Composite Decking Thickness – Why It Matters

Quality, colour options, and style are usually the main considerations when choosing composite decking products. But I’ll add another to the mix – deck board thickness.

The thickness of deck boards matters in decking installation and design. How thick a deck board is contributes to its sturdiness underfoot and how the deck is constructed.

I’ll explain about composite decking thickness, why it matters, and how you, as homeowners, can make the right decision for your deck project.

Why Composite Decking Thickness Matters

Composite deck boards come in different levels of thickness.

A standard deck board is about 2.5 cm thick. When referring to traditional timber decking, this same thickness may be called a 5/4 board, which usually means between 2.5 and 3.2 cm. Thicker composite boards are up to 3.8 cm thick or 2 x 6 boards when referring to timber. That thickness can range from 3 to 3.8 cm.

Regardless of how you refer to it, most composite boards range from 2.5 to 3.8 cm thick.

At Trex®, our composite boards are made from 95 per cent recycled plastics and timber fibres, ensuring the highest quality, durability, and reliability regardless of thickness. Added thickness, though, can make a difference in how the deck feels and is built.

Factors That Influence the Choice of Deck Board Thickness

Residential decks are usually constructed using standard boards about 2.5 cm thick. Depending on size, type, and usage, commercial decks may use the thickest boards.

I believe the average homeowner will find that a standard composite board is the perfect option for a typical deck size. But to help you make that decision, I’ll share a few more construction facts for your consideration.

Structural Support Needs

When looking at a deck, we see the beauty and design of the boards. Composite decking comes in various styles and colours and can closely replicate natural wood grain. You can even design your deck using different board widths to create visual interest and patterns.

But what you don’t see is just as important – perhaps more so.

Under those beautiful boards is the deck’s support system:

• ledger board — a horizontal board that secures the deck to its structure (house, building, etc.)
• flashing – acts as a water barrier where the ledger is attached
• joists – vertical bearers that go from the ledger board to the outer board
• joist hangers – the fasteners for attaching the joists
• blocks – small horizontal pieces between each joist
• posts and footers – the support posts that secure the deck to the ground

Joist Spacing

Evenly-spaced joists span across your decking frame. Joists are what your deck boards fasten to. Joist spacing is measured from one joist’s centre to the next’s centre. “On centre” is how you refer to this type of measurement.

Standard joist spacing is about 40.6 cm on centre. This construction is typical for most residential deck builds and accommodates standard board thickness. However, if you desire a more sturdy feel when using a 2.5 cm thick deck board, you can narrow your joist spacing to 30.5 cm on centre.

You can also opt for a thicker deck board, which may allow you to widen your joists to about 61 cm on centre. Use this option for commercial applications such as docks or boardwalks. You can also save some money on joists if the space between them is wider.

For the joists, I recommend using joist tape to provide a moisture barrier, helping to protect the integrity of the deck’s structure.

Environmental Conditions

A thicker composite board does not withstand weather conditions better than a standard board. They are made from the same materials and are usually capped in a protective plastic shield. Composite deck boards resist warping, rotting, splintering, and staining even under the harshest weather conditions – from strong UV rays to snow and ice. A thicker board may provide some additional sturdiness to how the deck feels underfoot, but its durability and reliability are the same.

Composite deck product lines carry warranties of 25 to 50 years.

Installation Ease

Installation is the same regardless of board thickness. The only difference might be in the joist spacing. Composite decks can be a DIY project, but you must follow an installation guide. If budget allows, I recommend a professional deck builder to ensure proper installation, especially if your deck project is elevated, multi-tiered, or expansive.

Best Applications for Thick Boards

All Trex® decking product lines – Trex Signature®, Trex Transcend®, Trex Enhance®, and Trex Select® – are low maintenance, durable, reliable, and beautiful. Standard board thickness is sturdy and fine for nearly all applications of an average homeowner.

But in certain situations, you may want to choose a thicker board.

• A large, expansive, second-story deck may benefit from the added sturdiness of a thicker board.
• You may live on the water and need decking for a dock. A thicker board can be ideal for this application.
• You may prefer the feel under your foot of a thicker board.
• Wider-spaced joists in your deck system may require thicker boards.
• Commercially, boardwalks are made from thicker deck boards.

As you plan your new deck, consider how you will use it, the space you have for it, and how you want it to look and feel. Answering these questions will help you decide on the right deck-board thickness for your project.