A Comprehensive Guide to Timber Sizes

Timber is sold in a wide array of sizes within the UK, for use in everything from building frames to the legs of a chest of drawers. In this guide from Building Materials Nationwide, we’ll examine the common timber sizes within the UK, comparing these to the commonly used imperial measurements. We’ll also look at the crucial difference between nominal and actual sizes – an important thing to consider when ordering timber.

What Are the Common Timber Sizes?

Within the UK all timber must be sold in metric sizes, with millimetres as the common unit. Across the world, and informally in the UK, imperial sizes continue to be used. Below are the approximate metric equivalents of the most popular timber sizes.

Within this, you’ll find smaller sizes that are perfect for use within fine or decorative projects, as well as thicker lengths that would be used within building framing, or as floorboards. For instance, our range of CLS timber sizes starts at 50mm x 75mm (2x3), while our structural timber starts at 47mm x 100mm (2x4) and goes all the way up to 75mm x 225mm (3x9). When using C16 or C24 structural timber for carcassing work, a thicker beam will be essential for load-bearing supports. Our planed timber (PSE) starts at much smaller standard sizes, with 25mm x 25mm (1x1) beams available, which are suited for more decorative uses. For joists, a 50mm by 100mm batten will commonly be used.

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What Are the Common Lengths For Timber?

The lengths that timber beams come in can vary dramatically, but standard lengths tend to escalate in 60cm increments, with our range including 2.4m, 3m, 3.6m, 4.2m, and 4.8m lengths.

Nominal vs. Actual Timber Sizes

When looking at timber sizes it’s important to understand the distinction between nominal and actual sizing. When timber is freshly cut it is known as “green timber” and will be at the nominal sizes stated. Rough-sawn green timber is not commonly used within buildings due to its high water content. Within the timber industry, the vast majority of timber will be dried and planed before being sold. Kiln-drying will shrink the size of timber, as excess moisture is removed. Almost all timber used within buildings will have been dried first, with high moisture content causing a suite of issues. Planing (sanding to a clean, flat surface) timber is not a necessity but will be needed in all visible timber, such as floorboards and window frames. This will also reduce the size of the timber as the edges are worn down to create a flat surface.

How Much Smaller Are Actual Timber Sizes?

For a finished timber beam that has been dried, planed, and potentially treated, the actual size is likely to be at least 10mm smaller on all edges. While it is necessary to have timber that can be used for buildings, it can be frustrating that the size written on the product is different from the true size. Below is a chart demonstrating the approximate finished planed sizes for all common timber beams.