Translation guide
A lintel is the horizontal beam across the top of a door or window opening. In Japanese, the most common word is 鴨居 (kamoi) for sliding doors, but other terms exist depending on the context.
The structural or decorative beam spanning the top of an opening.
Specifically the upper grooved track for sliding doors (fusuma or shoji) in traditional Japanese architecture. This is the most common word encountered for 'lintel' in a Japanese context.
A general architectural term for a lintel, often used for stone or concrete lintels in modern construction. Can be written as 目草 but is usually in kana.
窓の上に石のまぐさが使われている。
A stone lintel is used above the window.
The kanji form of まぐさ, meaning lintel. Rarely used; the kana form is preferred.
楣にひびが入った。
A crack appeared in the lintel.
Literally 'upper frame', this can refer to the top horizontal part of a door or window frame, functioning as a lintel. Common in carpentry and DIY contexts.
ドアの上枠を取り付ける。
Install the door's top frame (lintel).
If you are talking about a traditional Japanese room with sliding doors, use 鴨居. For Western-style or modern construction, まぐさ or 上枠 may be more appropriate.
Dust has accumulated on the lintel.