What koshi-yane means
A koshi-yane is a small roof raised one step above the ridge of a main roof. The ridge of the lower roof is opened and a short superstructure built up over it, and the sides between the two roofs are fitted with windows, lattices or board shutters. The arrangement lets smoke, damp and hot air escape upward and admits light at the same time.
Its size depends on its purpose. Where it serves only to vent the smoke of a hearth or kitchen stove, it sits on a short section of the ridge; the record for the gatekeeper's lodge of the Iida family describes a smoke-vent koshi-yane placed at the middle of a pantiled ridge. Buildings for raising silkworms, which needed controlled warmth and humidity, carry a long raised roof along the whole ridge. The silkworm house of the Sato (Mitsubiki) family has one running the full length of its ridge, with windows on the north and south sides of each upstairs room.
Such rooftops shape the look of whole districts. The record for the main house of the Sakai family, Rikyudo, describes it as having the appearance of a silkworm-raising farmhouse crowned by a koshi-yane. Breweries, miso storehouses, silk-reeling and tea-processing workshops, all of which gave off steam or heat, frequently have them too.
Reading the records and the building
Records add the feature after the roof form and material, noting for instance two koshi-yane on the ridge. The number, length and position can say something about how the building was used: a short one on part of the ridge usually served for smoke or ventilation, while one along the whole ridge points to work such as silkworm raising.
A related word, kemuridashi, is the general term for any opening or small roof for letting out smoke. Some take the form of a koshi-yane, while others are simply a small hole or hood in the roof slope. Where a record says only kemuridashi, a photograph will show whether it is a raised roof.
On site the two-tier ridge is easy to recognise. Many were closed up or removed in later alterations, and an odd seam along a ridge can be the trace of one that has gone.
References
- 佐藤家住宅（三ツ引）蚕室｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00013789
- 佐藤家住宅（三ツ引）主屋及び奥上段｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00013788
- 飯田家住宅門番所｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00005179
- 利久堂酒井家住宅主屋｜国指定文化財等データベース（文化庁）
- https://kunishitei.bunka.go.jp/heritage/detail/101/00002541
Last verified: 2026.09.26