A box sitting on a pallet is supported across its whole base. The same box in pallet racking is supported on two beams at two edges, with a gap in the middle, and everything above it is hanging off those two bearing points. That is a completely different load case and it is the reason boxes that work perfectly on a pallet sag when they go into a rack.
The base is the whole question
What matters is whether the base spans the gap between the beams without deflecting into it, and whether the two bearing edges can carry the load without crushing. A flat, ribbed PP base will span a standard beam gap at moderate loads; at higher loads it will dish into the gap, and once it dishes the contents shift and the box stops locating properly in the stack above. If the loads are real, put the boxes on a pallet or a rack deck rather than directly on beams.
Beam gap and bearing width
Check two dimensions before ordering: the clear gap between the beams and the width of the bearing surface each beam gives. A box needs the majority of its base length supported, and the bearing edges need to land on the flat part of the base rather than on the corner radius. If the beam gap is wide relative to the box, use a rack deck – a sheet of corrugated PP or a purpose-made deck panel – which converts the rack into a pallet and removes the whole problem for a few pounds per position.
Deflection over time, not at first
The failure is not immediate. A box goes into a rack on Monday within tolerance and is still there on Friday, and PP creeps under sustained load. Over a week of standing load the base takes a permanent set, and the box that was fine on Monday no longer locates in a stack. If boxes are going to live in racking rather than pass through it, specify for sustained load rather than for transit load, and ask the supplier for a creep figure rather than a momentary one. The trade-off is discussed for a larger container in whether sleeve boxes go in pallet racking.
When the answer is a rack deck
A rack deck – a sheet that converts two beams into a shelf – costs a few pounds per position and removes every load-case problem in this article at once. The box sits on a continuous surface again, the load is spread, the deflection disappears, and boxes that would not have survived racking at all become entirely serviceable in it. Against a heavier box, a deck is usually the cheaper answer.
It is also worth considering a purpose-made deck in corrugated PP rather than a steel panel: it is lighter to handle during reconfiguration, it does not rust, and if it is damaged it is replaced for a fraction of the cost. The constraint is load rating, and that should be checked against the heaviest box the position will ever see rather than the typical one. Where the racking is adjustable and the layout changes regularly, a light deck is also far easier for one person to move than a steel panel.
Loaded boxes that stand for months
A box loaded and put into racking for a week is one load case. A box loaded and left there for six months because it is buffer stock is a different one, and PP creeps under sustained load. The deflection that is invisible at a week can be permanent at six months, and a box that has taken a permanent set will not locate in a stack afterwards even though nothing about it looks damaged.
Where boxes are used for long-term storage rather than transit, reduce the standing load: fewer boxes per stack, lower fill weight per box, or a deck that spreads the load. Ask the supplier for a creep figure at the intended duration rather than a momentary load rating – it is a standard measurement and any competent supplier can produce it. If the answer is that the box will creep at that duration, the fix is usually a deck rather than a different box.
What we can do
We supply PP corrugated turnover boxes and rack decks, and we will ask where the box sits before specifying the base. Tell us the beam gap and how long a loaded box stands in the rack, and we will tell you whether the box needs a deck – sometimes the honest answer is yes.