Rafters

In a rafter truss structure, it is placed on top of the purlin, and connects the ridge beam to the nokigeta or hanamoya (purlin placed near the eaves) along the flow of the roof. Usually, a back board is laid on top of the rafter, and then roofing material is placed on top of it, but sometimes a lattice is assembled and used instead of the back board. In either case, the slope of the rafters is the slope of the roof. Looking up at the back of the eaves, a rafter protruding from the purlins and running directly to the eaves is called a hitonoki, and when this rafter is cut halfway up the eaves and another rafter is placed on its tip to extend to the eaves, it is called a futanoki. In the case of a two-ridge structure, the original rafter is called the ji rafter, and the rafter at the tip is called the hien rafter. In temple architecture, this is called "ground circle flying form" and there are many examples where the cross section of the ground rafters is circular and the cross section of the flying rafters is rectangular. When the eaves of one or two houses are visible from below, it is called a decorative eaves, and the rafters used there are called decorative rafters. In Japanese architecture, a decorative eaves with a gentle slope is sometimes built separately on the underside of the eaves of the roof that covers the entire building, and is used for the verandas of houses. In this case, the roof rafters are hidden and not visible, so they are called field rafters.

Rafters are normally arranged at equal intervals (about 45 cm) on the main building, but in decorative eaves, special arrangements may be used due to design requirements. Rafters arranged closely together are called shige rafters, of which those with the same rafter spacing as the rafter width are called honshige, and those with the same spacing as the height are called koma kaeshi, and are used in temples and other buildings. In addition, an arrangement in which two rafters are placed close together to form a pair and the spacing between the pairs is appropriately widened is called fukiyose, and is preferred in sukiya-style buildings. All of the above have rafters arranged in parallel, but those that spread out radially from the center of the building are called fan rafters, and those that are radial only at the four corners of the building and parallel at the rest are called corner fan rafters, and are seen in Zen-style architecture.

The material for the rafters is usually wood, but in the case of decorative eaves, bamboo may also be used depending on the design requirements.

[Yamada Koichi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

小屋組みにおいて母屋(おもや)の上に置き、屋根の流れに沿って棟木から軒桁(のきげた)または鼻母屋(はなもや)（軒先近くに置かれる母屋）までをつなぐ部材。垂木の上には裏板を張り、葺(ふ)き材料を置くのが普通であるが、小舞(こまい)を組んで裏板にかえることもある。いずれにせよ、垂木の勾配(こうばい)が即屋根勾配となる。軒裏を見上げて、軒桁から突き出た垂木が軒先まで直通している軒の構成を一軒(ひとのき)といい、この垂木を軒の出の中間で切り、その先端に別の垂木をのせて軒先まで伸ばすものを二軒(ふたのき)とよぶ。二軒の場合、元の垂木を地(じ)垂木、先端の垂木を飛檐(ひえん)垂木という。寺院建築などでは「地円飛方」と称し、地垂木断面を円形、飛檐垂木断面を長方形にする例が多い。一軒、二軒にかかわらず軒裏が下からみえる場合を化粧軒裏といい、そこに用いられている垂木を化粧垂木という。日本建築では建物全体を覆う屋根の軒部分において、その下側に勾配の緩い化粧軒裏を別につくることがあり、住宅の縁側などに用いられる。この場合の屋根の垂木は隠れてみえないので野垂木とよぶ。

　垂木は母屋の上に等間隔（45センチメートル程度）に配列するのが標準であるが、化粧軒裏では意匠上の要請から特別な並べ方をする場合がある。垂木を密に配置する形を繁(しげ)垂木といい、そのうち、垂木間隔を垂木幅と等しくするものを本繁(ほんしげ)、間隔を成(せい)（高さ）と等しくするものを小間返(こまかえ)しとよび、寺院などに採用される。また、垂木2本を接近させて一組とし、その組と組との間隔を適当に開く配置を吹寄せといい数寄屋(すきや)造などで好まれる。以上はすべて垂木を平行に配列するものであるが、建物の中心から放射状に広げていく形を扇垂木、建物の四隅のみを放射状とし他を平行にするものを隅扇垂木といい、禅宗様建築などにみられる。

　垂木の材料は通常は木材であるが、化粧軒裏の場合は意匠上の要求から竹などを用いることもある。

［山田幸一］

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