CMSA with in-rack sprinklers is the classic warehouse approach. Where ESFR is ruled out by roof slope or skylights, or where budget is constrained, CMSA plus in-rack is chosen instead. It is, however, harder to maintain and more exposed to flue space obstruction. This article covers the DS 8-9 in-rack design detail.
CMSA Ceiling Sprinklers
- K115: standard activation at moderate density
- K160: intermediate density
- K200: higher density
- K240: the largest CMSA ceiling option
- Design operating count of 15 to 30 sprinklers, against 12 for ESFR
In-Rack Sprinkler Arrangement
- K11.2: the standard in-rack head, at low operating pressure
- K14: for higher hazard in-rack applications
- Horizontal spacing: typically 1.5 to 3 m depending on commodity and rack
- Vertical spacing: every level, or at defined height intervals
- Plastics storage: in-rack sprinklers at every level are mandatory
- The purpose is to put water into the array so the fire is controlled inside the rack
Flue Space Management
- Transverse flue: at least 150 mm, running across the rack
- Longitudinal flue: at least 75 mm, between back-to-back rows
- Obstruction checked by annual photographic survey
- Product overhang at the rack edge kept to a minimum
- Operator training and painted rack limit markings
CMSA plus In-Rack vs ESFR
- CMSA tolerates a steeper roof slope and accepts skylights, where ESFR does not
- On existing buildings where the roof cannot be changed, CMSA is often the only route
- Capital cost is higher than ESFR once in-rack pipework and labour are included
- Maintenance is harder: in-rack access is difficult and annual inspection is more complex
- Over a twenty year life ESFR is generally the lower burden
Quick Checklist
- Ceiling CMSA K-factor selected for the storage height
- In-rack K11.2 as standard, K14 for higher hazard
- Flue spaces maintained at 150 mm transverse and 75 mm longitudinal
- Annual photographic flue inspection scheduled
- In-rack sprinklers at every level for plastics storage
Frequently Asked Questions
When is CMSA with in-rack sprinklers preferred over ESFR?
Where ESFR is ruled out by the building rather than by the commodity: a roof slope beyond the ESFR limit, skylights, or an existing structure whose roof cannot be altered. CMSA tolerates those conditions, which is why it dominates retrofit work on older warehouses even though ESFR is simpler on a new build.
Why do in-rack sprinklers matter more than ceiling sprinklers in tall storage?
Above a certain height the ceiling sprinklers cannot get water down into the array, because the plume deflects the spray and the upper loads shield the lower ones. In-rack sprinklers put water inside the rack at the seat of the fire, which is why they become mandatory at every level for plastics.
What are the flue space requirements and why do they fail in practice?
Transverse flues must be at least 150 mm and longitudinal flues at least 75 mm. They fail because cartons overhang the rack edge during normal operation, closing the flue. A fully compliant sprinkler design can then be ineffective, which is why annual photographic surveys and rack limit markings are part of the protection.
Is CMSA cheaper than ESFR?
Not usually, once the whole system is counted. The ceiling heads may be cheaper, but the in-rack pipework, hangers and labour add substantially, and the maintenance burden continues for the life of the building. CMSA is chosen because the building demands it, not because it is the economical option.
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Download MEP Calc on the App StoreFM Global Property Loss Prevention Data Sheet DS 8-9 (Storage of Class 1, 2, 3, 4 and Plastic Commodities); NFPA 13 in-rack sprinkler provisions; EN 12845-2 CMSA provisions.