EN 12845 Maximum Sprinkler Coverage Area

21 m² for LH, 12 m² for OH, 9 m² for HHP and HHS — the ceiling and sidewall spacing tables, the effect of obstructions, and a worked example on a 12 × 18 m room.

On a logistics building project the architect sends over the ceiling plan: a storage room with a clear span of 12 by 18 metres. The contractor says "we'll cover this with six sprinklers, four corners plus a centre line". Measure it against the table and the answer is different. EN 12845 limits the area covered by each sprinkler; in the HHS class each head protects no more than 9.0 m², so a 216 m² room does not come in below 24 sprinklers. This article works through the spacing tables and the errors we most often catch.

What the standard limits

The maximum area protected by each sprinkler is set through two tables:

• The ceiling table — all sprinklers other than sidewall (upright, pendent, conventional). It gives two things: an area limit in m², and limits on S (spacing between sprinklers) and D (distance between branch lines).
• The sidewall table — wall-mounted sprinklers, with different rules depending on room width and spacing along the wall.

Both area and spacing must be satisfied simultaneously. Even if 21 m² would fit in LH, you cannot set S at 5.2 m; spacing sets the floor, area sets the ceiling.

Ceiling, upright and pendent spacing

Hazard class	Max. area per sprinkler	Standard S and D	Staggered (S)	Staggered (D)
LH	21.0 m²	4.6 m	4.6 m	4.6 m
OH (1–4)	12.0 m²	4.0 m	4.6 m	4.0 m
HHP and HHS	9.0 m²	3.7 m	3.7 m	3.7 m

Staggered layout offsets the sprinklers like a chessboard. In OH it allows 4.6 m in the S direction and 4.0 m in D, which is slightly more relaxed — but the 12 m² area limit still applies. On site, mark the area first and the spacing second; satisfy both and you are within the table.

Sidewall sprinklers

Sidewall heads are used in hall-type spaces and in hotel and office areas where pipework must be concealed. The logic is different:

Class	Max. area	Spacing (S)	End distance to wall	Room width (w)	Rows
LH	17.0 m²	4.6 m	2.3 m	w ≤ 3.7 m	1 row
LH	17.0 m²	4.6 m	2.3 m	3.7 < w ≤ 7.4 m	2 rows
LH	17.0 m²	4.6 m	2.3 m	w > 7.4 m	2 rows plus ceiling sprinklers
OH	9.0 m²	3.4 m	1.8 m	w ≤ 3.7 m	1 row
OH	9.0 m²	3.4 m	1.8 m	3.7 < w ≤ 7.4 m	2 rows
OH	9.0 m²	3.4 m	1.8 m	w > 7.4 m	2 rows plus ceiling sprinklers

Four notes accompany the table: rooms wider than 7.4 m need ceiling sprinklers in addition to two sidewall rows; in OH the side-to-side spacing may go to 3.7 m under a ceiling with 120 min fire resistance; the deflector sits 100–150 mm below the ceiling and 50–150 mm clear of the wall; and there must be no ceiling obstruction within 1.0 m either side of the sprinkler axis and 1.8 m perpendicular to it. These are routinely forgotten in design — on site a T-bar hanger or an HVAC duct puts the sprinkler out of action.

Obstructions — when the area comes down

The table sets the limit, but the structural-element and obstruction rules usually pull it down in practice:

• Additional sprinklers under wide ducts and beams — anything wider than 0.8 m needs a head beneath it, and that head is still bound by the area limit.
• Partial-height partitions — where a partition drops more than 0.5 m below the ceiling, a separate layout is drawn beyond it.
• Voids above suspended ceilings — anything deeper than 0.8 m is calculated independently.
• Small compartments — areas under 4 m² do not require an extra sprinkler, but do not push that exception outside LH.

The practical rule to avoid rework: keep both the area and the spacing on the conservative side. A third-party auditor measures D first, then calculates the area; fail either check and the layout changes.

Worked example — a 12 × 18 m room

Take an area of 12 m by 18 m, i.e. 216 m². The sprinkler count for each hazard class:

Class	Max. area	Min. sprinklers by area	Spacing check	Recommended layout
LH	21.0 m²	216 / 21 = 10.3 → 11	4.6 × 4.6 m → S = 4.0 m, D = 4.5 m	3 rows × 4 heads = 12, i.e. 18 m² each
OH	12.0 m²	216 / 12 = 18	4.0 × 4.0 m → S = 3.6 m, D = 3.75 m	4 rows × 5 heads = 20, i.e. 10.8 m² each
HHP/HHS	9.0 m²	216 / 9 = 24	3.7 × 3.7 m → S = 3.6 m, D = 3.6 m	5 rows × 5 heads = 25, i.e. 8.64 m² each

Note that the contractor's "six sprinklers will do" needs 12 heads even in LH. In an OH3 warehouse you either fit 20 heads or add in-rack protection. HHS spacing is tighter because the fire load and flame spread rate are higher; beyond 9.0 m² per head, K-factor and pressure cannot keep up.

Our quick site check: derive the sprinkler matrix from (width / S) × (length / D), then confirm each cell area is below the table limit. Corner sprinklers must never be more than S/2 from the wall.

Common errors

• "I always use 4.6 m spacing for OH" — that applies only in the S direction of a staggered layout; a standard square layout is 4.0 m.
• Ignoring rack height in HHS — even if the ceiling sprinklers meet 9.0 m², in-rack heads are still required once storage height exceeds 3.5 m.
• Using sidewall heads in an atrium — prohibited; sidewall does not work where the ceiling is above 5 m.
• No sprinkler beneath a wide HVAC duct — a 0.8 m duct casts a shadow, and flame passes through the gap in the water curtain.
• Placing corner sprinklers hard against the wall — the sprinkler-to-wall distance is a minimum of 0.1 m and a maximum of S/2, otherwise the spray distribution is distorted.

Comparison with NFPA 13

NFPA 13 gives area limits in ft²; converting to SI:

• NFPA light hazard: 200 ft² ≈ 18.6 m²  |  EN 12845 LH: 21.0 m² (EN is more generous)
• NFPA ordinary hazard 1/2: 130 ft² ≈ 12.1 m²  |  EN 12845 OH: 12.0 m² (essentially identical)
• NFPA extra hazard 1/2: 100 ft² ≈ 9.3 m²  |  EN 12845 HHP/HHS: 9.0 m² (essentially identical)

The difference at light hazard lets European designs use slightly fewer sprinklers in office and residential spaces. At ordinary hazard and above, the outcome is the same.

Link to Turkish regulation

BYKHY bases sprinkler design on TS EN 12845, and a spacing table breach is one of the most frequent findings in third-party audits — particularly because drawings are often set out on a 4 m square grid in CAD, producing 16 m² cells in OH2 and OH3 areas. Area is always checked at approval, and showing the assigned area for every sprinkler in a separate table speeds the process up.

Frequently asked questions

How many square metres can one sprinkler protect?

For ceiling, upright and pendent heads: a maximum of 21.0 m² for LH, 12.0 m² for OH, and 9.0 m² for HHP and HHS.

Does the same table apply to sidewall sprinklers?

No. Sidewall heads have their own limits — 17.0 m² for LH and 9.0 m² for OH — plus rules on single or double rows depending on room width.

What is the maximum spacing between sprinklers?

In a standard layout, S and D must not exceed 4.6 m for LH, 4.0 m for OH and 3.7 m for HHP and HHS. Staggered layouts limit the D direction differently.

How are sprinklers under obstructions counted?

Ducts and beams wider than 0.8 m require an additional sprinkler beneath, and those heads are still bound by the same area limits.

Are the NFPA 13 and EN 12845 coverage areas the same?

Close but not identical. NFPA 13 gives 18.6 m² for light hazard, 12.1 m² for ordinary and 9.3 m² for extra hazard. EN 12845 is more generous at LH (21 m²) and effectively the same at OH and above.

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