ROOF FRAME STRUCTURE
The usu-bari, the bottom members of the roof truss frames, are spaced regularly
at I ken (around 1.8-meter) intervals, resting on top of the horizontal
girders and beams of the body frame. After the usu-bari are set
in place, the other truss members are added to complete the frame.
The connection between the sloped truss rafter and the usu-bari
is a type of pin connec-tion, with the bottom ends of the truss
rafters carved into a pointed shape and inserted into slots in
the top of the usu-bari, and the top ends of the truss rafters are lashed together with
mansaku (Japanese witch hazel). The slope is quite steep, almost 60 degrees
in most houses that remain today -- but researchers have learned
that the roof slopes in earlier periods were not quite so steep,
and that over time the angle of the slope has gradually increased;
thus the older the house, the less steep is the angle of the slope.
For attachment of the roofing material, horizontal wooden members
are secured across the top of the sloped truss rafters, and another
layer of smaller sloped members is placed across these horizontals;
straw mats are then fastened on top to serve as the base plane
to support the thatching material (Fig.2-3, 4, 5).
To reinforce the overall structure, compensating for the inherent
weakness of this type of frame against lateral forces acting on
the gable end, diagonal bracing members (suji-kai) are used --large diagonal braces secured inside the plane of
the truss rafters with smaller cross-braces fas-tened to the outside
of the truss rafters (Fig.2-4). This type of diagonal bracing was another unique feature of
the Gassho-style.
As described above. the usu-bari simply rests on top of the girders
and beams, with only small dowel pins to prevent them from slipping
out of place, and is not structurally connected to it; therefore
the forces which are transferred from the roof structure to the
body frame are not torsional forces but simple vertical loads.
Also, the adoption of a thin member to use as the usu-bari is quite logical, since this member functions only in tension,
with no compression forces, and there is no need here for a thick,
heavy beam (Fig.2-2, Fig.3).
These points, as well as the adoption of a pin connection between
the usu-bari and the truss rafter, show that the Gassho-style
house is indeed an unusually rational structure, one which demonstrates
that the people in this area clearly understood the principles
of structural dynamics. This type of structure is very rare in
Japan, and it is appropriate to say that the Gassho-style house
represents the most advanced development of structural method
in Japanese architecture.
There are other important characteristics of the Gassho-style
roof structure which differ from those of ordinary Japanese farmhouses.
The most typical farmhouse roof types are hipped or hipped-gable
thatch roofs, with gable-roofed houses in the minority (Fig.5-1,2,3, Fig.6-1). In cases where the gable roof ~ adopted, however, truss structures
are not common; to resist external horizontal forces in the direction
of the longitudinal axis, the typical roof structure uses short
vertical posts, connected and stiffened by horizontal penetration
bracing. In such cases the roof slope is quite low, usually less
than 20 degrees, and the roofing material is not thatch but wooden
boards (Fig.6-2). Thus the particular combination of features which make up the
Gassho-style -- the adoption of a truss structure for a thatched,
steeply-sloped gable roof -- makes this style unique.
One of the reasons that the truss structure was adopted was to
make the roof slope as steep as possible to help shed snow, a
practical defense against the unusually heavy snowfall. Also,
there was a need to increase the volume of interior space inside
the roof structure -- fully open space unobstructed by internal
posts and beams -- to make the interior spatial volume of the
roof structure usable as an active functional space; this was
very unusual in Japan, where most farmhouse attics were either
unused or used only for passive functions such as storage. Compared
to the roughly 60-degree slope of the Gassho-style roof, other
generic thatched-roof houses had slopes of 45 degrees or less,
too low to provide adequate open volume inside.
To provide the necessary usable area inside the attic, bamboo-slat
flooring or wooden-board flooring is installed on top of the usu-bari and one or more layers of floor-support joists (called gassho-bari)
are added to divide the triangular volume into two or three levels
(up to four levels in the largest houses) for maximum utilization
of the available space. The ends of the gassho-bari are inserted into and lashed to the sloping truss rafters, and
bamboo or board flooring spans from joist to joist.
The adoption of the gable style instead of the more typical hipped-gable
style, in spite of the usual inherent weakness of the gable type,
was primarily due to the need for adequate lighting and ventilation
in the functional attic space. These attic levels were used primarily
for the raising of silkworms, but were sometimes used as storage
spaces for mulberry leaves (Fig.4). To make the space habitable, several large shoji-paneled window
openings were provided in the gable end wall for air circulation
and natural lighting. The exterior of the gable end wall is surfaced
with wood siding; in Shirakawa-go the end wall inclines outward
toward the top, but in the Gokayama area this wall is vertical.
The combination of this particular structural system and roof
type has resulted in another characteristic which sets the Gassho-style
house apart from the typical Japanese farmhouse: unlike the common
image of a low-profile house form which rests comfortably on the
earth, suggesting not confrontation with nature but rather passive
harmony with it (Fig.5-1,2,3), the Gassho-style house is an unusually tall, impressively large-scaled
form which projects a strong image of "confrontation" or "confident
resistance" against the harshness of nature (Fig. 1-1,2).
