Securing cargo in a container: blocking, bracing, lashing and dunnage bags

We export from Jaipur to 20+ countries, and Jaipur has no sea. Every export container that leaves our city reaches a port by road or rail first, then goes on a ship, and then often travels by road again at the other end.

Each of those legs shakes the load in its own way.

The method has four steps. Stow the cargo tight, so the container walls and the other cargo block it. Fill the gaps that remain with timber, empty pallets or dunnage bags. Lash only what cannot be blocked. Then make the door end firm, because that is where loads fall out on the people who open the container.

The reference I use for all of this is the CTU Code, the Code of Practice for Packing of Cargo Transport Units, written jointly by the IMO, the ILO and UNECE and issued by the IMO as MSC.1/Circ.1497 in December 2014. It is long and written for engineers. In this article we pull out the parts a factory exporter actually needs.

A revised edition is on its way. An industry submission to the IMO dated July 2026 says the restructured text is aimed at approval in December 2026. Until it is published, the 2014 Code is the one to follow.

I should be clear about one limit first. I am not a marine surveyor. For heavy machinery, steel coils or anything that needs a lashing calculation, please pay a surveyor to plan it. My advice below is for the common case: cartons, bags, drums and pallets.

The forces your load has to survive

Most of us assume the weight of the cargo holds it in place. It does not.

The CTU Code says plainly that mass alone, even with a high friction factor, does not secure cargo, because vibration can still walk it across the floor. It gives the forces as accelerations, in g. A figure of 0.8 g means the load pushes with 80% of its own weight in that direction.

Source: CTU Code, chapter 5. The sea areas are set by wave height. Area A includes the Mediterranean, the Red Sea and the Persian Gulf, and area C is unrestricted. If you do not know the route, plan for area C.

I find the g figures easier to understand with one example. Look at the first row. When a truck brakes hard, 10 tonnes of cargo pushes forward like 8 tonnes.

Think of a standing passenger in a city bus when the driver brakes suddenly. If he is not holding the rail, his own weight does nothing to keep him in place. Our cartons are that passenger.

Plan the load before the first pallet goes in

Most securing problems are decided during planning, before anyone touches a strap.

1. Read the CSC safety approval plate on the container. It shows the maximum gross mass, and the permitted payload must never be exceeded.
2. Keep heavy goods low and light goods on top. The centre of gravity should sit below half the height of the cargo space.
3. Distribute heavy items evenly over the floor. A heavy crate on a small footprint needs timber beams under it, so its weight reaches the container's floor beams.
4. Keep the weight centred along the length. For a container that will be lifted, the centre of gravity should in general be no more than 5% away from the middle. The Code's rule of thumb is 60% of the cargo mass within 50% of the length.

Point 4 is easy to miss when the heaviest goods happen to be loaded first, all at the front.

We can check it with a pencil. Multiply the mass of each group of cargo by the distance of its middle from the front wall, add those up, and divide by the total mass. That is the Code's own formula.

Let us take an example, with assumed numbers. Our 20 ft container has an inside length of about 5.9 m (check your shipping line's figure). We load 9 tonnes whose middle sits 1.5 m from the front wall, and 5 tonnes whose middle sits 4.4 m from it.

• (9 × 1.5) + (5 × 4.4) = 13.5 + 22 = 35.5
• 35.5 ÷ 14 tonnes = 2.54 m from the front

The middle of our container is at 2.95 m. The load sits 0.41 m forward, which is approximately 7% of the length, and the front half carries 64% of the mass.

Both numbers fail.

Moving one heavy group towards the doors fixes it, and it costs nothing at the loading dock. This matters on the road too. The Code warns that containers on trailers quite often carry their centre of gravity forward, which can overload the truck when the container is packed close to its full payload.

Blocking first: stow tight and fill the gaps

Blocking means the cargo cannot move because something solid is in the way: the container wall, another package, or a filler. It is usually the simplest and cheapest method. In the Code, lashing is the answer for cargo that cannot be blocked.

The rules on gaps are simple.

1. Small gaps that you need to get the goods in and out are acceptable.
2. The total of all voids in any horizontal direction should not exceed 15 cm. Measure across the width and along the length.
3. Between dense, rigid cargo such as steel, concrete or stone, keep voids even smaller.
4. Fill larger voids with empty pallets stood on edge, timber, or dunnage bags. Do not use rags, gunny cloth or weak foam, because they deform or shrink permanently.

I like one trick from the Code very much. In a container with strong side walls, stow the cargo tight against both walls and leave the remaining gap in the middle. Then each brace only has to resist the push from one side at a time.

Bracing with timber

When the gap is too big for a bag, timber does the job.

The advice here is practical. Spread the force with cross-beams where a brace meets the cargo, unless that point is a strong part of the package. Join the pieces with nails or clamps, and add diagonal braces, so the frame stays in place even if it goes slack for a moment.

Do not nail into the wooden floor of a closed container, because those floors are generally not suitable for nailing. Nail your blocks to timber laid under the cargo instead.

Securing material should also comply with ISPM 15, the international phytosanitary standard for wood packaging, so use treated timber that carries the ISPM 15 mark. At the other end, the Code says unmarked natural wood should be disposed of under that country's plant protection rules. That means extra cost and delay for your buyer.

Dunnage bags: where they work and where they do not

A dunnage bag is an air bag that you put into a gap and inflate, so it presses the cargo blocks apart and holds them. Used well, it is fast and clean. Used badly, it bursts or pushes the doors open.

Five rules matter here.

1. Follow the manufacturer's instructions on filling pressure and maximum gap. Every bag has both.
2. Do not use dunnage bags to fill the space at the doorway, unless you make sure they cannot force the doors open violently when someone opens them.
3. Smooth any rough or sharp surface in the gap, or the bag will chafe and puncture on the voyage.
4. Fill to a slight overpressure only. Too little and the bag works loose when the air cools. Too much and it can burst when the temperature rises.
5. When unloading, open the valve and let the air out before you touch the cargo.

There is a simple reason for point 1. The Code estimates a bag's holding force from its burst pressure multiplied by the area that touches the cargo, and that area shrinks as the gap gets wider.

Take a bag 1.0 m wide and 1.8 m high (an assumed size). By that formula, it touches approximately 1.0 m² of cargo in a 20 cm gap. In a 30 cm gap, the same bag touches only about 0.7 m². That is roughly 30% less hold from one bag.

If a gap is too wide, my answer is timber or a bigger bag, not more air.

There is a safety margin on top. The Code counts only 75% of the burst rating for a single-use bag and 50% for a reusable one.

Lashing: know what the container can take

Lashing ties the cargo to the container with webbing, wire, chain or steel band. We use it for items that cannot be blocked: a crate standing alone, a machine, a top layer that might tip.

Before planning any lashing, I would look at the rings in the actual container. The weak point is often the container, not the strap. Where lashing rings are fitted, the Code says floor points have a maximum securing load (MSL) of at least 10 kN, and newer containers often have 20 kN. Points on the top side rails have at least 5 kN. Many general-purpose containers have only a few of them.

MSL is not the breaking strength. The Code gives MSL as a share of breaking strength: 50% for shackles, rings and chains, 50% for reusable web lashings, 75% for single-use web lashings and 70% for single-use steel band. So a web lashing that breaks at 1,000 daN should be counted for no more than 500 to 750 daN in service, depending on whether it is reusable.

Two rules are easy to break on a busy dock. Do not tie web lashings with knots, and do not tighten any lashing so hard that it damages the goods. On cartons, put an edge protector under each lashing. Our guide to edge protectors and corner boards explains which kind to use.

Pallets inside a container

Palletised cargo is easier, if the pallet loads are firm. Gaps between pallets need not be filled if the goods are firmly secured to the pallets, the pallets are stowed tight, and they cannot tip.

There is a catch for sea freight. The Code warns that repeated heavy loads in bad weather can fatigue film wrapping and reduce its hold. I would not rely on film alone for a heavy pallet at sea. The pallet itself has to be built for the voyage, and our guides on stretch film and PP, PET and steel strapping cover how.

Make the door end firm

The last few rows matter most for safety, and this is the part I would check myself before the doors close.

Build a firm face of cargo at the doors, so nothing falls out when the container is opened. If you have any doubt, strap the top layers back to securing points, or build a fence of timber battens between the rear corner posts. The Code also points out that a container on a trailer usually tilts towards the doors, so vibration pushes the cargo that way during the trip.

The doors are not a wall to lean cargo on. A general-purpose container's door end is built to withstand 40% of the permitted payload as an even load, against 60% for each side wall.

Safety for the people who load and unload

Loads can kill people. These points come from the Code, and I would make them site rules.

1. Wear a helmet, safety shoes and gloves, and teach people how to lift heavy parcels.
2. A forklift working inside a container needs a short mast, a low overhead guard and lights. Do not use LPG forklifts inside a closed container, because a leak can build up an explosive mixture.
3. The container floor may be rated for an axle load of only 5,460 kg, the minimum under the CSC. Ask the line before you drive a heavy forklift in.
4. Before anyone enters at the receiving end, ventilate the container. Some ordinary goods give off harmful fumes on a long voyage, and fumigated containers must be checked even when the warning label is missing.
5. Before opening, tap the flat surface of both steel doors. The Code says a dull sound with no ring means cargo is probably resting against the door. Then open slowly, with a safety strap around the inner locking rods or a safety chain across the doors, so the first door cannot swing open freely.

Check before you close the doors

This is the list I would give a loading supervisor. Each step takes minutes.

1. The total gross mass is weighed and declared. Under the SOLAS rules in force since 1 July 2016, the verified gross mass is a condition for loading a packed container onto a ship (IMO). If you weigh the parts instead of the whole container, the IMO counts pallets, dunnage and securing material in that weight too.
2. The centre of gravity has been checked with the formula above.
3. Voids are measured with a tape, and no direction adds up to more than 15 cm.
4. Every dunnage bag is at the manufacturer's recommended pressure and is not touching the doors.
5. The door face is firm, or fenced, or strapped back.
6. Photos are taken at the half-way stage and at the door, before sealing.

If I had to keep only one habit from this list, it would be the photos. If there is ever a claim, they are your evidence.

Send us your load plan and a few photos through our contact page, and we will tell you where we see a gap. Sometimes it is a 20 cm void that one empty pallet would have filled.