Rebar sizes explained! What each bar size is used for — an easy guide to choosing steel for building a house

When you build or extend, the steel structure is one of the most important parts — the 'backbone' of the house. You may have heard tradespeople talk about No. 6 or No. 9 bars, DB12 or DB16, and wondered what the numbers mean and why each size matters. Choosing the right bar size for each type of work not only helps the house carry its load safely, it also keeps your budget from running over.

1. Round bars (RB) for light-load work

Round bars have a smooth surface and are used in small structures or as secondary reinforcement where loads are light. Common sizes are:

• RB6 (6 mm): commonly used for 'stirrups' around columns and beams, holding the main bars in position
• RB9 (9 mm): used for stirrups in larger beams or columns, as reinforcement in concrete slabs, or as anti-crack bars
• RB12 (12 mm): suits light structures such as sign frames and single-storey buildings, or for tension at specific points

2. Deformed bars (DB) for heavy loads

Deformed bars have ribs along their length to grip the concrete better, and are used as the main bars in heavy-load work:

• DB12 (12 mm): main bars for columns, beams and slabs in 1-2 storey houses
• DB16 (16 mm): suits columns and beams carrying heavy loads, such as long-span beams or houses of 2-3 storeys or more
• DB20 and DB25 (20 and 25 mm): used in high-rise buildings, foundations (footings/pile caps) or piles that take very high compression

Checklist for choosing and inspecting rebar before use

For a structure that meets standards and is safe, homeowners can use this simple checklist on site:

• Clear TIS mark: check for the TIS (Thai Industrial Standard) mark with the grade (e.g. SD40, SD50) and the size stamped on the bar
• Use full-size bars: choose bars from major mills with standard diameter and weight — not under-sized 'light' bars
• No pitting rust: a dull red surface rust can be brushed off, but if the rust has pitted deep into the steel, never use it

Important caution: Structural and building engineering work is a matter of life safety. Even if you know the basic bar sizes, the size, number and spacing of the bars in a real house must be calculated and approved by a qualified structural engineer only. Never change bar sizes on your own or take a tradesperson's word without an engineer's drawings to confirm it.