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The instruction manual for some of my kitchen equipment says "Dishwasher safe (top shelf)".

What difference is there between what an object on the top shelf of a dishwasher experiences, and what happens to an object on the bottom shelf?

What are the risks of putting one of these items on the bottom shelf?

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    Great question. After having seen one of our neighbors putting all their top rack dishwasher safe stuff in the bottom of their dishwasher without incident, we figured it would be reasonably safe to experiment with such since we had the same model. Anecdotally, one place where I've lived in the past, I had a tupperware lid deform/melt from falling into the bottom rack. But with our current dishwasher, even the disposible tupperware seems to survive the bottom rack unscathed....so I've been kind of curious why in some dishwashers, it's fine, but not others.
    – Jessica Brown
    Commented Jun 4, 2013 at 22:33

3 Answers 3

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In most dishwashers the heating elements are on the bottom only. Lightweight plastic items might melt (soften and distort, not drip away) on the bottom, but they don't mind being in the steamy environment or being sprayed with hot detergent-laced water. They are therefore only dishwasher safe on the top shelf.

I've melted a plastic bowl or two in my time; there is a definite difference between top and bottom.

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    That's definitely true of older dishwashers, however newer ones have a heating element that is designed to make sure that all the water is of an even temperature. With newer washers it's generally ok to put plastics on the top or bottom, as long as they aren't soft plastics.
    – GdD
    Commented May 30, 2013 at 13:02
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    I don't think the problem was ever with the temperature of the water, but rather the hot dry option. My apartment has an old dish washer; if I enable the hot dry option the electric heating element in the bottom glows red hot like the one in an oven.
    – Dan Is Fiddling By Firelight
    Commented May 30, 2013 at 14:12
  • Yes, I definitely know from experience that it will melt plastic dishes during drying.
    – Petah
    Commented May 31, 2013 at 5:30
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Differences include:

  • higher heat (water's had a chance to air cool before it gets higher up)
  • higher pressure

The pressure isn't as big of a deal typically (it could scrub off some finishes, but generally those items aren't marked as being 'dishwasher safe' at all. But for lighter weight items, it often means that they flip over or get thrown (possibly to the bottom of the dishwasher, where the heaters for drying are).

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This only applies to dishwashers with heated dry cycles. However, many brands of dishwashers use condensation drying rather than heated drying, in which case your plastics are safe in the lower rack (example).

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    I think this could be considered as purely advertising.
    – Johannes_B
    Commented May 11, 2020 at 4:44
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    @Johannes_B Good point, I rewrote it to say "many brands" instead.
    – stjudyscomet
    Commented May 12, 2020 at 14:42

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