An open top container has a removable tarpaulin roof, so cargo can be lowered in from above by crane or loaded with a high-reach forklift. It is the go-to box for machinery, tall crates, stone and other heavy items that will not pass through a door. The floor is almost the same as a dry container, but the inside height is lower and the loading routine is quite different.

20OT and 40OT dimensions

In the ISO type code an open top carries the letter U, for example 22U1 for a 20-footer and 42U1 for a 40-footer. Typical values from the Stowage.id catalogue:

Specification (typical values) 20OT (22U1) 40OT (42U1)
Inside dimensions (L × W × H) 5,898 × 2,352 × 2,348 mm 12,032 × 2,352 × 2,348 mm
Door opening (W × H) 2,340 × 2,280 mm 2,340 × 2,280 mm
Internal volume 32.6 m³ 66.4 m³
Tare 2,350 kg 3,850 kg
Max gross mass 30,480 kg 30,480 kg
Max payload 28,130 kg 26,630 kg

Compared with a 20ft dry container, which is 2,393 mm high inside, an open top loses a little height to its top rails and roof bows. Units from different owners vary, so check the CSC plate and the door markings.

Tarpaulin, roof bows and the door header

Three components set an open top apart from an ordinary box:

  • Tarpaulin. A PVC (or similar) cover tied down with a rope through eyelets along the top edge. The rope is normally threaded through customs fittings (a TIR cable) so the container can be sealed.
  • Roof bows. Transverse bars that hold the tarpaulin up so it does not sag or collect rainwater. They come out for top loading and go back in afterwards. Some units stow the bows inside the container, and that space has to be allowed for in the plan.
  • Swing-away door header. On many units the top beam above the doors can be swung or removed, so a tall item can be pushed in through the doors by forklift and the header refitted.

Inspect the tarpaulin before stuffing. A small tear can let water in for weeks at sea.

How to load from the top

Top loading needs more planning than ordinary stuffing:

  1. Remove the tarpaulin and roof bows on level ground, away from strong wind.
  2. Lay out timber and dunnage on the floor where the cargo will sit, so the weight is spread and the crane slings can be pulled out after the item is landed.
  3. Lift with the right gear. Use a spreader bar or slings on the manufacturer's marked lifting points, and keep slings clear of the container's top rails.
  4. Centre the item lengthwise and across the width; see centre of gravity and weight distribution.
  5. Secure the cargo before refitting the bows and tarpaulin.

Open tops have lashing points on the bottom rails and often on the top rails too. Heavy cargo must never rely on friction alone: combine timber blocking, bracing and lashing with a suitable MSL rating. The principles are covered in the cargo lashing and securing guide, and the Lashing Calculator gives a first estimate of how many lashings you need.

Over-height and out-of-gauge cargo

Cargo taller than the container walls is over-height and counts as out-of-gauge (OOG). What that means in practice:

  • The tarpaulin cannot be closed normally, so the cargo needs its own weather protection (a separate cover or shrink wrap), and the carrier must accept that condition.
  • Nothing can be stacked on top, so the vessel loses slots. That is why OOG surcharges are common; amounts vary, so check with the carrier.
  • Stowage is usually limited to the top tier on deck or to dedicated positions.
  • The booking must state the actual dimensions and by how many millimetres the cargo projects.

If the cargo is also wider than the container, a flat rack container is usually the better fit.

Open top, dry box or flat rack?

The right choice depends on the shape of the cargo and the handling equipment at both ends, not just on size. A rough guide to start the conversation with your forwarder:

  • Dry container when the cargo goes through the doors by forklift or pallet jack and stays below roughly 2.2 m overall. Rates and equipment availability are usually best.
  • Open top when the cargo fits within the container width but is too tall or too heavy for the doors, or has to be lifted by crane because it has no forklift pockets.
  • Flat rack when the cargo is wider than the container, very long, or must be loaded from the side.

Availability matters too. Open tops and flat racks are special equipment with fewer units in depots, especially at ports outside Java. Book early and confirm that the discharge port has a crane able to lift the cargo out. Cargo craned in at an Indonesian factory but meant to be unloaded by forklift at destination is a classic problem that only shows up on arrival.

Worked example: a machine crate in a 20OT

A factory in Cikarang exports one machine crate of 3,200 × 2,100 × 2,200 mm weighing 7,800 kg. It is too tall to drive through the doors on a forklift, so the shipper books a 20OT and a crane lift.

  1. Space check: width 2,352 − 2,100 = 252 mm of side clearance (about 126 mm each side when centred); height 2,348 − 2,200 = 148 mm. The crate fits inside, no OOG.
  2. Weight check: 7,800 kg plus about 150 kg of dunnage and lashing = 7,950 kg, far below the 28,130 kg payload.
  3. Floor pressure: a footprint of 3.2 × 2.1 = 6.72 m² averages about 1,161 kg/m². If the crate stands on two narrow skids, spread the load with timber beams and check the carrier's concentrated-load guidance.
  4. Estimated VGM: 7,950 + 2,350 kg tare = 10,300 kg.
  5. Variation: if the crate were 2,600 mm tall it would rise about 2,600 − 2,348 = 252 mm above the inside height. That is over-height; ask the carrier how it measures the excess (from the floor or from the top of the container).

These figures are a simulation; the official weight still comes from weighing or an approved VGM method. Check the remaining weight capacity with the Container Payload Calculator.

Common mistakes

  • Leaving roof bows out, so the tarpaulin sags, tears or pools water.
  • Lashing to roof bows or tarpaulin fittings; they are not cargo securing points.
  • Assuming an open top takes wider cargo; the side walls are still there.
  • Not declaring over-height on the booking, so the terminal refuses the box at the gate.
  • Concentrated loads without dunnage, such as a machine on small feet that damages the timber floor.
  • Planning right up to the height limit without allowing for the roof bows under the tarpaulin.

Plan your open top load

Check the unit's code with the ISO Type Code tool, then simulate the crate position, clearances and centre of gravity in the 3D Load Planner. A planner layout is a simulation and does not replace a cargo survey, official weighing or carrier and operator approval.