how the design copes with a range of gap sizes, and whether the support can be adjusted on site without being cut or improvised.
Compatibility is the other half. The materials that touch one another at the edge, including the sealing material, the support, the sealants on the facade and the surface of the slab, must not harm each other. Ask the supplier to confirm that the products are intended to be used together, and that the surface will be clean and sound enough to take the seal.
A coordination problem more than a product problem
The difficulty with this detail is rarely the material. It is that several trades meet at the same place. The structure contractor forms the slab edge. The facade contractor installs the anchors and the frame. A specialist trade may install the seal. Interior finishers may arrive after and cover everything. Each trade can reasonably believe that another will deal with the gap.
The remedy is to write the responsibility down. The contract documents should say who supplies the material, who installs it, who inspects it and who records it. They should also say when in the programme the work happens. If the seal has to be fitted after the anchors but before the ceiling line is closed, the programme needs to leave that window open and protect it.
Sequence and access
The gap is easiest to reach before the interior finishes are fitted and from the floor itself, not from outside. If the facade is installed first and the seal comes later, there needs to be a safe way to get at the full length of the edge. Work done blindly from a distance is the work most likely to be incomplete.
- Agree the order of work between the facade, the structure and the sealing trade before installation starts.
- Keep the edge clear of debris, dust and loose material so the seal can be fixed to a clean surface.
- Leave a safe working space along the edge, and do not close the ceiling or the finishes over it until it is inspected.
- Protect the finished seal from later trades who might cut, drill or strip it for other work.
Penetrations and interruptions
The edge is not a clean straight line. It is interrupted by anchors, brackets, mullion ends and, at times, by services that pass through. Each one is a point where a seal can be cut around and left imperfect. The detail should show how the seal is closed around these items, and it should be checked at these points first.
Corners and changes in direction need the same attention. Where one length of seal ends and another begins, there should be an overlap or a joint that is part of the design, not a gap that arises by chance. Ask the facade supplier to show these junctions on the shop drawings rather than assume they will be worked out on site.
Inspection and photographs
Because the seal is hidden once the ceilings are in, inspection has to happen while it can still be seen. A sensible routine is to inspect each floor edge before it is closed, to record the result against the floor and the grid line, and to take dated photographs that show the full length and the support. Photographs taken up close, with a tape or marker for location, are more useful than general views.
The record should also say who inspected and who accepted the work. A pattern of problems on one floor or one elevation is a signal to look more widely, and the photographs allow it to be found before the opening is covered. This is also the kind of record that a building owner or an insurer may later ask to see.
Later work deserves a note of its own. Cables, pipes and ceiling fixings are often added after the facade is finished, and each can disturb a seal that was correctly installed. The handover documents should show where the seal is, so that later trades know it is there. If a seal is cut for any reason, it should be repaired by the same method and recorded again, not patched with whatever material is to hand.
Questions to ask before work starts
- Who designs, supplies, installs and signs off the seal at the slab edge?
- What supports the seal, and how is that support fixed?
- How does the detail allow for movement between the slab and the facade?
- How are the anchors, mullion ends and corners dealt with?
- When in the programme is the work done, and how is access kept open?
- What will be photographed and recorded, and who keeps the record?
These questions help a team find a gap in responsibility before it becomes a gap in the building. They also fit well with a mock-up review, where the edge detail can be built and checked once in full before it is repeated across every floor. For wider context on movement at this location, see movement joints in facades.
General guidance for orientation, not a specification. Applicable standards, values and details are confirmed per project with the facade consultant.
