Storage protection is the largest part of NFPA 13 and the one where most mistakes are made. The reason is simple: in a warehouse the fire load is set by operations, not by the architectural drawings. The general storage requirements define the questions every warehouse design must answer first, whichever protection option follows (density/area, CMSA, ESFR or in-rack).
What this part of the standard covers
The general requirements standardise the input data for storage design. You cannot enter any protection table until you have answered six questions:
- What is stored? Commodity class and packaging.
- How is it stored? Palletised, solid-piled, racks, shelving, bin boxes.
- How high? Maximum storage height.
- Where is the ceiling? Highest ceiling point and roof slope.
- How much space in between? Clearance from deflector to top of storage.
- How does water get down? Rack flue spaces, aisle widths, horizontal barriers.
Commodity classification
NFPA 13 classifies stored goods by how they burn:
| Class | General description | Example |
|---|---|---|
| Class I | Non-combustible product on wooden pallets, in single-layer corrugated cartons or light wrapping | Metal parts, drinks in glass bottles |
| Class II | Class I product in wooden crates or multi-layer corrugated cartons | Crated machine parts |
| Class III | Products of wood, paper, natural fibre or Group C plastics; limited plastic content | Paper products, wooden furniture |
| Class IV | Products or packaging with a limited amount of Group A plastics | Small appliances with plastic parts |
| Group A plastics | Highest heat release plastics; split into cartoned/exposed and expanded/unexpanded | Polystyrene foam, PE/PP products |
Group B and C plastics and special commodities such as rubber tyres, roll paper and baled fibre have their own rules. For the logic behind classification see commodity classification in NFPA 13.
Pallets and mixed commodities
Pallets are fuel too. Plastic pallets can raise the commodity class unless they are listed and shown to be equivalent to wood. In mixed storage the design is normally based on the most hazardous commodity; a lower class is only allowed where the quantity and arrangement limits in the standard are met.
Storage arrangement and geometry
The same commodity behaves very differently in different arrangements. In solid piles the fire travels over the outer faces; in racks the flue spaces act as chimneys and the fire spreads vertically very quickly. That is why the tables are split by arrangement.
| Parameter | General criterion / note |
|---|---|
| Deflector to top of storage (standard spray) | At least 457 mm (18 in) |
| Deflector to top of storage (CMSA and ESFR) | At least 914 mm (36 in) |
| Longitudinal flue in double-row racks | Nominal 152 mm (6 in) |
| Transverse flue spaces | Nominal 76 mm (3 in) |
| Aisle width | Affects design density in some tables |
| Miscellaneous storage | Generally up to 3.7 m (12 ft); may be protected under occupancy hazard rules in set conditions |
Flue spaces are vital, especially in racks protected by ceiling sprinklers only: they let water run down into the rack and let heat reach the sprinklers. Pallets pushed back until they close the flues is one of the most common operational problems.
Heights: storage and ceiling
Storage tables have two axes: storage height and ceiling (building) height. Higher storage means a bigger fire; a higher ceiling means later sprinkler operation and a harder journey for the drops. As the gap grows, protection gets harder, which is why many tables limit ceiling height as well as storage height. Always design on the highest values: the roof ridge, the top rack level and the storage height operations could reach in the future.
Special cases
- Idle pallets: empty wooden and especially plastic pallets are a very high fire load with their own tables. High-piled idle plastic pallets indoors exceed the limits of most protection options.
- Encapsulation: plastic sheeting over the top and sides of a pallet load stops water reaching the product and calls for more protection in some tables.
- Dry systems: in storage, too, a dry system enlarges the design area, and it is generally incompatible with ESFR.
- Solid shelving: shelves that block water cut off ceiling sprinkler discharge and, above certain area limits, require in-rack sprinklers.
Choosing the protection option
Once the input data is fixed, the options are compared: density/area (CMDA), CMSA, ESFR and, where needed, in-rack sprinklers. Water demand, roof construction, operational flexibility and ease of maintenance are weighed together. The ESFR suitability checker gives a quick view on ESFR, and the fire water tank sizing calculator helps with volume.
Common site defects
- The actual commodity differs from the design commodity (particularly more plastic packaging).
- Storage height rises above the design and clearance disappears.
- Flue spaces closed up.
- Temporary solid piles in aisles.
- Idle pallets stacked between racks or against walls.
- Sheet metal or boards laid on rack levels after handover.
Handover note: put the design assumptions (commodity class, maximum storage height, rack arrangement, flue spaces) in writing on the system information sign and in the warehouse operating procedure. The weakest link in storage protection is a shift manager who does not know them. For field observations see warehouse protection field findings.
Frequently Asked Questions
At what height do the NFPA 13 storage rules apply?
Storage above 3.7 m (12 ft) is generally protected under the storage requirements. Limited miscellaneous storage up to that height can, in set conditions, be protected under the occupancy hazard approach.
How much clearance is needed between deflector and storage?
Generally at least 457 mm for standard spray sprinklers and at least 914 mm for CMSA and ESFR sprinklers. The clearance lets the sprinkler's spray pattern develop.
Do plastic pallets change the commodity class?
They can. Unless listed and shown equivalent to wooden pallets, plastic pallets may require the commodity class to be raised. Check the product listing.
How is mixed commodity storage designed?
Normally on the most hazardous commodity. A lower class is only allowed when the quantity and arrangement limits defined in the standard are met.
Why do flue spaces matter?
They let ceiling sprinkler water run down into the rack and let heat reach the sprinklers. Closing them seriously weakens protection in racks protected only at ceiling level.
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App StoreNFPA 13 (current edition) – general requirements for storage and commodity classification. Numeric values are general criteria; confirm against the edition in force and the authority having jurisdiction for each project. The findings here are typical defect patterns, not an account of events at any particular site.