The area of operation is the largest area assumed to have sprinklers operating simultaneously in the hydraulic calculation. It represents the area a real fire would cover, and its placement cannot be left to chance.
Why rectangular?
EN 12845 clause 13.4.3.1 requires the area, at the hydraulically most unfavourable position, to be as rectangular as practicable and symmetrical with the sprinkler layout. The aim is to imitate the circular or square spread of a real fire.
If the area is chosen as a narrow strip at right angles to the range pipes (a few sprinklers on each of many ranges), the same number of sprinklers is shared across many lines; the friction losses in the ranges fall unrealistically and the pump pressure comes out smaller than it should. That is why the L ≥ 1.2 × √A rule below requires the area to be long enough along the ranges. That produces a system that is coherent on paper and inadequate on site.
The 1.2 root A rule
On a gridded layout, the side length L of the area parallel to the range pipes must not be less than 1.2 times the square root of the area of operation:
L ≥ 1.2 × √A
For example, A = 216 m² for OH-3, so L must be at least 1.2 × root 216, about 17.6 m. Where the ranges run parallel to the ridge of a roof sloping more than 6 degrees, or along bays formed by beams deeper than 1.0 m, the factor rises from 1.2 to 2.0 (clause 13.4.3.1 b and c). Terminal and looped layouts have no such formula; the far side of the area is defined by a range, or a pair of ranges in an end-centre layout (13.4.3.1 a).
NFPA 13 uses the same 1.2 factor but does not restrict it to gridded systems: under the density/area method the design area is a rectangle with a side parallel to the branch lines of at least 1.2 √A (NFPA 13-2025 28.2.4.2.1). The terminology differs: EN says "area of operation", NFPA says "design area".
How the hydraulically most remote position is found
- Identify the distribution pipe furthest from the riser.
- Target the range lines at the far end of that pipe.
- Place the remote side of the area on the last range or pair of ranges.
- Group the sprinklers that do not form a full range on the next upstream range row, as close as possible to the distribution pipe.
- On a gridded system, shift the area by one sprinkler pitch in each direction along the ranges and verify the highest pressure requirement (clause 13.4.2.1).
The last point matters: on a gridded network water can reach the same sprinkler from two directions, so the "most remote" point is not always the geometrically furthest one. A table cannot solve that; EN 12845 clause 13.1 requires full hydraulic calculation for gridded and looped layouts.
The corridor problem: sprinklers in a single row along a narrow corridor necessarily break the rectangle rule — the area is already a strip. In that case take all the sprinklers along the corridor and group the remainder on the next row up in the adjacent room, as close to a rectangle as possible. Making the corridor a hydraulic zone on its own does not reflect the pressure loss realistically.
The relationship between area and sprinkler count
The sprinkler count is found by dividing the area by the coverage per sprinkler. Because the maximum is 12 m² per sprinkler in the OH class (Table 19), the 216 m² of OH-3 gives about 18 sprinklers. But where additional sprinklers have been added because of obstructions at ceiling level (beams, ducts), that number rises and the pre-calculated tables lose their validity.
Area increase on a dry system
On dry and alternate installations the area of operation increases by 25 percent: EN 12845 Table 3 gives it in a separate column for OH1 to HHP3 (dry and alternate systems are not allowed for LH and OH4, which use OH1 or HHP1), and for HHS storage the notes to Tables 4 and 5 recommend a 25 percent increase. The 216 m² of OH-3 becomes 270 m²; at the same 12 m² per sprinkler, the count rises from 18 to 23 and the design flow rises accordingly.
Frequently Asked Questions
Why must the area of operation be rectangular?
To imitate the circular spread of a real fire. If a narrow strip at right angles to the ranges is chosen, friction losses fall unrealistically and the pump pressure comes out inadequate.
What does the 1.2 root A rule say?
On a gridded layout, the side of the area parallel to the range pipes must not be less than 1.2 times the square root of the area. For OH-3 at 216 m², L must be at least 17.6 m.
Does the factor change with a steep roof?
Yes. On a gridded layout where the ranges run parallel to the ridge of a roof sloping more than 6 degrees, or along bays formed by beams deeper than 1.0 m, the factor becomes 2.0 instead of 1.2.
How is the rectangle rule applied in a corridor?
Take all the sprinklers along the corridor, then group the remainder on the next row up in the adjacent room, as close to a rectangle as possible.
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Download MEP Calc on the App StoreEN 12845:2015+A2:2026 · NFPA 13 (2025) · NFPA 20 (2025) · EN 12259-1. Definitions are for information; the full text of the standard governs in design.