Dunnage Bags: Filling the Gap That Wrecks Freight

Every damage claim that crosses a dispatch desk with the phrase "shifted in transit" started as a measurable gap. Somebody loaded two pallet rows with fifteen centimetres between them, closed the doors, and handed the freight a runway. Three hundred kilometres later a cloverleaf ramp at highway speed collected on that decision, and the receiver photographed the result before the driver had the second door open.

Void space is not a cosmetic problem. It is stored energy waiting for an input. A pallet with room to accelerate arrives at whatever is in front of it carrying momentum it never should have had, and the difference between a clean delivery and a claim file is usually two minutes of air that nobody wanted to spend at the dock.

Why does void space wreck freight when the load looked fine?

The securement standard is written around honest numbers: restraint has to hold cargo through 0.8 g of forward deceleration — a genuine panic stop — plus 0.5 g rearward and to each side. Palletized goods sitting on a smooth trailer floor do not come close to resisting that on friction alone, and the loaded trailer works against itself all day. Vibration walks pallets. Shrink wrap relaxes. Cartons settle and compress. A load that was tight at the shipper is measurably looser two hundred kilometres later, and every centimetre that opens gives the freight a longer run at the next hard input.

That is why lateral events do more damage than braking events on properly blocked loads. A hard stop pushes freight into freight; a highway ramp taken at posted speed pushes freight sideways, where nothing is holding it and where a top-heavy pallet only needs to lean far enough for the wrap to fail. Side-by-side pallets with a centre void are the classic setup: two rows that lean into the same empty space, then into each other.

Concealed damage is the version of this that costs carriers the most, because nobody sees it at delivery. The pallet gets signed clean, broken down at the consignee two days later, and the carrier spends five weeks arguing over whose handling crushed the bottom-row cartons. Filling voids does not just prevent the visible avalanche — it prevents the argument you cannot win with a signature.

What does a dunnage bag actually do?

A dunnage bag is a bladder that converts empty space into a load-facing surface. Inflated between two pallet faces, it spreads force across the freight instead of concentrating it, holds both rows in position, and absorbs vibration that would otherwise walk them apart. It is blocking and bracing done with air rather than lumber, which is why it took over rail and intermodal loading decades ago and then moved into dry van work.

Bags are rated by level — a strength classification tied to how much load the bag is expected to restrain and how wide a void it can span. Level 1 is the common dry-van and LTL grade for palletized consumer goods and mixed freight. Heavier grades exist for rail and dense industrial loading. A poly-woven bag resists puncture better than paper-ply, which matters when the void is lined with pallet splinters, banding ends, and the occasional protruding nail. The rating is not marketing: a bag asked to hold more than its level, or to span a void wider than it was designed for, fails by bursting or by simply rolling out of the gap under pressure.

How do you size and place a bag without guessing?

Measure the void, not the trailer. Bag size is chosen so the inflated bag fills the gap under modest pressure rather than being stretched to its limit — a bag straining to bridge a void that is too wide is a bag that will find the weakest point of its seam somewhere in northern Ontario. If the gap exceeds what a single bag comfortably spans, the answer is not more air. It is a second bag, a piece of blocking, or repositioning the freight so the void closes.

Placement follows the freight's centre of mass. On standard palletized rows, the bag sits vertically centred in the void with roughly two-thirds of its height against the upper portion of the load — that is where the leverage is when a pallet wants to tip. Keep the bag off the floor edge, off sharp corners, and away from banding. On mixed-height loads, that often means a bag placed against the taller neighbour with a filler on the short side rather than one bag doing both jobs badly.

Inflation discipline separates operators who use bags from operators who buy them repeatedly. Inflate until the bag is firm and the load faces are held — not until it is drum-tight. Overinflation is the single most common failure: it stresses seams, crushes the carton faces you were trying to protect, and leaves no margin for the air expansion that happens when a trailer sits in the sun or crosses a mountain pass. Underinflation is the other half of the same coin, leaving a bag that slumps out of the gap on the first hard input and lands on the floor looking useless, which it now is.

Deflate deliberately at the receiver. Slashing a bag is fast, ruins a reusable item, and puts a driver's hand near a knife inside a trailer full of leaning freight. Use the valve, let it dump, fold it, and put it back in the nose. Inspect on the way in: a bag with a pinhole, a nicked seam, or heat damage from a reefer duct is dunnage for the garbage, not for the next load.

Bags or bars — which one does this load actually need?

The division is geometric, not preferential. A bag needs two surfaces to push against, so it lives inside the load: the centre gap on side-by-side pallets, the mismatched-depth gap mid-trailer, the space left when a stop comes off in the wrong order. The back of the load has only one surface, which is exactly where a ratcheting load bar earns its keep — jammed wall-to-wall across the last row, holding the door area that no bag can address.

The honest caveat on both: neither is a rated tie-down. Bags and bars are blocking and bracing, and neither contributes working load limit to the securement math. Heavy, dense, or high-value freight gets rated straps to anchor points on top of whatever fills the voids. When a load does move, the first question an adjuster asks — and the theme running through cargo shifted in transit — is whether the restraint matched the weight of the freight, not whether something was present.

Where do bags belong in a reefer or a partial?

Reefers add one rule that dry vans do not have: nothing may dam the return-air path. A bag pinned between a pallet and the sidewall in a refrigerated trailer can block the airflow the temperature control depends on, and a temperature claim is materially worse than a scuff claim. Keep bags in the interior voids between rows where they belong, stay off the sidewall channels and the bulkhead, and load with the airflow discipline described in the reefer pre-cool and loading guide. Cold freight also means condensation, which means valve stems and seams want a look before reuse.

Partials and multi-stops are where the discipline slips, because the load geometry changes at every door. The freight that was blocked perfectly at the shipper becomes a partially emptied trailer with new voids after the first delivery, and the driver who is behind schedule re-closes the doors on a gap that did not exist that morning. That is the load that generates the claim, and it is why the bags and the bars both live at the nose where they can be reached at every stop rather than buried under the last pallet.

What does a five-minute void check prevent?

The recurring dry-van claim file is boring and expensive: crushed carton faces where a row accelerated into the row ahead, toppled top stacks that leaned past their wrap, the door avalanche that greets a receiver and doubles as a safety incident, and the concealed-damage dispute that arrives by email a week after a clean signature. Every one of those begins with a gap somebody decided not to fill.

The desk rule is simple enough to hand a new driver: before the doors close, walk the load and look for daylight. Any daylight between rows gets a bag, any daylight behind the last row gets a bar, and anything heavy or dense gets rated straps regardless of what else is in the box. Then note it — a photo of the loaded, blocked trailer taken at the shipper is the cheapest evidence a carrier can generate, and it is the document that decides who pays when the receiver's photos arrive.