A shaft closure is an assembly, not a product. The framing, the board type and number of layers, the fixing centres, the joint treatment, the perimeter detail, the access panels and every service passing through all form one tested system. Change one element and the tested performance of the whole assembly no longer applies.
Why the shaft is the highest-risk route
Everything else in a building fails locally. A bad seal on a compartment wall lets fire into the next room. A bad shaft lets it into every floor above.
The physics is not complicated. Hot smoke is buoyant. Give it a vertical route with no resistance and it rises, fills the shaft, and pushes out at each floor through whatever opening it finds. People three or four floors above the fire are affected before anyone tells them there is one.
That is why we treat shaft work as the first thing to check in any building, and why the five-step process we follow is the same on every project regardless of how small the opening looks.
The parts of the system
- Framing. The structure the closure is built on. Section size and spacing are part of the test, not a site decision.
- Boards or mortar slab. Type, thickness, and number of layers. Two layers of the specified board is not the same as one thicker one.
- Fixing centres. How often it is screwed or tied. Wider spacing than tested changes how the assembly behaves as it heats.
- Joints. Staggered, taped or filled as the system requires. Joints are where untreated assemblies open first.
- Perimeter detail. Where the closure meets the structural slab or wall. A perfect panel with an untreated perimeter is an open gap.
- Access panels. Rated to match the assembly, not a plasterboard hatch cut in afterwards.
- Service penetrations. Every cable, pipe and tray through the closure needs its own tested seal, as covered in the penetrations guide.
Seven elements, and the tested rating belongs to all seven together. This is the point people miss when a shaft closure is quoted by the square metre.
Where a shaft closure quietly fails
| What changed on site | Why it seemed fine | What it actually did |
|---|---|---|
| A thinner board was substituted | Same manufacturer, same product family | Moved the assembly outside its test |
| An access hatch was added later | It is only 400mm square | Created an unrated opening in a rated assembly |
| Fixings spaced wider to save time | The panel felt solid | Changed how the assembly holds together under heat |
| The perimeter was left unfilled | It is a hairline gap | Smoke does not need more than a hairline |
| Cables pulled through after closing | Only three cables | An unrated patch in the middle of the closure |
| Mortar applied thinner than specified | It looked finished | Reduced the fire resistance period, invisibly |
Scroll sideways on a phone.
Every row of that table is something we have found during audits. None of them look wrong at handover. All of them are found only by opening the shaft or reading a record that was never made.
A shaft that is sealed on some floors and not others. The contractor sealed the levels with easy access and never came back to the two with a plant room in the way. From the ground floor riser door, the building looks done.
Before you close it: coordinate
The work has to happen in a window that is narrower than most programmes allow for: after the services in that riser are complete, and before the ceiling and finishes go in.
Three questions worth asking before the first seal goes in:
- Are all the services in this riser final, or is another pull expected?
- Does anything need routine access through this closure later?
- Who owns re-sealing if a service is added after we finish?
If the answer to the first is “probably another pull”, the right decision is a re-enterable system such as fire safety pillows rather than a solid mortar seal that somebody will break.
After you close it: record it
Once the riser door shuts, the shaft becomes invisible for years. What the next team needs is a label on the door or the closure itself, and a file entry: floor, system used, product, date, and photographs at each stage.
It sounds like paperwork. In practice it is the difference between a maintenance team who knows how to re-instate the closure after they open it, and one who patches it with whatever is in the van.
