Most detailed definition I could find:


Picture from google:

enter image description here


The heavy rain flooded my house up to 30cm below the floorboards.

However, what I don't understand from the above is:

Does the water rise within 30cm distance of the wooden floorboards (upper limit) or 30cm above the cement floor (lower limit, like on the picture)? Does 床下 refer to the whole space? In any case, what exactly happens in the example sentence?

  • +1 for revealing an unexpected aspect of Japanese usage :D Jan 5 '16 at 12:39

Upper limit.

Though 床下【ゆかした】 stands as a noun by itself, it literally means "under-floor". Therefore, 床下30センチ always means "-30cm from the floor level". 床 in this phrase is understood as the floorboards you live upon. If you want to tell you have 30cm deep water in the space between the base and the floorboards, you have to say 床下30センチ (so that you grammatically treat 床下 as a noun).

Similarly, 床上【ゆかうえ】 is used for "above the floor level" as well.

Or more generally, there are lots of words with ~上/下 that means "(distance) above/below X".

  • 桁上【けたうえ】/桁下【けたした】 above/below the girder (see the pic)
  • 股上【またがみ】/股下【またした】 (of bottom wear) rise / below the crotch
  • 地上【ちじょう】/地下【ちか】 above/below ground
  • 氷点下【ひょうてんか】 below freezing (= minus degree Celsius)

enter image description here


"床下 + number" is possibly open to two ways of interpretation (see the comments)... But at least I've found a J-E dictionary entry that says:

The flood waters rose to within ten centimeters of the floor of his house.

  • Hmm, now I am not so sure about what I wrote I was pretty sure in my answer…. Jan 5 '16 at 12:09
  • @TsuyoshiIto えっマジですかこれ…床下… Jan 5 '16 at 12:18
  • 正直なところ、自信がなくなってきました。そもそも厳密さが要求される場面ではこんな言い方はしないのではないかという気も……。 Jan 5 '16 at 12:24
  • 3
    – marasai
    Jan 5 '16 at 12:35
  • 1
    @broccoliforest 床下が60センチもあれば「半分も浸水している!」ってなるんですけど、上の写真みたいに一般的には床下ってそんなに高くないので30センチも水面から離れているとあまり浸水していないように感じますね。
    – h2so5
    Jan 5 '16 at 12:46

I am pretty sure that 床下30センチ in your sentence means 30 centimeters above the ground, with additional information that it is below the floor level. It does not mean 30 centimeters below the floor level (that is, the distance from the floor level was 30 centimeters), which would be 床の30センチ下.

床下 means the space under the floor, as in 床下に収納スペースがある家 (a house with an underfloor storage).

Note/Warning/Excuse. I was pretty sure when I posted this answer, but I am no longer sure. I admit that I could not find evidence that my interpretation was correct. broccoli forest’s answer proves that at least some people interpret it differently from what I wrote above. Maybe my interpretation is simply wrong, or maybe the sentence is ambiguous.


I understand the sentence, “床下30センチまで水につかった” implies that the water below the floor came up to 30 centimeters high from the ground. I casted this question to several members of English speaking circle I'm with to monitor other's view. The answer was unanimously 'the height from the ground.' Reason? It’s easier to fathom the depth of water from the ground than measuring the length of gap between the crest and floor level by creeping into 'below-floor.'

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