Schütz and Mauser print clear stacking rules. We follow them. But there's a layer of nuance under them that's important to know if you're actually moving totes around.
The printed rule
Most manufacturers print: two-high indoors on level concrete with base tank ≤90% full; one-high outdoors; never on mixed-material or mixed-size stacks. These rules are correct.
The unprinted rule
In practice, almost nobody follows the indoor two-high rule strictly because warehouses are temperature-cycled, slabs settle, and forklift operators occasionally bump stacks. The realistic indoor stacking rule is: two-high on flat slab, in temperature-controlled space, on pallets you've inspected within the last quarter.
Translation: don't two-high stack on rented warehouse floor unless you walk the slab with a level.
The temperature one
HDPE softens at elevated temperature. In a warehouse that hits 95 °F sustained (think south-facing roof in August), a fully filled tote near the top of a two-high stack can experience pressure on the bottom tank's bottle that exceeds the design pressure of 3.5 PSI by a meaningful margin. We've seen bottle deformation at the corner where the stack pressure meets thermal softening.
Translation: in hot warehouses, single-high stack in summer. Two-high in winter is fine.
The outdoor one
Manufacturers print one-high outdoors because UV and freeze-thaw cycles make even single-stacked tanks degrade faster than indoor stock. Two-high outdoors compounds the UV problem (the bottom tank is now shaded but the top one is hammered) and adds a thermal-cycling stress on the bottom tank's cage.
We see field failures on outdoor two-high stacks roughly 4x more often than outdoor one-high stacks. Don't do it.
The mixed-size one
The 275 and 330 have the same footprint. They do not have the same dome shape, weight distribution, or cage rigidity. Stacking one on the other concentrates pressure on the corners of the lower tank's cage in ways neither was engineered for. Mixed stacks fail more often. Just don't.
The composite-on-HDPE one
Don't ever stack composite or stainless on HDPE. The flex characteristics are wrong. You'll tip over the stack the first time the forklift bumps anything.
The printed rules are conservative. The actual failure modes are mostly thermal and UV-driven. Pay attention to which warehouse the totes are sitting in, not just how many high they're stacked.