7th Dec 2022 -

4x1 timber serves a variety of visual uses within a building project, despite being unsuitable for load-bearing installations. In this quick guide from Building Materials Nationwide, we’ll cover the actual sizing of a 4x1 timber beam in the UK, the different finishes available, and some of the popular uses within construction.

What Size is 4x1 Sold at in the UK?

Within the UK all products must be sold in metric sizes, such as mm, cm, or m. This includes timber, despite the prevalence of imperial measurements within the UK still. While imperial measurements continue to be used informally, you'll be looking for 100mm x 25mm when shopping for a 4x1 timber in the UK. This isn’t an exact match, but will be sold as the equivalent. When buying everything except rough sawn green timber, it’s important to note that the actual size will be lower than the size it is being sold at.

The Difference Between Nominal and Actual Sizing

Nominal sizing is the size of the beams that a felled tree will be originally cut to, in a rough sawn wood known as green timber. Green timber has a high moisture level and will shrink over time as it dries out. This makes it unsuitable for a lot of construction purposes, so most 4x1 timber sold will have been kiln-dried to take the moisture level below 20%. This drying process will shrink the wood below the size listed. Planing will also reduce the size of wood, a process which involved smoothing out surfaces of timber to make it more attractive and ensure that it won’t give splinters to those who touch it. This means that the size of a 4x1 timber in the UK is likely to be around 89mm x 19mm.

What Lengths of 4x1 Timber Are Commonly Sold?

Standardised lengths of timber tend to scale in increments of 30cm or 60cm, with the lowest lengths typically over a metre and the longest around 4m.

What Types of 4x1 Timber Can I Buy?

When purchasing 4x1 timber within the UK, it will usually be kiln-dried at a minimum, with a series of other finishing options that each bring their own benefits.

PSE 4x1 Timber

PSE, or planed square edge timber, is a piece of timber that has been planed on all sides, leaving sharp edges between each facing. This serves two purposes, giving it an attractive and smooth look, and making it safe for touch. This allows 4x1 timber to be used within visible or touchable elements of a house or piece of furniture.

Pressure- Treated 4x1 Timber

When looking to use timber externally, or in locations with a high level of moisture risk, opting for a pressure treated timber is a necessary step. Pressure-treated timber, sometimes called tanalised timber, has been coated and filled with preservatives, offering resistance to both moisture and insects. Water can cause both rot and decay to timber, while insects can burrow through timber beams, causing aesthetic and structural damage. If your 4x1 timber is at risk of moisture or insects, you should opt for a model that has been tanalised.

How is 4x1 Timber Used Within Construction?

4x1 timber has a variety of uses within a building project, though its low strength means it cannot be used for structural or load-bearing components. 

Barge Board and Fascia Boards

Barge boards and fascia boards are installed externally below the roof of a house, running along the lower edge. Fascia boards run horizontally and have the gutters fixed to them, while barge boards are diagonal, used within properties with gabled roofs. Due to the external insulation and the use of guttering, opting for a pressure-treated 4x1 timber is essential in this instance.


Planed square edge 4x1 timber can be used as floorboards within a property, supported by the floor joists to hold all weight. It’s important to opt for a planed wood when looking for flooring, due to its attractive appearance compared with rough sawn, and the risk of splinters that could come from using rough sawn timber as floorboards.

Non-Structural Use

A 4x1 timber beam can be used for many of the same non-structural applications as a 4x2, though it is unsuitable for load-bearing applications. This means that it can be used for non-load bearing applications within buildings or furniture. If being used externally you should opt for a pressure-treated 4x1 timber, and if you want the timber to be visible you should opt for a planed square edge timber beam.

Our Range of Quality Timber For Sale

Building Materials Nationwide works with builders and contractors across the country, supplying a range of quality building materials, including timber. We stock CLS timberPSE timberstructural timber, and pressure-treated timber, all of which can be dispatched nationwide at competitive prices. Customers working within the industry can take advantage of our trade accounts. A building Materials Nationwide trade account gets you access to a dedicated account manager, who can source building products for you through our wide network of suppliers. Sign up for a trade account online today.