A traditional Japanese wood-frame floor may look simple on the surface,
but underneath the flooring it is built from three essential components:

• Joists (Neda / 根太)
• Girders (Obiki / 大引)
• Floor Posts (Tsuka / 束)

Understanding these three basic elements makes it much easier to grasp:

• how a wooden floor is supported,
• why floors sometimes feel soft or bouncy,
• why floor-raising is needed in renovations.

No structural formulas or complicated engineering are required—just basic concepts.

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■ 1｜What Is a Joist (Neda)?

A joist is a narrow horizontal member that directly supports flooring materials such as plywood or floorboards.

It is the most numerous component in the wooden floor structure.

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● Typical Spacing of Joists

The standard spacing is:

• 303 mm (roughly 12 inches)
  (Sometimes 303–455 mm depending on the site.)

This frequent spacing prevents the floor from sagging or vibrating when walked on.

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● Standard Joist Sizes

Common dimensions used in modern construction:

• 45 × 45 mm
• 45 × 60 mm
• 45 × 75 mm (for longer spans)

Joists are small but crucial because they carry the finished flooring.

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■ 2｜What Is a Girder (Obiki)?

A girder is a much thicker horizontal member that supports the joists above it.

It determines the overall strength and stiffness of the floor.

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● Roles of the Girder

• Supports the joists
• Forms the main structural frame of the floor
• Transfers loads into posts (floor supports)
• Helps maintain floor level and stability

Compared to joists, girders are like heavy-duty horizontal beams.

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● Standard Girder Sizes

Typical dimensions include:

• 90 × 90 mm
• 105 × 105 mm (most common)
• 120 × 120 mm (for high-load areas)

Girders must be large because they span longer distances and carry greater loads.

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■ 3｜What Is a Floor Post (Tsuka)?

A floor post is a short vertical support placed under the girder.

Unlike regular long building posts, these are short structural supports under the floor.

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● Roles of Floor Posts

• Hold the girder from below
• Adjust floor level
• Transfer loads downward into the foundation or ground
• Prevent floor sagging or vibration

If the floor feels soft or bouncy,
floor posts are often the first suspect.

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● Types of Floor Posts

Three common types exist:

① Wooden Posts

Traditional method, now rarely used.

② Adjustable Steel Posts (most common today)

• Made of steel
• Height adjustable with threaded mechanisms
• Excellent durability
• Standard in modern renovations

③ Stone Block + Wooden Post (older homes)

Used in traditional houses where the ground or crawl space is simple.

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■ 4｜Understanding the Floor Structure (Text-Based Diagram)

Imagine looking at a wooden floor from the bottom up:

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① Ground or soil

↓

② Concrete foundation

↓

③ Floor posts (vertical supports)

↓

④ Girders (thick horizontal beams)

↓

⑤ Joists (narrow horizontal members)

↓

⑥ Flooring material (plywood, boards, finish flooring)

This simple vertical → horizontal → horizontal sequence
creates the basic wooden floor system.

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■ 5｜Why Does a Floor Sag or Feel Bouncy?

Since this is a beginner’s guide, here are the common causes (very simple):

● Weak or deteriorated joists

→ Flooring feels soft or uneven.

● Girders bending from long spans

→ Floor feels sloped or unstable.

● Floor posts sinking or loosening

→ The entire floor becomes bouncy.

● Old-style stone supports shifting

→ Common in very old traditional homes.

These problems all relate to the basic structure explained above.

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■ 6｜Summary

A wood-frame floor is built from three essential components:

• Joists = narrow horizontal members supporting flooring
• Girders = thick horizontal members supporting joists
• Floor posts = short vertical supports under girders

Once you understand this simple structure,
you can visualize how a wood floor works—even without diagrams.

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■ Disclaimer (English Edition)

This article explains general aspects of Japanese wood-frame floor construction.
Actual floor systems vary depending on building age, region, structural method, and renovation conditions.
Always consult qualified professionals before modifying structural elements.