Gasshō-Style Thatched Roofs
The most distinctive feature of gasshō-style houses is their steep, triangular roofs, and the construction and maintenance of these roofs is a fundamental part of Shirakawa-gō’s history and culture.
While most of a house’s ground floor was built by professional carpenters, the roof was a collaborative effort by the villagers themselves. This was, in part, an effort to reduce construction costs in a region without many financial resources. In addition, building their own roofs allowed the villagers to customize their houses to suit the local climate. The considerable snowfall in this region necessitated roofs with a sharper angle than that of typical Japanese farmhouses. This helped snow to slide off more easily, rather than accumulating and causing structural damage. The steeper angle also allowed the roofs to absorb more of the sun’s heat in winter, when it hangs low in the sky, and less in the summer, when it is directly overhead—thereby mitigating seasonal temperature extremes. Additionally, the thick bundles of miscanthus grass used to thatch the roofs gave the attics excellent insulation against both cold and heat. Finally, the steep roofs created attic spaces ideal for sericulture (silkworm cultivation), which required large workspaces.
The process of creating a thatched roof begins with thin crossbeams called usubari, which are placed at equal intervals along the house’s support posts and fixed with small stoppers to prevent them from shifting. Next, trusses are created by lashing together two long logs using twisted witch hazel branches (neso). Sharpened at both ends, the tied logs are lifted onto the usubari crossbeams and slotted into holes at each end. Together, the usubari and two logs form a strong triangle, and horizontal dowels are placed at the end of each hole to keep the truss in place. A series of these trusses span the width of the house to support the weight of the finished roof.
Next, the trusses are prepped for thatching. Diagonal braces are roped to the inside of the trusses for structural support. Horizontal roof supports (purlins, known as yanaka) are then laid perpendicular to the trusses and affixed with rope, creating a grid-like base frame. The topmost, central beam of the roof is called the karasu odori (“where the crows dance”). The final step before thatching is to bind the rafters (kudari) to the base frame with more neso.
Before thatching begins, the base frame is covered with reed mats to act as a foundation to support the bundles of thatch. Next, one of two types of miscanthus grass is used as thatch: kariyasu or susuki. The former is preferable, because its thinner, hollow stems allow roofs to dry more quickly after rain or snow. Roofs thatched with kariyasu therefore last longer than those thatched with susuki. However, kariyasu fields are difficult to maintain and grow more slowly than susuki. As a result, roofs are now rethatched with susuki, and rethatching takes place every 20 to 30 years. The miscanthus for repairs was once grown in Shirakawa-gō, but the fields are no longer maintained, and the grass for rethatching is brought in from outside.
A distinctive visual feature of Shirakawa-gō houses is their neatly trimmed thatch on the gable ends. The four corners of the roof are the most difficult to rethatch, and the work is usually left to those in the village with the most experience. The first thatching to be placed on the roof is carefully trimmed and bound together at an angle before being tied diagonally onto the vertical poles of the base frame. The work then moves upward along the roof, with bundles added diagonally to further shape the gable end. In a process that resembles sewing, a worker inside the attic uses a giant wooden needle (nuibari) to pass rope upward through the roof, where a worker outside ties it over the thatch and sends the nuibari back through, stitching the bundles to the base frame. As more bundles are added, their ends are pounded into a uniform shape with a wooden paddle.
Moving from the gables toward the center of the roof, the angle of the thatch bundles slowly changes from diagonal to vertical. As the work proceeds, more bundles are added to increase the roof’s thickness. They, too, are “sewn” onto the base frame using the nuibari and rope. Throughout this process, both the bundles and the ropes are repeatedly tugged and pounded with mallets to ensure they are tight. More bundles are added and sewn onto the roof until they reach the top; then the process is repeated on the other side of the roof.
