An empty crate is still a structure doing work
A crate with nothing in it looks like it is resting. It is not. The panels carry their own weight, the box carries the weight of whatever is stacked on it, and the whole assembly takes load from the surface it stands on. Over weeks, those three loads have time to act, and plywood deforms under sustained load rather than springing back.
That is why crate storage is worth a procedure. The failure modes are slow, silent, and only discovered when the box is needed: a panel with a permanent bow, a base that no longer sits flat, a box that was square when it was built and closes with a gap now.
Storage position decides the shape
Three positions, three different outcomes.
Flat on a face. The panel that is horizontal carries the weight of the box above it, spanning whatever is underneath. If the box is empty, there is nothing inside to support that panel, so it sags into the box. Long storage flat on a face is the most common cause of a bowed lid.
On edge. The box stands on one edge, and the load path runs through the panel edges. This is the structurally sensible position — the edge is designed for exactly this — but it needs the box to be supported so it does not lean and rack.
Nested or stacked empty. Empty boxes stacked inside each other put load on the panels of the box below, in a place the box was never designed to take it. On thin panels this produces exactly the kind of local crush that a raw edge cannot resist.
The general rule: store a box the way it will be loaded — on its base, with its edges carrying load, and with nothing pressing on a panel face.
The floor is half the problem
A concrete floor is a moisture source. It wicks water and holds it against whatever touches it, and plywood in contact with damp concrete takes up moisture through the bottom panel and the skids. The result is a box that swells at the base, loses stiffness there, and no longer sits flat — which then shows up as a stack problem or a closure gap rather than as a storage problem.
A bearer or a pallet under the box costs little and removes the contact. It also allows air to move under the base, which is the actual mechanism for drying.
Outdoors, and what happens to the steel edge
Outdoor storage is a decision, not an accident, and it should be taken deliberately.
Plywood exposed to rain takes up water at the exposed edges fastest, because the edge is where the panel's end grain is. That is the same edge a steel profile is pressed onto, so an exposed edge is either protected by the profile or open to the weather — and where the profile is formed, the panel edge is covered but the water still reaches the joint line.
Steel edge corrodes where its coating does not reach: cut ends and fastenings. A galvanised strip and stainless fastenings raise the tolerance for damp conditions, but they do not make a plywood box an outdoor structure. If outdoor storage is unavoidable, the box should be off the ground, under cover where possible, and covered on top rather than wrapped tight — trapped moisture under a tarpaulin is worse than free air.
Distortion is not recoverable
The important property of creep in panels is that it is not elastic. A panel that has bowed under sustained load for a month does not return to flat when the load is removed; it keeps most of the bow. That is why storage damage is different from handling damage: handling damage is visible at the time, and storage damage is discovered later.
The practical consequences are specific:
- A bowed lid no longer seals, so a moisture plan that assumed a closed box no longer applies.
- A base that is not flat rocks, and a rocking base racks the walls when the box is stacked.
- A box out of square does not stack predictably, and its declared internal dimensions no longer describe it.
A storage rule that fits on a wall
Five lines, all of which can be checked in a walk-round:
1. On the base or on edge, never flat on a face with nothing inside. 2. Never on bare concrete — bearers or a pallet underneath. 3. No load on a panel face; stack on the structure, not on the skin. 4. Identical boxes in one place, so the oldest stock goes out first and nothing sits for a year. 5. Under cover, with air movement, rather than wrapped tight.
What to send us
Tell us how the boxes will be stored between build and use: how long, indoors or out, stacked or single, and on what surface. On a long storage period the construction changes — a stiffened edge and a base designed to sit flat are worth more than an extra millimetre of panel.
Storage is where flatness is lost. A plywood folding crate stores flat by design; a rigid crate stood on edge does not, which is why distortion usually shows up first on the panels that were leaned against a wall rather than stacked.