Flue Space Requirements for In-Rack Fire Sprinkler Systems (NFPA 13)

Flue spaces are the open vertical gaps in rack storage that let heat, smoke, and sprinkler water move through the stored goods. For rack storage protected by in-rack fire sprinklers, NFPA 13 uses these spaces as part of the fire protection design, not as leftover warehouse space. This guide explains the main flue space requirements NFPA 13 applies to racks, how those requirements affect sprinkler placement, and what facility teams should watch during daily operations.

Written and reviewed by the rack engineering and permitting team at Quality Material Handling, Inc. (QMH) — a material handling contractor with more than 32 years of experience designing, permitting, and installing pallet rack and high-piled storage systems, licensed in California as Contractor #731100 (B, D34) and serving facilities in 30+ states. Our project teams work directly with fire protection engineers, plan checkers, and fire marshals on rack permit submittals, so the guidance below reflects what actually gets flagged during plan review and field inspection. Last reviewed: September 2026. This article is educational and does not replace a stamped fire protection design, your insurer’s requirements, or a ruling from your local Authority Having Jurisdiction.

What are NFPA 13 flue space requirements for rack storage?

NFPA 13 flue space requirements for rack storage are rules for keeping clear vertical openings between pallet loads, rack rows, and rack uprights so the sprinkler system can work as designed. For racks protected by in-rack sprinklers, NFPA 13 committee material states that nominal 6 in. (150 mm) transverse flue spaces between loads and at rack uprights are to be maintained, while random variations in flue width or vertical alignment are permitted. Local enforcement can vary by adopted edition and by the Authority Having Jurisdiction, often called the AHJ.

NFPA 13, Standard for the Installation of Sprinkler Systems, is a design and installation standard. It does not replace the building code, fire code, insurance requirements, or the AHJ’s final decision. In the United States, many jurisdictions adopt a specific edition by law, so a warehouse may be reviewed under NFPA 13 2019, 2022, 2025, or another adopted edition. The 2025 edition is available as the current edition through standards sellers, but the legally enforced edition for a project depends on the local code adoption and permit documents.

Key terms used in rack flue space design

A flue space is an open vertical space within rack storage. Think of it like a narrow chimney inside the rack. It gives heat a path upward and gives sprinkler water a path downward and sideways into the burning area.

A transverse flue is the vertical gap between stored loads along the rack row, often at rack uprights. In simple terms, it is the gap you see between pallet loads when looking down the length of a rack aisle. NFPA material describes the transverse flue as the space between rows of storage parallel to the direction of loading.

A longitudinal flue is the vertical gap between back-to-back rows of storage. In a double-row rack, this is usually the center gap between the rear faces of pallets. NFPA material describes a longitudinal flue as the space between rows of storage perpendicular to the direction of loading, with a width not exceeding 24 in. (610 mm) between storage.

A rack upright is the vertical steel frame member that supports the rack. NFPA 13 often ties transverse flue spaces to these uprights because they are regular points where gaps can be planned, marked, and inspected.

Why flue spaces matter in in-rack sprinkler systems

Flue spaces matter because in-rack sprinklers are placed inside the storage array, not only at the ceiling. If pallet loads block the intended gaps, sprinkler spray may not reach the seat of the fire, and heat may not move in the way assumed by the design. This can change the fire behavior and may make the installed system different from the approved fire protection design.

In-rack sprinklers are often used when ceiling-only protection is not enough for the storage height, commodity type, rack depth, ceiling height, or shelf arrangement. Where a ceiling-only approach can carry the hazard, designers often evaluate early suppression fast response (ESFR) sprinklers instead, which shifts the conversation toward ceiling clearance and obstruction rules rather than in-rack flue alignment. NFPA 13 also includes provisions for added in-rack protection when clearance to the ceiling becomes too large for certain rack storage conditions; some criteria call for supplemental quick-response in-rack sprinklers below the top tier and at flue space intersections.

For operations teams, the practical point is simple: the sprinkler drawings and rack layout are connected. Changing pallet size, adding decking, pushing loads too far back, or filling the center gap can turn a compliant layout into a problem.

Common rack storage configurations

Single-row racks

Single-row racks are one rack row served by aisles. NFPA material describes single-row racks as racks with no longitudinal flue, depth up to 6 ft (1.8 m), and aisles at least 3.5 ft (1.1 m) between loads on racks. The main flue concern is usually the transverse flue between loads and at uprights.

In-rack sprinklers for single-row racks are commonly coordinated with transverse flues because there is no back-to-back center flue. The design team must check the commodity, storage height, rack depth, and approved sprinkler arrangement before deciding where sprinklers belong.

Double-row racks

Double-row racks are two rack rows placed back to back. NFPA material describes them as racks up to 12 ft (3.7 m) in aggregate depth, with aisles at least 3.5 ft (1.1 m) between loads. The key added issue is whether a longitudinal flue exists or is required.

For Class I through IV and Group A plastic commodities in double-row open racks, NFPA committee material states that a longitudinal flue is not required for storage up to and including 25 ft (7.6 m), while nominal 6 in. (150 mm) longitudinal flues are provided for double-row racks over 25 ft (7.6 m). Always confirm the adopted edition and exact commodity classification before using that rule.

Multiple-row racks

Multiple-row racks are deeper rack arrangements with more than two rows or wider storage blocks. These layouts can be harder to protect because water and heat must move through a larger storage mass. NFPA research material notes that multiple-row open-rack treatment has depended on 6 in. (150 mm) transverse flues at maximum 5 ft (1.5 m) intervals, with rack depth and aisle limits also affecting the arrangement.

For facility teams, multiple-row racks deserve extra care. If loads bridge over flues, if shelf decking blocks openings, or if aisles are narrowed by stored goods, the rack may no longer match the approved design.

Clearance dimensions to plan and maintain

The following dimensions are common checkpoints, but they must be verified against the adopted NFPA 13 edition, approved plans, sprinkler listing, and AHJ direction:

• Transverse flues for rack storage: Maintain nominal 6 in. (150 mm) transverse flue spaces between loads and at rack uprights for single-row, double-row, and multiple-row racks where the NFPA 13 criteria apply.
• Longitudinal flues in taller double-row racks: For certain Class I through IV and Group A plastic storage over 25 ft (7.6 m), provide nominal 6 in. (150 mm) longitudinal flue spaces in double-row open racks.
• Multiple-row transverse flue spacing: Recent NFPA material describes 6 in. (150 mm) transverse flues at maximum 5 ft (1.5 m) intervals as part of the open-rack approach for certain multiple-row racks.
• Ceiling sprinkler clearance to storage: OSHA 29 CFR 1910.159 requires a minimum 18 in. (45.7 cm) vertical clearance between sprinklers and material below, and NFPA 13 applies comparable clearance-to-storage criteria, with exceptions that depend on the sprinkler listing, hazard, and evidence such as large-scale fire testing.
• In-rack sprinkler clearance: NFPA committee material includes a minimum 6 in. (152 mm) vertical clear space between in-rack sprinkler deflectors and the top of a tier of storage.

These dimensions are not a complete sprinkler design. They are field conditions that help preserve the design assumptions made by the fire protection engineer or sprinkler contractor.

How should flue spaces be managed in daily warehouse operations?

Flue spaces should be managed as controlled safety zones, not as flexible storage space. The best practice is to make the required gaps visible, train operators on why they matter, and inspect them often enough to catch changes before they become habits.

Useful controls include:

1. Mark rack beams or uprights. Use durable labels, painted marks, or load stops to show where 6 in. flues must remain open.
2. Match pallet size to rack design. Oversized pallets can close transverse flues even when operators place them neatly.
3. Use pallet stops where suitable. Stops can help prevent loads from being pushed into a longitudinal flue, but they must not obstruct sprinkler discharge or required openings.
4. Review new commodities. A change from paper goods to plastics, aerosols, tires, or mixed high-hazard storage can change the sprinkler basis.
5. Control temporary storage. Do not place cartons, returns, empty pallets, or equipment in flues, aisles, or near in-rack sprinklers.
6. Keep drawings available. Supervisors should be able to compare the actual rack layout with the approved fire protection drawings.

Common compliance pitfalls

One common pitfall is treating “nominal 6 in.” as a suggestion. NFPA material allows random variations in flue width or vertical alignment, but that does not mean flues can be closed, filled, or ignored. A pattern of blocked flues is different from small field variation.

Another pitfall is changing rack parts without review. Solid shelves, close-mesh decking, carton flow systems, pallet racking back stops, signs, or added barriers may block openings that were part of the sprinkler design. NFPA research material explains that solid shelf conditions can create shielding and may require more in-rack sprinkler protection.

Other frequent issues include:

• In-rack sprinklers and drops installed where forklifts can strike them.
• Pallets pushed back into longitudinal flues.
• Load overhang closing transverse flues at uprights.
• Storage placed above the approved height.
• Aisles narrowed below the design basis.
• Missing water shields where required by the design.
• In-rack piping used as a hanger point for signs or product.

The best way to avoid these problems is to review rack changes before installation, train forklift operators, and include flue spaces in routine safety walks.

What we see most often in the field: on the rack projects our teams inspect and repair, closed flues are rarely a design error. They are almost always an operations drift problem — a pallet footprint that grew by two inches, a new back-to-back row installed without re-checking the center gap, or seasonal overflow parked in the aisle-end bays. The rack was compliant on the day it was signed off. The question a fire marshal asks is whether it is still compliant today.

Inspection and maintenance considerations

Inspection should cover both the sprinkler equipment and the storage arrangement. NFPA 25 is the main U.S. standard for inspection, testing, and maintenance of water-based fire protection systems, while NFPA 13 is the installation and design standard. NFPA 25 guidance also emphasizes action when building, occupancy, storage, or water supply changes could affect sprinkler performance.

A practical inspection checklist should include:

• Confirm transverse flues are open at rack uprights.
• Check that longitudinal flues, where required, are not blocked.
• Look for damaged in-rack sprinklers, guards, piping, and braces.
• Verify storage height is within the posted or approved limit.
• Confirm ceiling sprinkler clearance is maintained.
• Check that rack decking and shelf materials match the approved design.
• Report damaged sprinklers or closed valves at once.
• Document repeated storage violations and retrain the affected team.

Maintenance staff should not relocate, cap, remove, or add in-rack sprinklers without a qualified fire sprinkler contractor and design review. Even a small relocation can affect hydraulic calculations, spacing, obstruction rules, and AHJ approval.

When to consult a fire protection engineer or AHJ

Consult a fire protection engineer or the AHJ when the storage arrangement changes, the commodity changes, the rack layout changes, or the existing design basis is unclear. You should also seek review before adding rack levels, installing solid or semi-solid shelves, changing aisle widths, increasing storage height, modifying in-rack sprinklers, or using a new pallet type that affects flue openings.

The reason is not only paperwork. Rack storage fire protection depends on many connected items: commodity class, packaging, ceiling height, storage height, sprinkler type, water supply, aisle width, rack depth, flue spaces, and sprinkler placement. If one item changes, the rest of the design may need to be checked.

The AHJ has the final local enforcement role, and insurers may apply added property-protection rules. A qualified fire protection engineer can help interpret NFPA 13, compare editions, prepare code-compliant options, and explain what is needed for permit approval and safe operation.

In California, this review is usually not optional. Rack storage above 12 ft generally falls under high-piled storage fire code requirements, which trigger a plan submittal covering commodity classification, aisle widths, storage height, smoke removal, and fire department access. Our permitting team handles those submittals routinely, and flue spacing is one of the details plan checkers return comments on. If you are scoping a new building or a rack expansion, it helps to understand the broader warehouse fire sprinkler requirements in California before the rack layout is finalized, because sprinkler design and rack design constrain each other.

Key takeaways for compliant rack storage

NFPA 13 flue space requirements are a core part of fire protection design for rack storage and in-rack sprinklers. In many common rack layouts, nominal 6 in. (150 mm) transverse flues between loads and at uprights are a central requirement, and some taller or deeper rack arrangements also depend on longitudinal flues or closely spaced transverse flues. These requirements can vary by commodity, storage height, rack type, sprinkler arrangement, edition, and AHJ interpretation.

For a safer and more reliable warehouse, keep flues open, protect in-rack sprinklers from damage, maintain required clearances, and review storage changes before they are made. When in doubt, do not guess from a sign on the rack or a past inspection. Confirm the approved plans, the adopted NFPA 13 edition, and the AHJ’s requirements before changing the storage layout.

How this guide was researched and reviewed

This article was prepared by QMH’s rack engineering and permitting staff and reviewed against the following primary sources: NFPA 13, Standard for the Installation of Sprinkler Systems, including technical committee material on transverse and longitudinal flue criteria; NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems; OSHA 29 CFR 1910.159; and California Fire Code provisions for high-piled combustible storage. Dimensions cited here are drawn from published NFPA criteria and are reproduced as general guidance only.

Known limitations. NFPA 13 flue criteria change between editions, and the edition that governs your facility is the one adopted by your jurisdiction, not necessarily the most recent one. Criteria also vary by commodity classification, storage height, rack depth, sprinkler listing, and whether the design relies on ceiling-only or in-rack protection. Nothing here should be used in place of the approved plans for your building. If a figure in this article conflicts with your stamped drawings, the adopted code, or your AHJ, those govern.

Corrections and updates. We review this page at least annually and after any NFPA 13 edition change. If you believe a detail here is out of date, tell us and we will verify it against the current standard.

Get a flue space and in-rack sprinkler review

QMH has spent more than three decades designing, permitting, installing, and repairing rack systems in high-piled storage facilities, and we hold California Contractor License #731100 (B, D34). Our teams coordinate rack layout with fire protection design, prepare high-piled storage permit submittals, and correct field conditions that put a facility out of compliance. If you are unsure whether your current rack arrangement still matches your approved in-rack sprinkler system design, contact a QMH expert for a walkthrough before your next inspection.