7th Dec 2022 -

3x3 timber is a sturdy option and is commonly used for household features and as fence posts. In this overview from Building Materials Nationwide, we’ll look through the accurate sizing of a 3x3 timber beam, the different finishes available in the UK, and the key uses for it within a building project.

What Size is 3x3 Timber Sold As?

Within the UK, timber will be sold in metric sizes. This is government regulated, and though imperial sizes are still used informally, the actual measurements will likely be listed in mm. The size 75mm x 75mm is the UK equivalent of a 3x3 timber, though this is actually around 2.95 x 2.95. It should also be noted that this nominal sizing for timber will likely be larger than the actual size of any timber beam you purchase.

How Much Smaller Will the Actual Size Be In mm?

Timber is sold in the sizes it was originally cut to, despite usually having been dried and potentially sanded to a smaller size. Rough sawn timber will be cut to the listed size, but the high water content of this green timber makes it unsuitable for many building applications. Kiln-drying will make timber stronger and more consistent, but by lowering the moisture level to below 20% it will effectively shrink the size of any timber beam. Planing is another option that will reduce the size, sanding faces to a consistent flat look. While 3x3 timber will be listed at 75mm x 75mm the actual size in mm will be around 64mm x 64mm.

How is 3x3 Timber Sold In the UK?

Alongside kiln-dried rough sawn options, 3x3 timber can be bought in a couple of different finishes, each bringing specific benefits to a project:

PSE 3x3 Timber

Either called PSE (planed square edge) timber or PAR (planed all round) timber, this is a 3x3 timber beam that will have been planed on each side, leaving square edges. Planing gives the wood a smooth and attractive appearance, allowing it to be used within visual features. It also means that people won’t get splinters from handling PSE timber, allowing it to be used throughout a building.

Pressure Treated 3x3 Timber

When looking to use 3x3 timber externally, untreated softwood is prone to rot and decay from water, with rain and the moisture within soil causing serious issues. It is also prone to damage from wood-boring insects, which can harm both the look and the integrity of your timber beams. Pressure-treated timber, sometimes called tanalised timber, has been treated with preservatives to give it resistance to both moisture and insects. When looking to install softwood timber externally, you should be looking to buy a pressure-treated option.

How Can 3x3 Timber be Used?

3x3 timber can obviously be used in general purpose applications, but there are some specific areas within a house or garden where it offers specific benefits:

Fence Posts

The most popular use for 3x3 timber is as fence posts, with their sturdiness and square shape creating a stable and long-lasting support for thinner beams to be attached to. When selecting 3x3 timber for use as fence posts, ensure you opt for a pressure-treated option. Fence posts will be exposed to moisture from both rain and the soil they are placed in, and untreated timber could quickly rot.

Corner Studs 

Stud walls offer a quick and effective way to split a larger room into several smaller rooms, with solid timber frames ensuring they are built to last. If you’re looking to orchestrate a more advanced series of stud walls, such as two or three stud walls that connect together, corner studs can be used to ensure a stable join between them. 3x3 timber beams make a great choice for a floor to ceiling corner stud, ensuring that more complex stud wall designs are properly supported. As this will be internal and hidden from view, rough sawn timber can be used.


Handrails within a home need to be large enough to offer support, and 3x3 timber can make a great starting point for a variety of designs. Buying PSE 3x3 timber is a must due to the visual nature of handrails and the obvious risks of splinters. When installing handrails externally, such as for stairs up to decking, you should also ensure you use a pressure-treated option.

Our Range of Timber Supplies

Building Materials Nationwide aims to offer everything a builder or contractor could need from one handy location, and delivers a full range of timber across the UK. Within our timber and joinery category you’ll find quality treated timber and PSE timber, alongside sturdier options like CLS timber and structural timber – all at competitive prices. For customers in the industry we also offer trade accounts, getting you access to a dedicated account manager, our full product range, and our speedy delivery options. We work with our customers to save them time, money, and hassle. Sign up for a trade account through our website today.