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 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 a long, narrow strip is chosen, the same number of sprinklers spreads along a single line; the friction losses fall unrealistically and the pump pressure comes out smaller than it should. 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 roof slope exceeds 6 degrees, or where beams are deeper than 1.0 m, the factor rises from 1.2 to 2.0.
NFPA 13 uses the same rule, with the same 1.2 factor. 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 sprinkler remainders that do not form a full range on the previous upstream range, as close to a rectangle as possible.
- On a gridded system, shift the area by one sprinkler step and verify the highest pressure requirement.
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; a gridded system always requires full hydraulic calculation.
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, 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 is increased by at least 25 percent. 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 strip is chosen, friction losses fall unrealistically and the pump pressure comes out inadequate.
What does the 1.2 root A rule say?
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. Where the roof slope exceeds 6 degrees, or beams are 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 StoreBS EN 12845:2015+A1:2019 · NFPA 13 (2025) · NFPA 20 (2025) · EN 12259-1. Definitions are for information; the full text of the standard governs in design.