Ask how many layers a box can be stacked and the answer usually quotes the load rating of the bottom box. That is one of two limits, and for a tall, light load it is often not the binding one. The other limit is stability: at some height the load’s centre of gravity sits high enough that ordinary braking, or a fork taken round a corner too quickly, will tip it.
The two limits, separately
The first limit is structural. The bottom box carries the weight of everything above it, plus a dynamic allowance for the road, and its walls have to take that without buckling or slowly creeping. The second is geometric. A load tips when its centre of gravity passes outside its base, and the higher that centre sits, the less sideways acceleration it takes to get there.
Where each one bites
Dense loads hit the structural limit first, because the bottom box is overloaded long before the stack is tall enough to be tippy. Light, bulky loads are the reverse: they reach the tipping limit while the bottom box still has most of its capacity unused. Working out which regime a part is in tells you which number matters, and it genuinely changes the box you should buy.
The dynamic allowance
A static rating is measured with the load sitting still. In transit the load sees vertical accelerations that raise the effective weight on the bottom box, and the usual way to handle that is a factor rather than a fixed margin. Where a specification quotes a stack rating, ask whether it is static or dynamic; the gap between the two is where most stack failures live.
What the wrap is contributing
Wrap does not make a load stronger in compression. What it does is hold the boxes together so they move as one mass rather than as separate columns, and that resistance to small movements is what stops a tip from starting. A badly wrapped load leaves every box free to lean on its own, and the tipping limit arrives lower than the geometry suggests it should.
Practical height caps
Most operations settle on a stack height set by the vehicle, the rack or a doorway rather than by the box, which is fine provided the box rating has been checked against the resulting load with a dynamic allowance included. Where it has not, the first tall stack is also the first test, and it tends to happen during the busiest week of the quarter.
What to measure after the first legs
Three things: whether the bottom box has taken a permanent set, whether the stack has leaned, and whether the wrap has cut into the corners. Each takes seconds to check on arrival, and together they say whether the height is safe. Where one of them is wrong, the fix is one fewer layer rather than a different box.
Turning it into a number
Work out the loaded box weight, apply the route factor, and divide the bottom box rating by the result. Round down. That is the total number of layers, and it is usually lower than the number that fits in the vehicle, which is the useful piece of information.
Where the stack is taller than the layer limit, the options are a stronger box or fewer layers – and the second is free. The centre-of-gravity check is what tells you whether the height is a stability problem rather than a strength one.
What we can do
We build PP corrugated turnover boxes and we quote both the static and the dynamic stack rating, stating the allowance we used. Send us the part weight and the height you need and we will tell you where the limit falls. The securing side is covered in where to put the strap.