Why does 屋根 means roof and 天井 means ceiling? Shouldn't it be the contrary?


I was learning these words with my kanji list, and I thought that I made a
mistake and switched their meanings, but after checking the dictionary again I
realized that actually their definition are correct.

But what confuses me is that 屋根, that uses the kanji 屋, also used in 部屋
("room"), leads me to think in the top cover of a room (ceiling). And 天井, that
uses 天 ("heaven/sky"), leads me to think in the top cover of the house which
faces the sky/heaven (roof). But in reality, their meanings are the exact
opposite.

Is my logic flawed?




## Origins of 屋根{やね}

屋根{やね} is a compound of 屋{や} _"house, dwelling"_ + 根{ね} _"root → fundament,
base supporting the whole"_. This word is quite old, appearing already in
[poem 779](http://jti.lib.virginia.edu/japanese/manyoshu/Man4Yos.html#779) of
the _[Man'yōshū](https://en.wikipedia.org/wiki/Man%27y%C5%8Dsh%C5%AB)_ , one
of the oldest pieces of written Japanese dating to 759, with parts that appear
to be older still.

The age of this term might point to its origins: houses in ancient Japan were
dugouts, where the roof came right down to the ground:

![_yane_: the roof of a dugout, coming right down to the
ground](https://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/Yoshinogari-
iseki_tateanashiki-juukyo.JPG/320px-Yoshinogari-iseki_tateanashiki-juukyo.JPG)

There is more about these houses [here on the relevant Japanese Wikipedia
page](https://ja.wikipedia.org/wiki/%E7%AB%AA%E7%A9%B4%E5%BC%8F%E4%BD%8F%E5%B1%85).
As such, the thing that formed the basis of a house was the roof itself:
_"house + basis"_ , thus 屋根{やね}.

## Origins of 天井{てんじょう}

Meanwhile, 天井{てんじょう} was borrowed out of Chinese. At its root, this is a
compound of 天{てん} _"heavens"_ + 井{じょう} _"well; wellhead; wellhead shape"_. 井
read with the _kun'yomi_ of い refers to a _"well"_ , as in the place where you
lower a bucket to get water out of the ground. 井 read with the _on'yomi_ of
either じょう or せい can refer additionally to the 井桁{いげた}, the crossbeams at the
top of the wellhead. The crossbeams themselves look a lot like the shape of
the 井 kanji, which is probably where this character came from:

![_igeta_: the crossbeams at the top of the
wellhead](https://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Well_with_bucket%2C_Ichijodani_200507.jpg/180px-
Well_with_bucket%2C_Ichijodani_200507.jpg)

By extension, 井 read as じょう or せい can refer to something with this same shape
-- much like what rafters or crossbeams might look like, over which the
ceiling of a room would be laid: _"sky → the area above one's head + wellhead
crossbeams"_ , thus 天井{てんじょう}.




## Origins of 屋根{やね}

屋根{やね} is a compound of 屋{や} _"house, dwelling"_ + 根{ね} _"root → fundament,
base supporting the whole"_. This word is quite old, appearing already in
[poem 779](http://jti.lib.virginia.edu/japanese/manyoshu/Man4Yos.html#779) of
the _[Man'yōshū](https://en.wikipedia.org/wiki/Man%27y%C5%8Dsh%C5%AB)_ , one
of the oldest pieces of written Japanese dating to 759, with parts that appear
to be older still.

The age of this term might point to its origins: houses in ancient Japan were
dugouts, where the roof came right down to the ground:

![_yane_: the roof of a dugout, coming right down to the
ground](https://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/Yoshinogari-
iseki_tateanashiki-juukyo.JPG/320px-Yoshinogari-iseki_tateanashiki-juukyo.JPG)

There is more about these houses [here on the relevant Japanese Wikipedia
page](https://ja.wikipedia.org/wiki/%E7%AB%AA%E7%A9%B4%E5%BC%8F%E4%BD%8F%E5%B1%85).
As such, the thing that formed the basis of a house was the roof itself:
_"house + basis"_ , thus 屋根{やね}.

## Origins of 天井{てんじょう}

Meanwhile, 天井{てんじょう} was borrowed out of Chinese. At its root, this is a
compound of 天{てん} _"heavens"_ + 井{じょう} _"well; wellhead; wellhead shape"_. 井
read with the _kun'yomi_ of い refers to a _"well"_ , as in the place where you
lower a bucket to get water out of the ground. 井 read with the _on'yomi_ of
either じょう or せい can refer additionally to the 井桁{いげた}, the crossbeams at the
top of the wellhead. The crossbeams themselves look a lot like the shape of
the 井 kanji, which is probably where this character came from:

![_igeta_: the crossbeams at the top of the
wellhead](https://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Well_with_bucket%2C_Ichijodani_200507.jpg/180px-
Well_with_bucket%2C_Ichijodani_200507.jpg)

By extension, 井 read as じょう or せい can refer to something with this same shape
-- much like what rafters or crossbeams might look like, over which the
ceiling of a room would be laid: _"sky → the area above one's head + wellhead
crossbeams"_ , thus 天井{てんじょう}.

