Loading through the roof
Open tops can be loaded through the roof by crane or through the doors, and many have a door header that swings aside so tall items clear the opening. After loading, the roof bows go back and the tarpaulin is lashed down along the top rails. The type suits heavy or awkward cargo such as machinery, stone blocks, sheet glass and long timber that would be hard to forklift through the doors.
In gauge or over height
If the cargo stays below the top rails, an open top moves much like a standard box. If it rises above them, the load is out of gauge: the carrier must approve it, usually stows it on top of a stack so nothing sits on it, and may charge for the slots it blocks. Over-height bookings can be rolled when space is tight, and each handoff, including transshipment and the road leg, needs extra planning.
Booking scarce special equipment
Open tops are special equipment, so carriers keep fewer of them and availability varies by port. Book early, give exact cargo dimensions and weights, and send lifting and lashing plans if the carrier asks. In ISO 6346 size-type codes, an open top carries a U, as in 22U1 for a 20 ft unit or 42U1 for a 40 ft one, which helps confirm the right box was released.
Example
A machinery exporter lowers a press into a 40 ft open top by crane, and because the press rises above the top rails, the booking is handled as over-height cargo that needs the carrier's approval.
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Official sources
Questions
What does Open Top Container mean?
An open top container has no solid roof: removable roof bows and a tarpaulin cover the top instead. It lets tall cargo, or cargo that is easier to load from above, be lowered in by crane.
What is the difference between an open top and a flat rack?
An open top keeps its side walls and end walls and only lacks a rigid roof. A flat rack has end walls but no sides or roof, so it suits cargo that is too wide as well as too tall.
Is an open top container watertight?
Not fully. A well-fitted tarpaulin keeps most weather out, but it does not seal like a steel roof, so moisture-sensitive cargo needs extra protection.