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I'm staying in an apartment in Mexico and found this mysterious shower drain cover/cap/lid in the bathroom, made out of plastic:

enter image description here

Is it to prevent insects from crawling out? To prevent bad smells from leaching? To decoratively hide the shower drain? What's the name of this "device" and how is it supposed to be used?

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    I can't tell from your picture, but if the shower has high sides, it could be used as a plug to allow you to fill it like a tub for children, although the tiles would make it tricky to create a seal. – Midavalo Dec 29 '19 at 3:54
  • @Midavalo the sides are around 10cm high, not enough for a bath. – JonathanReez Dec 29 '19 at 3:56
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    It could just have been forgotten by the previous owner. I travel with such a stopper in case the apartment/hotel one is broken or does not otherwise function. Essential tool to collect water in a sink for on-the-road laundry. – ZeroTheHero Dec 30 '19 at 4:02
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The device is a universal drain stopper. It is designed to keep water in a bathtub or sink. Because it works by sitting on top of the drain, it is not necessary for the consumer to worry about the diameter of the drain opening, as is the case with traditional drain plugs that sit inside the opening.

These work by creating a seal against the smooth surface of the tub or sink around the perimeter of the drain, being held there by the weight of the water. They are not effective at keeping gases from escaping, therefore, unless the basin is full of water. Furthermore, they are not particularly effective with drains such as the one in the photograph, because of of the joints between the tiles. The material is not flexible enough to seal these joints.

The purpose for which this thing has been placed on this drain, therefore, is unlikely to be related to sewer gases, because it will not be very effective for that purpose. It might be marginally more effective at keeping water out of the drain, but probably only for a couple of minutes, so that is also unlikely to be the reason.

The speculation that it is to keep scorpions or vermin of a similar size from escaping the drain seems more likely. This would be effective against creatures that are able to swim through the water in the trap, small enough to pass through the metal drain cover, too large to crawl through the tile joints, and not strong enough to move the cover. If there are any local scorpions or other vermin that have these characteristics (cockroaches?), that's probably your answer.

Have you tried asking the owner of the apartment or other people who live in the area?

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    Thanks! I've found the device on Aliexpress by searching for "universal drain stopper" and edited it in your post. – JonathanReez Dec 29 '19 at 18:06
  • @JonathanReez: ah crap, you posted this link, everyone jumped to get one and now they are out of stock...* – WoJ Dec 30 '19 at 21:07
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Having lived in Texas, I would use that to keep out scorpions, which can crawl up drains and even survive the water coming down them. Though I'd also change the strainer to a different design that a scorpion could not so easily squeeze through...

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    Note to self: never go back to Texas, I was happy not knowing this. – Matt Douhan Dec 29 '19 at 17:45
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    @MattDouhan You hardly ever see them in cities, it's when you get out into the country that you'll run into them a lot. Keep your shoes on indoors, and just step on them if you see one. – Michael Hampton Dec 29 '19 at 20:00
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    Surely the Texas solution to this issue is more along the lines of motion activated machine guns pointed down the drain? – ceejayoz Dec 30 '19 at 15:18
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    @MichaelHampton, you forgot “and check your shoes before putting them on.” – WGroleau Dec 30 '19 at 16:58
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    And your towel when drying off. Had a friend who had a nasty surprise after a shower. – Robb Sadler Dec 30 '19 at 18:36
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My guess is that this would be to prevent sewage smell coming into your room. It's a common problem, see for example this Q&A online.

While I haven't seen such lids before, I've come across many such drains which allowed sewage odors into the hotel room. This was especially a concern in the following conditions:

  • A well sealed-off room, common with newer hotels in hotter countries to prevent hot air coming in while airconditioners are running.

  • A powerful bathroom exhaust fan, meant to let out steam after a hot shower to combat mould growth.

  • A lack of a proper siphon in the shower drain. I am not sure why they don't always make them properly, though it could be because the floors aren't thick enough to allow for high enough siphons.

The lid might be an attempt at closing off the drain so gasses cannot pass though, though I'm not sure if it provides enough of a seal to work.

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    I stayed in an Airbnb in nearby Guatemala recently which had a sign requesting that (shared) shower users plug the drain after use with a similar device, for odor reasons. – Spehro Pefhany Dec 30 '19 at 13:28
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    +1. It could also be that the U-bend dries off by evaporation when the apartment is empty for a long time - even a non gas tight cover will slow down evaporation. – jpa Dec 31 '19 at 19:52

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