Container Loading and Dunnage: Securing Cargo by the CTU Code

A container can arrive with damaged cargo even when nothing unusual happened at sea. The cause often lies in the way the container was packed: a gap left at the door, a heavy crate standing on a small patch of floor, a dunnage bag inflated against an uneven surface, or a pallet that was wrapped but never blocked. By the time the doors are opened at the destination, the cause is hard to prove and the claim is hard to win.

This guide is for factories, exporters and logistics teams that pack containers in Vietnam and elsewhere in Asia, or that buy the securing materials for someone else to use. It follows the international code of practice for packing containers, explains which materials do what, and gives a checklist you can use at the loading bay.

💬 Need dunnage bags, lashing straps or timber for loading? Send us your cargo types, container sizes and volumes, and we will look for suppliers in Asia that fit. Tell us what you load.

The reference: the CTU Code

The IMO/ILO/UNECE Code of Practice for Packing of Cargo Transport Units (CTU Code) is the international, non-mandatory code for packing and securing cargo in containers and other transport units; use it as the basis of your packing instructions.

The CTU Code was approved in 2014 by the IMO Maritime Safety Committee, the UNECE Inland Transport Committee and the ILO Governing Body, and published as MSC.1/Circ.1497. IMO describes it as a non-mandatory global code of practice. It gives advice to those who pack and secure cargo and to everyone in the supply chain up to unpacking. It says it is not intended to conflict with or replace existing national or international regulations, so national road, rail and port rules still apply. Its informative material is published separately as MSC.1/Circ.1498.

The Code’s key requirements for securing are short:

• fill void spaces when necessary;
• use blocking, lashing or a combination to prevent sliding and tipping in any direction;
• distribute securing forces over an appropriate area;
• use non-slip material where appropriate;
• do not overstress the cargo, the securing devices or the container structure;
• do not fasten web lashings by means of knots.

It also says to determine the correct gross mass of the unit on completion of packing, and that the verified gross mass should be communicated to the carrier as early as the carrier requires.

Forces: what the securing has to hold

Design the securing for the accelerations of the whole route, not for the weight of the cargo alone, because friction and weight do not stop cargo moving under vibration and impact.

The CTU Code expresses transport forces as fractions of gravity (g). Its tables give, among others, these horizontal acceleration coefficients:

The Code also gives rail values and vertical coefficients, and allocates sea areas to wave-height classes in a separate table. Use the full tables from the Code for your calculation.

Two consequences matter for loading teams. First, the forward value for road transport (0.8 g) is as high as the worst sea-area value sideways, so the front of the load needs as much attention as the sides, even for a sea shipment that starts and ends by truck. Second, the Code states that the mass of the cargo alone, even with a high friction coefficient, does not sufficiently secure cargo, because it can move under vibration. Where cargo cannot be secured by blocking, lashing is required.

Blocking and bracing: fill the space

The simplest securing is a tight stow against the container walls with remaining gaps filled; the CTU Code limits the sum of unfilled gaps in any horizontal direction.

Blocking, bracing or shoring means filling gaps between cargo and the solid boundaries of the container, or between packages, so forces pass through compression. The CTU Code lists timber beams and frames, empty pallets and dunnage bags as typical materials. Its practical points include:

• Stow tight to both sides where the walls are strong, leaving any remaining gap in the middle, so sideways forces from only one side are transferred at a time.
• Small gaps are acceptable. Small gaps that cannot be avoided and are needed to pack and unpack need not be filled, but the sum of void spaces in any horizontal direction should not exceed 15 cm. Between dense, rigid items such as steel, concrete or stone, gaps should be smaller still.
• Fill with material that holds. Vertically inserted empty pallets with timber battens work; rags or weak foam that deform or shrink permanently should not be used.
• Nail only to dunnage. Wedges should be nailed to dunnage or timbers placed under the cargo, not to the floor of a closed container, which is generally not suitable for nailing.
• Heavy items on small footprints need their load transferred to the container’s bottom girders, for example through timber bearers.

Any timber used for blocking, bracing or dunnage is wood packaging under ISPM 15 and must be treated and marked. The CTU Code says timber dunnage, blocks and braces should be checked for the IPPC mark. Our guide to export pallets and ISPM 15 explains the mark.

Dunnage bags

Dunnage bags (air bags) fill gaps between cargo blocks; follow the maker’s limits on gap and pressure, keep them away from the doorway unless the door is protected, and rate them with the CTU Code’s method.

The CTU Code gives clear rules for dunnage bags:

• Follow the manufacturer’s instructions on filling pressure and maximum gap.
• Do not use them to fill the space at the doorway unless precautions ensure they cannot cause the door to open violently.
• If the surfaces are uneven and could chafe or pierce the bag, take suitable measures to smooth them first.
• Estimate the blocking capacity by multiplying the nominal burst pressure by the contact area on one side, then by a safety factor of 0.75 for single-use bags or 0.5 for reusable bags. Appendix 4 to Annex 7 gives the full calculation.
• On unpacking, open the bags and release the air before removing other securing, and do not reuse a bag that cannot be safely reinflated.

A footnote in the Code also says air-inflated dunnage bags should not be used for dangerous goods on US railways. Check the rules for your route if you ship dangerous goods.

When you buy dunnage bags, specify size, ply construction (paper or woven polypropylene), the maker’s rated burst pressure and maximum gap, valve type, and whether they are single-use or reusable. Ask for the test basis behind the rating.

💬 Comparing dunnage bags or lashing straps from several makers? We can collect samples on one specification and the makers’ test data, and compare them with you before you change supplier. Ask about securing materials.

Lashing: points, straps and pre-tension

Lashings must be rated, attached to rated points and tensioned within limits; know the capacity of the container’s lashing points before you plan the lashing.

The CTU Code gives the capacity of standard general purpose containers. Where lashing rings are fitted, bottom anchor points have a maximum securing load (MSL) of at least 10 kN in any direction, and many recently built containers have anchor points of 20 kN. Lashing points on the top side rails have an MSL of at least 5 kN. The container operator can give exact values.

For web lashings the Code says:

• the permitted securing load is generally labelled and certified as lashing capacity (LC), which corresponds to MSL;
• the MSL of single-use web lashings is 75 % of breaking strength, and of reusable web lashings 50 %;
• elastic elongation at MSL should not exceed 9 %;
• knots reduce their strength considerably and should not be used;
• they should be protected against chafing at sharp corners and against chemicals.

For direct lashings, the Code says pre-tension should never exceed 50 % of the lashing’s MSL, keep lashings as short as practicable, and run them as close as possible to the direction of the restraint needed. For friction (tie-down) lashings, synthetic web lashings are preferred to chains or steel band because they hold pre-tension when the cargo settles.

Wrapping and strapping the load on the pallet is a separate step. The Code treats it as an auxiliary measure that must be combined with blocking or lashing; see our guide to stretch film and strapping.

A loading-bay checklist

Plan the stow before the container arrives, check the container on arrival, load to the plan, and record the result with photographs before sealing.

Bulk bags and machinery cases need their own planning. See our guides to FIBC bulk bags and export crates for heavy goods. When several suppliers’ goods share one container, our article on consolidating Asian suppliers in one shipment covers the planning.

Frequently asked questions

1. Is the CTU Code mandatory?
IMO describes it as a non-mandatory code of practice. National road, rail and port rules, dangerous goods regulations and your contracts may make parts of it binding in practice.

2. Can I leave a gap at the container door?
The Code allows small unavoidable gaps, with the sum in any horizontal direction not exceeding 15 cm. Dunnage bags should not fill the doorway unless the door is protected from opening violently.

3. How strong are the lashing points in a standard container?
Under the CTU Code, bottom anchor points where fitted have an MSL of at least 10 kN, often 20 kN on recent containers, and top side-rail points at least 5 kN. Ask the container operator for exact values.

4. Are dunnage bags reusable?
Some are designed for reuse. The Code applies a lower safety factor to reusable bags and says not to reuse a bag that cannot be safely reinflated.

5. Do you load containers for us?
We arrange securing materials and work with packers and forwarders in Vietnam and Asia to load to a plan, inspect the loading and send you photographs before the container is sealed.

Summary

Use the CTU Code as your reference: design for the route’s accelerations, stow tight and fill gaps, use rated dunnage bags and lashings within their limits, check timber for the ISPM 15 mark, and record the result.

Sunsho Trade is a trading company. We look for and vet makers of securing materials in Vietnam and Asia, supply them on one specification, and coordinate loading inspections with packers and forwarders. The CTU Code has been under review at UNECE; check whether a revised edition applies to your shipment.

💬 Ready to source loading materials? See our industrial packaging and logistics materials service, or send us your loading plan. For other plant supplies, start from industrial sourcing.

Sources

Checked on 27 October 2026.

• IMO, IMO/ILO/UNECE Code of Practice for Packing of Cargo Transport Units (CTU Code) page: https://www.imo.org/en/OurWork/Safety/Pages/CTU-Code.aspx
• CTU Code text, MSC.1/Circ.1497 (16 December 2014): chapter 1, chapter 3, chapter 5 (acceleration coefficients), chapter 6 (container lashing points), Annex 7 sections 2 and 4 and Appendix 4 (dunnage bags), as reproduced by the Hong Kong Marine Department: https://www.mardep.gov.hk/en/msnote/pdf/msin1669anx1.pdf
• IPPC, ISPM 15 Regulation of wood packaging material in international trade (adopted 2018, published 2019): https://www.ippc.int/en/publications/640/