Flue space is the vertical clearance left between pallets in a rack system. It does two jobs: letting hot gas rise in a fire, and opening a path for water from the ceiling sprinklers to reach down.
Two kinds of flue
- Longitudinal flue: the gap between two back-to-back rack rows, running along the long axis of the rack.
- Transverse flue: the gap between pallets sitting side by side in the same rack row, running through the depth of the rack.
EN 12845 minimum dimensions
| Flue type | Minimum clearance |
|---|---|
| Longitudinal (back to back) | 0.15 m |
| Transverse (side by side) | 0.10 m |
Those clearances are protected with pallet stops. A pallet stop is a mechanical stop that prevents a pallet being pushed too far back into the rack; without one, the forklift operator pushes the pallet all the way in and the flue closes.
What happens when the flue closes?
A failure chain begins in two directions:
- Upwards: hot gas cannot escape through the vertical channel and spreads horizontally. Ceiling sprinkler activation is delayed, or sprinklers beside the fire open instead of the one directly above it.
- Downwards: water from the ceiling cannot penetrate the stack, collects on the top surface and runs off sideways. The fire grows freely in the lower layers.
The result is a system that looks compliant on paper and is useless on site. That is why a closed flue is reported as a Major Recommendation in an FM Global survey.
An operational problem, not a design problem. Flue space is drawn correctly in the design and closes in operation: a pallet pushed too far, a carton wedged in the gap, spare material stored behind the rack. So warehouse staff must be told the flue rule, and a visual marker placed on the racking.
The relationship with in-rack sprinklers
In-rack sprinklers are placed in the flue space. If the flue is closed, the sprinkler water does not reach its target either; so a flue problem is not solved by adding in-rack sprinklers — the clearance must be opened first. EN 12845 limits the vertical spacing of in-rack sprinklers to a maximum of 3.5 m or two rack tiers, whichever is less.
Inspection checklist
- Is the longitudinal flue at least 0.15 m clear?
- Is the transverse flue at least 0.10 m clear?
- Are pallet stops fitted and sound?
- Is there any foreign material in the flue (cartons, stretch wrap, spare pallets)?
- Are the in-rack sprinklers unobstructed, with water shields fitted?
- Do the warehouse staff know the flue rule?
Frequently Asked Questions
What is the minimum flue space?
EN 12845 requires at least 0.15 m for the longitudinal (back to back) flue and 0.10 m for the transverse (side by side) flue, protected with pallet stops.
What happens when the flue closes?
Hot gas cannot rise and ceiling sprinkler activation is delayed; at the same time water from the ceiling cannot penetrate the stack. The fire grows freely in the lower layers.
Do in-rack sprinklers solve a flue problem?
No. If the flue is closed, the in-rack sprinkler water does not reach its target either. The clearance must be opened first.
Why are pallet stops mandatory?
A pallet pushed too far closes the flue. A pallet stop prevents that mechanically; without one, the flue inevitably closes in operation.
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 · EN 12259-1 · EN 12259-13 · NFPA 13 (2025) · FM Global DS 8-9. Definitions are for information only.