Introduction
Almost every container leaves the factory with empty space in it. Gaps between pallets, between the last row and the doors, and between cargo and the side walls. During a voyage, those gaps turn into runways. Cargo gains speed across them and hits whatever is next.
Exporters usually reach for one fix: dunnage bags or straps. The right answer is often both, used for different jobs. This guide explains what each does, when each works, and how to fill voids properly. It comes from our 14 years supplying lashing products from Rajkot. We cover comparison, use cases, a step-by-step method, and common mistakes.
Why Voids Matter
A void is any space that lets cargo move. Even a 10 cm gap lets a pallet build speed before it hits the next one.
What Happens at Sea
- Ships roll sideways and pitch forward and back
- Containers on trucks and trains face sudden braking
- Each movement pushes cargo into the gap
The damage usually shows up at the destination: crushed cartons, broken pallets, cracked tiles, and load pressing against the doors.
What Dunnage Bags Do
Dunnage bags are inflatable bags placed in gaps. Once inflated, they press against the cargo on both sides.
Strengths
- Fill gaps between pallets quickly
- Spread pressure over a wide surface
- Work well for sideways and lengthwise gaps
Limits
- Need a solid surface on both sides
- Can puncture on sharp edges
- Do not hold cargo down or tie it to the container
- Should not be placed against the container doors
What Cord Strap Lashing Does
Cord strap lashing ties cargo together or anchors it to container lashing points.
Strengths
- Holds cargo to the container structure
- Unitises pallets so they move as one block
- Controls tipping as well as sliding
- Works on irregular cargo shapes
Limits
- Does not fill space
- Needs lashing points or a stable anchor
- Must be tensioned properly to work
Dunnage Bags vs Cord Strap: Comparison
| Factor | Dunnage Bags | Cord Strap Lashing |
| Main job | Fill gaps | Tie and anchor cargo |
| Controls sliding | Yes, by blocking | Yes, by restraint |
| Controls tipping | Limited | Yes |
| Needs lashing points | No | Usually |
| Works on irregular shapes | Poorly | Well |
| Best gap size | Medium gaps between solid faces | Any, when combined with blocking |
When to Use Each
Use Dunnage Bags When
- You have gaps between uniform pallets or cartons
- Cargo faces are flat and solid
- The gap is too wide to close by shifting cargo
Use Cord Strap When
- Cargo is irregular, heavy, or tall
- The cargo can tip
- You need to tie the load to the container
- You want pallets to act as one block
Use Both When
- The container is not fully loaded
- Cargo is heavy and mixed
- The last row sits near the doors
Using Dunnage Bags and Cord Strap Together
A common, effective approach:
- Load cargo tight to the front wall.
- Unitise each row or stack with cord strap.
- Fill sideways gaps with dunnage bags.
- Fill the gap near the doors with timber bracing or a strapped door barrier.
- Anchor heavy units to lashing points with cord strap.
The cord strap keeps the cargo together. The dunnage bags stop the block from moving.
Here is a point many loaders miss. A dunnage bag pressing against a loose stack of cartons often pushes the top cartons over. Strap the stack first, then fill the gap.
Step-by-Step: Filling Voids in a Container
Step 1: Plan the Load
Sketch where pallets go. Aim for minimal gaps before you start.
Step 2: Load Tight
Push cargo against the front wall and side walls where possible.
Step 3: Unitise
Strap pallets, crates, or bundles into stable units using cord strap and wire buckles.
Step 4: Measure Remaining Gaps
Check each gap. Choose the right dunnage bag size for the gap width.
Step 5: Place and Inflate
Place bags between solid faces. Add cardboard or board if a surface is uneven. Inflate to the recommended pressure.
Step 6: Secure the Door End
Never rely on dunnage bags against the doors. Use bracing or a strapped barrier tied to the lashing points.
Step 7: Final Check
Push on stacks. Nothing should move by hand.
Mistakes to Avoid
Using Bags Instead of Lashing
Bags block sliding. They do not stop tall cargo from tipping.
Overinflating
Too much pressure can crush cartons or burst bags.
Placing Bags in Very Wide Gaps
Huge gaps need blocking or extra cargo, not one large bag.
Lashing Without Tension
A slack strap lets cargo move before it starts to hold.
Bags Against Doors
The doors are not built to carry cargo pressure.
A Worked Example: Half-Filled 20ft Container
Say you ship 10 pallets of tiles in a 20ft container. Ten pallets fill two rows of five, but they do not reach the doors. You are left with:
- A gap of about 15 cm along one side wall
- A gap of about 1.2 m at the door end
The Fix
- Strap each pallet with cord strap, at least two straps each way.
- Strap each row of five pallets together into one block.
- Place a dunnage bag in the side-wall gap.
- Build a timber frame or strapped barrier at the door end, anchored to floor lashing points.
The side gap is small and between solid faces, so a bag works well. The door gap is too wide for a bag and sits against the doors, so it needs a barrier.
Choosing Between Bags, Timber, and Strap
| Gap Type | Best Option |
| Narrow gap between pallets | Dunnage bag |
| Wide gap mid-container | Timber bracing or extra bags with board |
| Gap at the doors | Strapped barrier or timber frame |
| Tall or loose stacks | Cord strap first, then fill |
| Heavy single units | Blocking plus cord strap to points |
What Surveyors Look For
When a lashing surveyor checks a container, they usually look for three things: no free movement by hand, no load on the doors, and lashings that are tensioned and buckled. A container that passes all three rarely produces a damage claim from load shift.
FAQs
Can I use only dunnage bags with no strapping?
For uniform, tightly packed pallets, bags may be enough to stop sliding. For heavy, tall, or irregular cargo, you still need lashing.
What gap size suits a dunnage bag?
Bags are made for specific gap ranges. Pick the bag size that matches your gap, and avoid very large gaps.
Is cord strap enough to fill voids on its own?
No. Cord strap restrains cargo but does not fill space. Combine it with bags or blocking.
Do dunnage bags damage cargo?
They can if overinflated or placed against fragile surfaces. Use the right pressure and a protective board.
Conclusion
Dunnage bags fill space. Cord strap holds cargo together and to the container. Most well-secured containers use both. Before your next shipment, review your load plan and see where each product fits. If you need help, send us your container layout and cargo details.
About Amass Strapping Solution
Amass Strapping Solution, Shapar, Rajkot, has manufactured cord polyester strapping, container lashing strips, and wire buckles since 2012. We help exporters build lashing systems that keep cargo in place from loading bay to destination.