Dangerous goods (DG) are a small share of container cargo but take a large share of a planner's attention. One misplaced DG container can lead to a fire, a chemical reaction or a refusal at the next port. This guide explains how DG fits into a vessel stowage plan at concept level. It does not replace the IMDG Code and does not reproduce any table or list from it.
The basics of the IMDG Code for shippers, such as classification, packaging and labels, are covered in our IMDG Code basics guide. Here we focus on the decisions made for the ship: where a DG container may go and how far it must be from other containers.
Dangerous goods classes in planner's terms
The IMDG Code groups dangerous goods into nine classes, and every substance is identified by a four-digit UN number. The summary below is written from a planning point of view, not as official definitions:
| Class | In short | Main planning concern |
|---|---|---|
| 1 | Explosives | Strict limits on position and quantity; often special port permission |
| 2 | Gases (flammable, non-flammable, toxic) | Gas leaks; well-ventilated positions |
| 3 | Flammable liquids | Keep clear of heat and ignition sources |
| 4 | Flammable solids, spontaneously combustible, dangerous when wet | Heat, moisture and fire-fighting water |
| 5 | Oxidisers and organic peroxides | Intensify fire; some need temperature control |
| 6 | Toxic and infectious substances | Away from foodstuffs and living quarters |
| 7 | Radioactive material | Distance from crewed spaces; transport index limits |
| 8 | Corrosives | Leaks that damage structure and other cargo |
| 9 | Miscellaneous, including certain marine pollutants and lithium batteries | Varied; read the special provisions per UN entry |
To practise, explore the DG classes and the UN number lookup. Official data must still come from the IMDG Code edition in force.
Segregation concepts
Segregation keeps apart goods that could react dangerously if they mixed, for example oxidisers and flammable liquids, or acids and certain other substances. The IMDG Code uses several levels of separation, each stricter than the last. In plain terms:
- The mildest level keeps the goods far enough apart that they cannot interact in an accident, while still allowing them in the same space or on the same deck.
- The next level asks for more distance, generally a different hold or compartment under deck, or a set horizontal distance on deck.
- A stricter level requires a complete compartment or hold between the two.
- The strictest level requires longitudinal separation by an intervening complete compartment or hold.
On container ships these levels are translated into distances counted in container slots, fore and aft and athwartships, with or without a bulkhead between. The rules also distinguish closed and open containers, and on-deck and under-deck positions. On top of the class there are segregation groups (for example acids and several other groups) that add requirements of their own.
A planner does not need to memorise the tables but must know the flow: take the required level from licensed data, translate it into slot distances on that ship, then check every pair of DG containers. An educational simulation is available in the DG segregation trainer; its results do not replace an official check.
Stowage categories: on deck or under deck
Every entry in the Dangerous Goods List carries a stowage category. It states whether the goods may go on deck, under deck, or on deck only, and the answer can differ between cargo ships and passenger ships. Class 1 has its own category scheme. In addition there are stowage and handling codes, for example a requirement to be protected from heat or kept away from living quarters.
Practical points on a container ship:
- On deck: better natural ventilation and access for inspection, but exposure to sun, seawater and, in the top tiers and outer rows, the risk of container loss.
- Under deck: sheltered from the weather, but only for classes permitted by the ship's Document of Compliance (DOC) for that hold, under SOLAS II-2/19. Different holds may be approved for different classes.
- Distance from accommodation and bridge: some DG must be kept away from crewed spaces and air intakes.
Reefers and heat sources
Heat sources on board include engine-room bulkheads, heated fuel tanks, the funnel and hatch covers above the engine room. Goods that must be protected from heat should not be placed next to them, and where the provisions say so, not in top tiers exposed to direct sun.
Reefer containers matter in two ways. First, the refrigeration unit and its power cable are electrical equipment and a potential ignition source, so their position relative to flammable DG deserves attention. Second, some organic peroxides (class 5.2) and self-reactive substances (class 4.1) travel in reefers with a control temperature and an emergency temperature. Such containers need a monitored reefer slot, a plan for power failure and crew access. Temperature set points can be checked with the reefer tool.
Example: checking one bay
An illustrative case on a fictional feeder: three DG containers are planned for the same bay.
| Container | Class | Questions to check |
|---|---|---|
| A | 3, flammable liquid | Does the DOC permit class 3 in this hold? Is it next to the engine-room bulkhead? |
| B | 5.1, oxidiser | Which segregation level applies against A? What slot distance is required under deck or on deck? |
| C | 8, corrosive | Any special segregation group? Is it stowed above foodstuffs? |
If B may not share the hold with A, the planner moves B on deck at a compliant distance or to another hold. Each move is checked again, because DG containers in the neighbouring bays are affected too.
Common mistakes in the stowage plan
- Checking the primary class only. Subsidiary risks and segregation groups also drive the distance required.
- Forgetting the neighbouring bay. Segregation applies in every direction, including across bulkheads and into the bays fore and aft.
- Ignoring changes at transit ports. New DG loaded at the next port can conflict with DG already on board.
- Mismatched declaration data. The UN number, class and packing group in the booking must match the Dangerous Goods Declaration and the placards on the container.
- Not checking the DOC. Hold approvals can differ between ships on the same service.
DG manifest and licensed data
SOLAS Chapter VII requires the ship to carry a special list or manifest of dangerous goods, or a detailed stowage plan, showing the class and position of every DG item. The core data comes from the shipper's Dangerous Goods Declaration: UN number, proper shipping name, class, packing group, quantity and emergency contact. DG data also travels to the next terminal in the BAPLIE message.
Software that checks segregation automatically must use official, licensed IMDG data that is in force. STOWAGE.ID Vessel Stowage does not redistribute IMDG Code text or tables; automatic checks run on licensed data held by the user's company. STOWAGE.ID is a shore-side planning and decision-support tool. The ship's class-approved loading computer, DOC and stability booklet remain the official reference, and accepting DG remains the master's decision. See how it works on the container ship stowage page.