Ebara Riverside Theatre | 江原河畔劇場

Located in Toyooka City, the former town hall of the old Hidaka Town was a Western-style wooden government building completed around 1935. Having undergone two major renovations over the years, it has continued to serve the community in various functions and stands as a local architectural landmark. The Ebara Riverside Theater is a renovation project that transformed this historic building into a theater, serving as the base for the theater company “Seinendan,” led by playwright Oriza Hirata.
The most significant and extensive change in this renovation was the conversion of the first-floor hall into a theater. To ensure sufficient ceiling height for the new theater space, the second-floor slab above was raised approximately 1.2 meters from its original level. Conversely, the second-floor studio (formerly the main conference room) was left with a reduced ceiling height, so the existing coffered ceiling was removed to relieve the sense of compression, exposing the timber roof trusses to create an open and airy atmosphere.
The theater spans 11 meters in the short direction, originally supported by large pine beams with diameters ranging from 450 to 600 mm, placed at 4-meter intervals. To increase the ceiling height on the first floor, these beams were cut and lifted at both ends, then supported by newly installed reinforcement frames along the walls. Additionally, new steel beams were introduced to meet the structural requirements of the studio above.
While the majority of the existing timber frame remained in good condition, some localized deterioration—likely due to water infiltration—was found and replaced with new timber members. Furthermore, the reinforced concrete foundation was strengthened, and additional shear walls were installed, significantly enhancing the building’s seismic performance.

Architectural Design: Atelier Mobile
Structural Design: Eureka+Takuo Nagai Lab
Program: Theater
Year of completion: 2020
Photo courtesy of Hideharu Asanuma

豊岡市に位置する旧日高町町役場は1935年頃に竣工した洋風木造庁舎であり，これまで2回の大規模な改修が行われ，用途を変えながらこれまで現役で使われ続けてきた地元のシンボルのような建築物である。荏原河畔劇場はこの建築物を平田オリザ氏が率いる劇団「青年団」の拠点の劇場にリノベーションしたプロジェクトである。
本改修における最も重要かつ大掛かりな変更は，1階ホールを劇場に用途変更したことである。1階の必要階高を確保するため，劇場用途となる範囲の2階床（旧大会議室）を既存のレベルに対し約1.2 mほど持ち上げた。一方，2階のスタジオ（旧大会議室）は天井高が低くなるため，既存の格子天井を撤去して圧迫感を軽減し，屋根の木造トラスを現しにした開放的な意匠としている。
劇場の短辺方向スパンは11mで，直径450～600mmの松の大梁が4 m間隔で架けられていた。1階の階高をあげるため，この大梁を両端で1度切断して持ち上げ，壁際に新設した補強フレームに支持させるとともに，スタジオ用途に対応するために補強の鉄骨梁を新設している。
木造の既存骨組みの大部分は健全であったが，漏水等によると思われる腐食や劣化が部分的に確認されたため，新品の部材に交換した。また，RC基礎の補強と耐力壁の増設を行い，耐震性を大幅に向上させている。

意匠設計：アトリエモビル
構造設計：Eureka + 永井拓生研究室
用途：劇場
竣工年：2020年
写真：浅沼秀治