How far apart do fire sprinklers need to be?

Fire sprinklers in the UK can be up to 4.6 m apart, but how far depends on what the building is used for. BS EN 12845, the standard commercial systems are designed to, sorts every space into a hazard class, and each class has its own maximum spacing and maximum area per head. The rules also set a minimum distance between heads and limits from walls and ceilings. Sentry Sprinkler Protection checks this layout on every survey, because a change of use inside a building can leave the heads too far apart for what is now stored beneath them.

The spacing table

For ordinary ceiling sprinklers (everything except sidewall heads), BS EN 12845 Table 19 sets:

Hazard class	Typical buildings	Max area per head	Max distance apart
Light hazard (LH)	Low fire load areas split into rooms of no more than 126 m², such as some offices and schools	21 m² (about 226 sq ft)	4.6 m
Ordinary hazard (OH)	Most commercial buildings: offices, hotels, hospitals, schools, car parks, shops and shopping centres, most factories	12 m² (about 129 sq ft)	4.0 m, or 4.6 m along a staggered line
High hazard (HHP and HHS)	High hazard processes and storage	9 m² (about 97 sq ft)	3.7 m

The distances are measured between heads along a range pipe and between the range pipes themselves, and both limits apply at once. Heads 4.0 m apart in both directions cover 16 m², which is inside the light hazard limit but outside the ordinary hazard one, so an ordinary hazard layout has to bring the lines closer together.

“Per square foot” is how the question is usually typed, and the answer is the other way round: it is not so many heads per square foot, it is one head per so many square feet, between about 97 and 226 depending on the hazard class.

The minimum: 2 metres

Heads must not be closer than 2 m (clause 12.3). A head that is too close to its neighbour gets wet when the neighbour opens, the spray cools its bulb, and it may then fail to open when the fire reaches it. The standard allows closer spacing only where baffles or the building’s structure stop one head wetting the next, for intermediate heads inside storage racks, and around escalators and stairwells.

Distance from walls

The furthest a head can be from a wall or partition is the smallest of these (clause 12.4.1):

• 2.0 m for a standard layout, or 2.3 m for a staggered one;
• 1.5 m where the roof has open joists or exposed rafters;
• 1.5 m from the open face of an open-sided building;
• 1.5 m beside external walls that are combustible, or made of metal;
• half the maximum spacing in the table for that hazard class.

Walls matter because a fire against a wall is a fire at the edge of the pattern, where fewer heads can reach it.

Distance below the ceiling

Heat rises and spreads under the ceiling first, so heads have to be close to it. Where possible the deflector sits 75 mm to 150 mm below the ceiling or roof, and never more than 0.3 m below a combustible ceiling or 0.45 m below a non-combustible one (clause 12.4.2). Beams, ducts and lights that interrupt the spray have their own rules on top of these.

Sidewall heads

Sidewall sprinklers, which throw water out from the wall rather than down from the ceiling, are spaced by Table 20: up to 4.6 m apart and 17 m² each in light hazard, and 3.4 m apart and 9 m² each in ordinary hazard (3.7 m where the ceiling has at least 120 minutes’ fire resistance). They are mostly used in corridors and small rooms.

Warehouses, racks and special heads

Storage changes everything. High-piled goods, in-rack sprinklers and ESFR heads have their own spacing rules, set by storage height and commodity rather than by this table, and the LPC Rules allow certain extended coverage heads to be spaced further apart than Table 19 where the manufacturer’s data supports it (TB222). Our guide to sprinklers in warehouses covers those cases.

Why heads per pipe size is the wrong question

You will find tables online that say how many heads a 25 mm or 32 mm pipe can feed. Those are pipe schedules from older pre-calculated designs. A system to BS EN 12845 is sized by hydraulic calculation: the pipe has to deliver the design density over the assumed area of operation for that hazard class, from the actual water supply. Two buildings with the same number of heads can need different pipe sizes. Our BS EN 12845 guide explains hazard classes and design density.

Checking your own layout

If a building has changed use, been refitted with new ceilings or partitions, or now stores goods higher than it used to, the spacing that was right when the system was installed may not be right now. A sprinkler service includes a review of the hazard against the installation, and where heads have to be added or moved, our sprinkler installation team designs the change to BS EN 12845.