To understand wood-frame construction, you only need to start with three basic structural elements:
posts, sills, and beams.

These components form the fundamental “bones” of a traditional Japanese wooden structure.
By learning their roles and typical dimensions, you can understand how wooden buildings stand and how loads move through the structure.

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■ 1｜What Is a Post?

A post is a vertical structural member that carries loads straight downward.

Posts support:

• Roof
• Ceiling
• Beams
• Upper-floor framing
• Walls

They are the vertical backbone of a wood-frame building.

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■ 2｜Standard Sizes of Posts (Very Important)

In modern Japanese wood construction, posts are typically:

• 105 × 105 mm (most common standard size)
• 120 × 120 mm (used where higher strength is needed)
• 150 × 150 mm (mainly for decorative “exposed posts” or special applications)

The 105 mm square post is the universal standard.
120 mm posts are used for key structural points such as corners or long-span beam supports.
150 mm posts are rare and mostly aesthetic.

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■ 3｜Types of Posts (Only the Basics You Need to Know)

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● Tsūshi-bashira (通し柱) – “Continuous Post”

A post running continuously from the first to the second floor.

• Acts as a primary vertical spine
• Critical for structural stability
• Typically 120 mm square
• Not removable without structural redesign

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● Kuda-bashira (管柱) – “Standard Floor-Level Post”

A single-story post used on each floor independently.

• Most common post in modern houses
• Typically 105 mm square
• Forms the basic vertical grid of interior and exterior walls

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● Sumi-bashira (隅柱) – “Corner Post”

Posts located at the four corners of the building.

• Subject to the highest stresses
• Reinforced with multiple metal connectors
• Usually 105–120 mm square

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▼ Simple Summary of Post Types

• Continuous post = main post connecting both floors
• Standard post = typical one-floor vertical member
• Corner post = high-strength post at building corners

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■ 4｜What Is a Sill (Dodai / 土台)?

A sill is a horizontal timber placed directly on top of the concrete foundation.

It forms the base frame of the building and supports the bottom of each post.

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● Roles of the Sill

• Supports vertical posts
• Establishes the first rectangular frame of the building
• Transfers seismic forces into the foundation
• Made from anti-termite and anti-rot treated lumber

● Standard Sill Sizes

• 105 × 105 mm (typical)
• 120 × 120 mm (depending on the foundation layout)

The standard sill section is 105 mm square.

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■ 5｜How Sills Are Fixed to the Foundation

Sills are firmly anchored to the foundation—not simply placed on top.

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● Anchor Bolts

Anchor bolts are embedded in the concrete foundation and extend upward.

The sill is placed over them and tightened with nuts to fix the structure securely.

• Spaced approx. every 910 mm
• Mandatory at corners
• Prevent uplift during earthquakes

This connection makes the wood frame and the foundation act as a unified structure.

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■ 6｜What Is a Beam (Hari / 梁)?

A beam is a horizontal structural member that connects posts.

It receives loads from:

• Roof
• Ceiling
• Second-floor joists/floor

A beam transfers these loads sideways to the posts at both ends.

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● Standard Beam Sizes

Beam sizes vary depending on span and load, but common examples include:

• 105 × 180 mm
• 120 × 240 mm
• 105 × 300 mm (long spans)

Beams are always larger than posts because they must handle bending forces.

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■ 7｜The Core Structure of Wood-Frame Buildings

Wooden construction may look complex, but the core concept is very simple:

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① Place sills (horizontal) on the foundation

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② Stand posts (vertical) on the sills

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③ Connect posts with beams (horizontal)

These three elements create the essential structural skeleton.
Other components—such as diagonal braces, connectors, or roof members—are simply reinforcements added on top of this basic framework.

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■ Summary

To understand wood-frame construction, focus first on:

• Posts = vertical members (mostly 105 mm square)
• Sills = horizontal base timbers on the foundation (105 mm square)
• Beams = large horizontal members carrying upper loads

These three components form the minimum structural framework of a wooden building.

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■ Disclaimer (for English Edition)

This article explains general concepts of Japanese wood-frame construction.
Actual building specifications vary depending on region, structural method, foundation type, and professional design requirements.
Always consult qualified professionals when planning or modifying structural elements.