Decking timber sizes: a dimensions guide with FAQs

Timber decking is a popular choice for outdoor spaces, providing a natural look that complements a variety of landscapes. Whether you're planning a new deck or replacing an old one, understanding the standard sizes and dimensions of timber decking boards and joists is important for a successful project.

This guide will help you navigate the common sizes, factors to consider when selecting dimensions, and how to choose the right size for your decking project.

Timber decking boards come in a variety of widths, thicknesses, and lengths, allowing for flexibility in design and installation. Here are some of the most common sizes:

• Widths: The standard widths of decking timber boards are usually 90mm, 120mm, and 140mm. Narrower boards can make a small space appear larger, while wider boards tend to offer a more contemporary look and feel.
• Thicknesses: Decking timber boards typically range from 19mm to 28mm in thickness. Thicker boards are stronger and can span longer distances between joists, but they are also heavier and more expensive.
• Lengths: Lengths can vary widely, but common decking timber lengths are 2.4m, 3.0m, 3.6m, 4.2m, and 4.8m. Choosing the right length can minimise waste and cut down on installation time.

Selecting the right size timber for your decking project is crucial for getting your deck right. To help you make an informed decision, here's a simple step-by-step guide to calculate how many decking boards you need:

Step 1: Calculate the area of your deck

First, you need to know the total area you'll be covering. For a rectangular or square deck, simply multiply the length by the width to get the area in square meters (m²).

Formula: Deck Area = Length (m) × Width (m)

Step 2: Determine single board coverage

Find out how much area one board will cover. Take the width of the board in meters (remembering that 100mm = 0.1m) and multiply it by the length of the board you plan to use.

Formula: Board Coverage (m²) = Width (m) × Length (m)

Step 3: Include spacing between boards

Spacing is necessary for drainage and expansion. Convert spacing width from millimetres to meters (5mm = 0.005m), add this to the board width, and use this adjusted width to calculate the number of boards needed.

Formula: Adjusted Width = Board Width (m) + Spacing (m)

Step 4: Calculate total number of boards

Now, calculate the total number of boards needed by dividing the deck area by the coverage of one board, adjusted for spacing. Always round up to the nearest whole number, as you can't purchase part of a board.

Formula: Number of Boards = Deck Area / (Board Coverage + Spacing Adjustment)

Example Calculation:

For a deck measuring 20m², using 120mm wide boards (0.12m), 3m in length, with 5mm spacing:

1. Adjusted Width: 0.12m (board width) + 0.005m (spacing) = 0.125m
2. Board Coverage: 0.12m × 3m = 0.36m²
3. Number of Boards: 20m² / (0.125m × 3m) = Number of Boards Needed

The size of the joists supporting your decking boards is just as important as the boards themselves. Common dimensions for decking joists include:

• Widths: Joist widths usually match the narrow dimension of the boards and are most commonly found in sizes like 45mm, 70mm, and 90mm.
• Depth: The depth of the joists can vary, but common sizes are 140mm, 170mm, and 200mm. The depth will depend on the span between supports and the load the deck needs to carry.
• Lengths: Like decking boards, joist lengths can be found in a variety of sizes to suit different projects. Common lengths include 2.4m, 3.0m, 3.6m, 4.8m, and sometimes longer. The choice of length often depends on the size of the deck and the layout of the support structure.

Related article: 13 decking timber options - which type is best for you?

To ensure your deck is structurally sound, follow these steps to calculate the correct joist sizes:

Step 1: Measure Span Between Supports

• Formula: Span = Distance Between Support Points.

Step 2: Choose Joist Depth

• Use building code guidelines: 140mm depth for spans up to 2m, 170mm for 2-2.5m spans, and 200mm for spans over 2.5m.

Step 3: Select Joist Width

• Match the width to the depth for strength, typically 45mm, 70mm, or 90mm.

Step 4: Calculate Joist Length

• Include length for spans and any overhangs. Measure the distance between supports and add for overhangs.

Step 5: Determine Joist Spacing

• Base spacing on decking board thickness: 400mm for thinner boards and up to 600mm for thicker boards.

Example:

For a deck with a 3m span using 28mm thick boards:

1. Depth: For a 3m span, use a depth of 200mm.
2. Width: Select a width of 70mm or 90mm.
3. Length: For a 3m span with 0.5m overhangs, use 4m long joists.
4. Spacing: Use up to 600mm spacing for 28mm boards.

When you're selecting timber decking and joist sizes, consider the following factors:

Design and aesthetic

The choice of timber species plays a significant role in the design and aesthetic of your deck. Popular options like Spotted Gum offer unique colours and textures that can complement your home and garden beautifully. The width and thickness of your decking boards can alter the deck's appearance, influencing the overall vibe of your outdoor living space.

Wider boards might give a more contemporary feel, whereas narrower boards can suit a traditional or intricate design. Consider how the sizes and species of timbers, like Spotted Gum for decking or Pine for gates and fencing, will integrate with your existing outdoor elements and desired style.

Span and load

The structural requirements of your deck, including the span between joists and the expected load, will significantly impact the required thickness and depth of your decking boards and joists. Timber species vary in strength and durability; for instance, Spotted Gum is known for its high density and strength, making it suitable for areas with heavy furniture or high foot traffic.

Consulting building codes and guidelines is essential to ensure your deck's safety and compliance, especially when determining the appropriate sizes for spans and loads. This consideration is crucial for decks, gates, and fencing that are in constant contact with the ground, as the durability and resistance of the timber to environmental factors are paramount.

Material availability and cost

The availability of certain timber species and set lengths can greatly influence your project. Some timbers, like Spotted Gum, may be readily available in a variety of set lengths, which can reduce waste and lower costs. On the other hand, more exotic or less commonly used species might come at a higher price or may not be available in the desired sizes.

The cost per linear meter for decking timbers like Spotted Gum and Pine can vary significantly. Larger and thicker boards are generally more expensive and might not always be necessary for your project's structural needs. It's important to balance your design preferences with your budget, considering both the initial installation cost and the long-term maintenance and durability of different timber species for outdoor living spaces.

Hopefully, this short guide has given you some insights into the different sizes decking timber boards typically come in. As you design your deck or seek professional advice, consider the size of the deck you're aiming to create. The choice of dimensions will impact both the visual and practical purposes of your deck, influencing your overall satisfaction with the end result.