Tsugite and shiguchi

A Japanese timber frame is built by laying sills (dodai) on the foundation, standing posts on them, and spanning beams and girders across the tops of the posts. This article divides the places where two members meet into two cases: members running in the same direction that meet end to end, and members running in different directions that meet at an angle. A joint of the first kind is called tsugite; a joint of the second kind is called shiguchi^[1][2][6].

Tsugite: adding length

Horizontal members such as sills, beams and girders are sold in standard lengths of 4 to 6 m^[20], so a long run cannot be covered by a single piece. A tsugite is used to extend a member when its length is not enough^[5][6].

There are guidelines for where a tsugite is placed. Several sources say the joint is made at a point projecting out from a post^[13][20]; the projection is given as about 15 to 30 cm from the post^[13] or about 15 cm^[26]. The reason given is that beams and girders carry bending, and the joint should avoid the places where that bending is large^[13][26].

Types of tsugite and how they are chosen

Different types of tsugite transmit different forces. According to Shimoyama, a member joined with an okkake-daisen-tsugi (sloped scarf splice with square pegs) or a kanawa-tsugi (rabbeted scarf splice with a square peg) can be treated as equivalent to a single piece. A koshikake-ari-tsugi (housed dovetail scarf joint) with metal reinforcement, on the other hand, cannot transmit force, and the member is treated as a simple beam supported at the joint^[13]. Noguchi also states that the okkake-daisen-tsugi is stronger than the koshikake-ari-tsugi and the koshikake-kama-tsugi (housed gooseneck scarf joint)^[20].

The types are also used in different places. Noguchi recommends the okkake-daisen-tsugi for beams and girders in shinkabe construction (where posts and beams are left visible in the wall)^[28], and uses the koshikake-ari-tsugi or koshikake-kama-tsugi for sills. The reason given is that sills are bolted to the foundation, so a simple joint is sufficient^[20]. Araki Komuten, a builder, likewise describes the koshikake-ari-tsugi as a simple joint used mainly for sills, and not very strong in tension or bending^[23].

In these sources, the choice of tsugite is explained by how force is transmitted and by where the joint is used^[13][20].

Shiguchi: joining members at an angle

Where a post stands on a sill, or a beam rests on a girder, members running in different directions meet. A shiguchi is used here.

Examples include the watari-ago, in which two members crossing at a right angle are notched into each other at slightly different heights^[24][25], and the arikake, in which a dovetail tenon on the end of one member is set into a groove cut in the side of the other^[10]. Both are classed as shiguchi^[4].

How to tell them apart

Whether a joint is a tsugite or a shiguchi depends on whether the two members run in the same direction or meet at an angle. Whether the member stands or lies flat does not matter. In netsugi, where only the decayed foot of a post is replaced with new timber, the kanawa-tsugi or the shiho-kama-tsugi is used^[9][22]. The post stands upright, but the upper and lower pieces run in the same direction, so the joint is a tsugite.

The name is a clue but not a deciding factor. Dovetail (ari) and gooseneck (kama) shapes are used in both tsugite and shiguchi^[5][21][23]. Shimoyama notes that a joint connecting beams on both sides of a through post is a shiguchi but is called "...-tsugi"^[13]. The word tsugite itself is also used in a broad sense that includes shiguchi^[3]. This site uses "tsugite" in that broad sense.

See them in 3D

• Tsugite used for beams and girders: Okkake-daisen-tsugi, Kanawa-tsugi
• Tsugite used for sills: Koshikake-ari-tsugi, Koshikake-kama-tsugi
• Shiguchi: Watari-ago, Ooire-ari-kake
• All joints