In-Rack Sprinklers in NFPA 13: When They Are Needed and How to Place Them

Beyond a certain point ceiling sprinklers cannot reach a fire inside a rack: the storage is too high, shelves block the water, or the commodity burns too fast. In-rack sprinklers then take water to where the fire is, inside the rack structure. The in-rack requirements of NFPA 13 set out when these sprinklers are needed, where they go and how they work together with the ceiling system.

What this part of the standard covers

• When in-rack sprinklers are required, and the options available
• Sprinkler type, K-factor, temperature rating and response
• Vertical levels and horizontal locations (flue spaces, rack face)
• Water shields and mechanical protection
• Hydraulically combining in-rack and ceiling demand
• Control valves, drains and maintenance access

When are in-rack sprinklers required?

In-rack sprinklers are not "extra safety"; usually they are mandatory because ceiling protection alone falls short. Typical triggers:

When you meet these situations, the first question is whether the storage can change: open-grating shelves or a lower storage height sometimes remove the need for in-rack sprinklers entirely. For how storage inputs are defined, see general requirements for storage.

Sprinkler selection

In-rack sprinklers are usually small or medium K-factor (for example K80 / K5.6 or K115 / K8.0), quick-response and ordinary temperature. Heat builds up quickly inside a rack, and a fast element catches the fire early. Minimum operating pressure is in many cases around 1 bar (15 psi); confirm the exact value and required flow for the chosen protection option in the edition in force. The sprinkler temperature rating selector helps with rating choice.

Placement: levels and flue spaces

In-rack effectiveness depends on location. The core principles:

• In flue spaces: sprinklers sit in longitudinal or transverse flues, preferably at intersections, the natural path for water and heat.
• Vertical clearance: generally at least 152 mm (6 in) between deflector and the top of the load below.
• Levels: number of levels and vertical spacing follow storage height and commodity; tall racks need several levels.
• Face sprinklers: some commodities and rack arrangements need sprinklers at the rack face as well as in the flue.
• Horizontal spacing: spacing along the rack is limited by table and protection option and must be coordinated with bay dimensions.

Water shields

An in-rack sprinkler with other sprinklers above it needs a water shield so that water from above does not cool it and delay operation. Intermediate level sprinklers with an integral shield can be used for this.

Mechanical protection

The forklift is the in-rack sprinkler's worst enemy. Sprinklers and pipe must stay within the protection of the rack members and never project into the aisle. Listed guards are used where needed.

Horizontal barriers

For some high-challenge commodities and tall racks, in-rack sprinklers are used together with horizontal barriers at rack levels. The barrier keeps heat within a limited vertical zone so the in-rack sprinklers beneath it open quickly, and it delays fire jumping above it. In barrier designs the barrier material, gaps and sprinkler positions are interdependent; removing or cutting barriers in operation invalidates the design.

Hydraulics: ceiling and in-rack together

In-rack demand is not independent of ceiling demand. In a fire both systems operate at once, so the calculation balances the two demands hydraulically at their point of connection. Typical steps:

1. Set the ceiling design area and density (from tables that change when in-rack sprinklers are present).
2. Set the number and layout of in-rack design sprinklers (the most demanding level and zone).
3. Balance both demands at the supply point, adjusting the flow of the lower-pressure side to the higher pressure.
4. Add the hose allowance to the total and apply the duration.

For the general method see the sprinkler hydraulic calculation guide.

Valves and maintenance

Because in-rack sprinklers are damaged often, it helps to be able to repair them without shutting the ceiling system. Many designs give in-rack sprinklers their own indicating control valve and drain; check the conditions that make this mandatory in the edition in force. The valve should be outside forklift traffic and easy to reach. Routine inspection must check in-rack sprinklers for damage, paint, dust and contact with loads; see visual inspection of sprinklers.

Common site mistakes

1. Rack layout changes (bay width, beam levels) without moving the in-rack sprinklers.
2. Loads touching the sprinklers; the 152 mm clearance lost.
3. Standard sprinklers used where a water shield is needed.
4. Pipe bent and sprinklers broken by forklifts, left unrepaired for long periods.
5. In-rack valve left closed, with no supervision.
6. Ceiling design based on the reduced table that assumes in-rack sprinklers, which are later removed.

Frequently Asked Questions

When are in-rack sprinklers mandatory?

When storage height, commodity or rack arrangement falls outside the ceiling-only tables: for example solid shelving, open-top containers, or a building ceiling above ESFR/CMSA limits.

Why do in-rack sprinklers need water shields?

Water from sprinklers above can cool the heat-responsive element of a lower sprinkler and delay it. A water shield, or an intermediate level sprinkler with an integral shield, prevents this.

How are in-rack and ceiling demands combined?

Each demand is calculated separately and the two are balanced at the supply point. The hose allowance is added to the total flow and the duration applied.

Can in-rack sprinklers be combined with ESFR?

The purpose of a standard ESFR design is to avoid in-rack sprinklers, although some special arrangements define combined use. Confirm with the edition in force and the product listing.

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