Containers are built to stack — they cross oceans six and seven high. Almost everything that goes wrong with a stack on dry land is about the ground under it, not the boxes on top of it.
We get asked this most often by transport yards, laydown operators and blokes on industrial blocks around the bayside who are out of room sideways and looking up. The short answer is yes, containers stack. The longer answer is that a stack is only ever as good as what it's standing on, and that a container sitting on a ship is in a very different situation to one sitting on your yard.
The corrugated walls of a container carry almost nothing vertically. They keep weather out and they stop the box racking sideways. The load path is the four corner posts and the eight corner castings — those steel blocks with the oval holes at every corner. Everything a container is rated to lift, carry or stack goes through those.
Two consequences fall straight out of that, and they're the two rules that matter:
Corners must sit directly on corners. Offset a top container so its castings land on the roof panel of the one below, and you're loading a sheet of steel that was never meant to carry anything but rainwater and a bloke's boots. The roof will dish, and then it ponds, and then it rusts from the inside of the dish outwards.
Heavy on the bottom, light on top. The bottom unit carries everything above it through its corner posts. That's what the posts are for — but it's also why nobody stacks a loaded 40-footer on top of an empty one and walks away.
The fittings that go between the castings of two stacked containers. Cones locate the top box so it can't walk; twist-locks locate it and lock it down so it can't lift. They're cheap, they're the difference between a stack and a pile, and they're the first thing an insurer or a WHS inspector looks for.
If containers are going two high anywhere people work, park or walk, they should be locked, not just placed. It's not a big cost and there is no good argument against it.
This is what actually bites people, and it's why a stack that would be routine at a port can be a bad idea on a five-acre block.
A ship's deck and a hardstand yard are flat, engineered and don't move. A paddock, a gravel pad or a slab poured for something else are none of those things. Concentrate the weight of two containers onto four small footprints and the ground under one corner settles a bit more than the ground under the others. Now the bottom container is being twisted along its length — and a container that's out of square stops doing the one thing it's brilliant at. The doors go first: they bind, then they won't latch, then they won't shut. Once a stacked unit has racked, unstacking it doesn't necessarily bring it back.
So before anything goes two high: firm ground, level across all four corners, and the load spread — concrete pads, heavy sleepers or steel plates under each casting, not milk crates and good intentions. Check it again after the first wet season, because that's when soft spots announce themselves.
An empty container is a big steel sail with very little holding it down. Loaded, weight does the work. Empty and up high in a coastal blow, it's a different proposition — which is another reason for twist-locks rather than gravity, and a reason to think hard before stacking empties near a boundary, a shed or somewhere people park.
Placing a container on top of another one is a lift, not a slide. A tilt-tray can put a container on the ground; it can't put one on top of a second one. That's crane work — a crane truck with the reach and the room to swing, which usually means more clear space than a ground-level delivery needs, and an operator who does this for a living.
Access decides feasibility far more often than weight does. If you're weighing up a stack, take a couple of photos of the spot and the approach and send them through before you order anything — it's the cheapest thirty seconds in the whole exercise. There's more on truck types and access on our deliveries page.
Some of this stops being a storage question and becomes a structural one. Get proper advice when: people will work, sit or walk underneath; the stack is part of a structure rather than storage; you're going three high or higher; it's a workplace and WHS obligations apply; or you're on soft, filled or sloping ground and you're not certain what's under it. That's an engineer's sign-off, and it's cheap next to the alternative.
Here's what we tell about half the people who ring about stacking: two 20-footers side by side, doors facing the same way, beats one on top of the other for nearly every ordinary storage job. Everything stays at ground level, everything's reachable without a ladder, nothing needs a crane, nothing needs locking down, and the ground work is far more forgiving. Stacking earns its keep when you're genuinely out of footprint — a tight industrial block, a yard where the fence line is the fence line. Not before.
If you're thinking about going up, ring 0418 179 002 and describe the site. We'll tell you straight whether it's a stack, a second unit alongside, or a conversation with an engineer first — and we'd rather talk you out of it now than sort out a racked container later. Or see the full container range if you already know what you're after.
One call, one delivered number, seven days a week.