EN 12845 Concealed Space Pipework and Sprinklers

Above suspended ceilings, beneath raised floors, in roof voids and intermediate landings — invisible spaces become the silent route for fire spread. Clause 7.7 keeps its exemption criteria tight, and a single millimetre outside the limit means sprinklers.

In one shopping centre fire, flame spread not through the retail units but through the 1.1 m ceiling void carrying services between the shops and the plant level above. The design had applied the "below 0.8 m, no sprinklers required" exemption; the dimension existed on paper, but on site the void reached 1.1 m. The shopfronts held, while the concealed space above turned into a flame machine. The Clause 7.7 exemption criteria cannot be accepted without being verified once more on site.

Clause 7.7 — concealed space exemption criteria

A concealed space may be left without sprinklers only if it satisfies all of the following:

• (a) Height: the void is less than 0.80 m high
• (b) Combustible content: no combustible material inside, or only limited amounts — combustible cable insulation counts
• (c) Construction: the enclosing construction is of a fire-resisting class (non-combustible or REI rated)

All three at once — no "approximately" version of any of them is acceptable. If the height exceeds 0.80 m at any point, that area needs sprinklers; "an average of 0.75 m" is not a valid defence.

Typical concealed space types

Void type	Typical height	Fire risk	Sprinklers required?
Suspended ceiling (office)	0.4–0.9 m	Low (PVC cable)	Yes if above 0.8 m
Suspended ceiling (retail/restaurant)	0.6–1.5 m	Moderate (ductwork, cable)	Usually yes
Raised floor (data centre)	0.3–0.9 m	High (cable, airflow)	Yes where combustible load exists
Roof void (timber)	0.5–3.0 m	High (timber)	Always
Roof void (concrete)	0.1–0.4 m	Low	Usually exempt
Intermediate landing	1.0–2.5 m	Variable	Yes, depending on use

Choosing the sprinkler type for a void

• Pendent or upright: dropped into the void from the main pipework. Standard response, K115.
• Fast response (RTI ≤ 50): in voids with high airflow, response time matters and quick-response heads are recommended.
• Temperature rating: 68 °C is typical. Where the void runs hot — a plant space, or a roof void in summer — use 79 °C or 93 °C.
• K-factor: generally K80 for LH, OH and HHP; K115 may be appropriate in voids above high-hazard storage.

Pipework design detail in voids

• Branches into the ceiling void are taken off the main by riser nipples.
• Use a dry system where freezing is possible; wet where the void is heated.
• Support spacing: no more than 2.4 m for 25 mm pipe and 4.0 m for 50 mm.
• Fall of about 0.4° on wet pipework to avoid air traps, with the drain point accessible below the ceiling.
• Access hatches of 600 × 600 mm every 9 m for inspection.

Field error — the cabling under a raised floor was overlooked

A bank data centre's first-phase design had a 0.55 m raised floor void, densely cabled. It was accepted as exempt because it was below 0.8 m. But the cable insulation was PVC rather than halogen-free, so the combustible load criterion was breached. Six months later an insurance audit recorded the deviation and the system was retrofitted — 240 sprinklers, a new alarm valve and connections, and two and a half months of additional project work. Skipping criterion (b) means criterion (a) alone is not enough.

Testing and drainage in voids

Void sprinklers are included in normal system testing through the zone test connection, but physical access is limited:

• Heads are reached through access hatches for the annual visual inspection.
• Pipe drainage is taken below the suspended ceiling, and the low-point drain must be genuinely accessible.
• Corrosion risk is high because of humidity and condensation — galvanised pipe or internal coating is recommended.
• Voids are sampled during the three-yearly internal inspection.

Ductwork and obstructions

A void is usually full of ductwork, cable ladders and mechanical services. The obstruction rules apply:

• Additional sprinklers beneath any horizontal obstruction wider than 0.8 m.
• Where the gap between duct and ceiling is under 0.3 m, place the sprinkler on the side of the duct.
• In densely cabled areas, sidewall sprinklers are often the better choice.

Comparison with NFPA 13

NFPA 13 sets its concealed space exemption on similar criteria: under 6 in (about 0.15 m) with non-combustible construction, or under 32 in (about 0.81 m) with no combustible load. The numbers are close. NFPA is stricter on combustible construction — a timber-joisted roof void almost always requires sprinklers. The structural fire resistance criterion in Clause 7.7(c) is interpreted more tightly on the NFPA side, and the European-American difference in interpretation should be stated in the design documents.

Link to Turkish regulation

BYKHY refers to EN 12845 and NFPA 13 and has no separate clause on concealed spaces. However, its compartmentation provisions require voids not to defeat fire spread barriers, so a concealed space left without sprinklers must maintain compartment continuity through REI 60/90 dampers and firestopping at service penetrations. Sprinkler exemption and firestopping are considered together; without one, the other is incomplete.

Quick check list

• Void height verified by real measurement, not from the drawing.
• Combustible content and cable insulation classified.
• Structural class (non-combustible or REI) documented.
• Sprinklers added wherever all three criteria are not met.
• Ductwork and cable tray obstructions marked on the sprinkler layout.
• Access hatches every 9 m and reachable.
• Low-point drain accessible, freezing risk assessed.
• Compartment continuity maintained by firestopping.

Frequently Asked Questions

When can a concealed space be left without sprinklers?

Only when all three criteria are met at once: the void is under 0.80 m high, contains no meaningful combustible material, and is enclosed in fire-resisting construction.

Does cable insulation count as combustible load?

Yes. PVC cable insulation is enough to breach the combustible content criterion, which is why densely cabled raised floors usually require sprinklers even below 0.8 m.

What sprinkler type suits a ceiling void?

Pendent or upright heads at K80 to K115, typically 68 °C, with fast-response bulbs where airflow is high and a higher temperature rating in hot voids.

How is a void system inspected?

Through access hatches every 9 m. Corrosion risk is high because of condensation, so galvanised or internally coated pipe is recommended and voids are sampled at the three-yearly internal inspection.

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