Timber Lengths | Timber
Buying timber is straightforward once you know what the measurements, finish and strength grade actually mean. Whether you are framing a stud wall, building a roof, fitting a floor or making shelves, the right timber length will make the job quicker, neater and more reliable. At JJ Building Materials, our timber lengths range covers kiln-dried CLS, pressure-treated C24 joists, roofing battens, planed redwood boards and tongue-and-groove boards in practical sizes for trade and DIY projects.
Timber is commonly described using a mixture of metric and imperial measurements. You might hear a length called “4x2”, for example, while the product is listed as 47mm x 100mm. Both are useful ways of identifying the material, but the metric dimensions on the product listing are the ones to work from when setting out a job. Before ordering, check the section size, length, timber grade and whether it is treated or planed. Those details matter far more than simply choosing the nearest-looking piece of wood.
Timber sizes explained: metric, imperial and finished dimensions
In the UK, timber is sold in metric sizes, normally millimetres for thickness and width, and metres for length. Imperial names are still widely used on site, so it helps to know the usual equivalents. A 4x2 is normally supplied around 47mm x 100mm, a 6x2 around 47mm x 150mm, and a 3x2 CLS around 38mm x 63mm finished. These are approximate trade descriptions rather than an instruction to round every measurement up or down.
| Common name | Typical metric size | Typical use |
|---|---|---|
| 3x2 CLS | 38mm x 63mm finished | Light studwork, partitions and framing |
| 4x2 CLS | 38mm x 89mm finished | Stud walls and internal frames |
| 4x2 structural timber | 47mm x 100mm | Joists, rafters and general construction |
| 5x2 structural timber | 47mm x 125mm | Heavier framing and joist work |
| 6x2 structural timber | 47mm x 150mm | Joists and structural applications |
| 8x2 structural timber | 47mm x 200mm | Longer spans where specified |
Always use the listed measurements when your work needs to fit a precise opening, lining, fixing or bracket. Timber may be rough-sawn, regularised, kiln-dried or planed all round (PAR/PSE). Drying removes moisture and planing smooths the faces and corners, so finished timber is smaller than the original sawn size. That is normal, not a fault. It is also why you should not assume a timber piece described informally as 4x2 will be exactly 50mm x 100mm.
Common timber lengths and how to choose them
The most common stock lengths on this page are 2.4m, 3.0m, 3.6m, 4.2m and 4.8m. The best length is not automatically the longest one. Choose a length that reduces joins and waste, but is still sensible to lift, transport and cut safely. For repeated work, such as studs or noggins, calculate a cutting list first. You will usually save money by planning cuts around the available lengths rather than ordering excess material and hoping it can be used elsewhere.
Featured timber lengths
Shop these popular timber lengths directly from the guide.
For a 2.4m floor-to-ceiling partition, 2.4m CLS can be a practical starting point where the finished size and allowances suit the design. For longer rafters, joists or continuous runs, a 3.6m, 4.2m or 4.8m length may reduce joints. Structural spans should never be selected from a general rule of thumb alone: the member size, grade, support spacing, imposed load and local building-control requirements must all be considered. Follow the structural specification or get professional advice where the timber carries load.
Choose the right timber for the job
CLS timber for stud walls and internal framing
CLS, short for Canadian Lumber Standard, is kiln-dried and eased on the edges. It is designed to be easy to handle and comfortable to work with, making it a popular choice for internal stud walls, partitions, boxing-in and timber frames. The smoother finish is especially useful when working around plasterboard, insulation and internal finishes. For a smaller section, see the 38mm x 63mm finished CLS timber. For a deeper stud section, choose the 38mm x 89mm finished CLS timber.
C24 treated timber joists for structural work
C24 is a strength grade used for structural softwood. It is commonly selected where a higher graded timber is required for joists, rafters, decking subframes and other construction work. Pressure treatment offers added protection against fungal decay and insect attack, but it does not replace proper design, ventilation or suitable detailing. Keep cut ends protected when the manufacturer recommends it, prevent timber from sitting in standing water, and use corrosion-resistant fixings for external work.
Our 47mm x 100mm C24 treated timber joist is the familiar 4x2 size for many general construction jobs. Where a deeper section is specified, shop the 47mm x 150mm C24 treated timber joist, or the 47mm x 200mm C24 treated timber joist. Do not substitute sizes because they look close: a small change in depth can make a significant difference to structural performance.
Roofing battens for tile and slate roofs
Roofing battens provide the fixing line for roof tiles or slates, so they need to be suitable for the roof covering and layout. A batten should not be chosen just by its colour or its price. Check the roof specification, rafter centres, tile manufacturer guidance and required batten standard. Our treated Blue/Yellow Roofing Batten BS5534 – 3.6m is a useful option for compatible roofing applications. Store battens flat, supported and protected from excessive weather before installation to help keep them straight and dry.
Planed redwood boards and pine T&G for finishing work
For shelving, furniture, boxing, decorative joinery and internal repairs, planed boards give you a cleaner starting point than rough-sawn timber. The 25mm x 75mm planed redwood board is useful for smaller trims and general joinery, while the 25mm x 150mm planed redwood board gives you a wider face for shelves and wider details. For a profiled internal lining or board finish, consider the Richard Burbidge pine tongue-and-groove pack.
How to order the right amount of timber
Start with a cutting list. Write down every component, the quantity needed and the exact cut length. Group cuts by section size, then work out which stock length gives the least waste once you allow for the saw kerf. Add a modest allowance for offcuts, trimming and any pieces with visual defects that you do not want in a prominent finished area. The right allowance depends on the job; a repeated framing layout typically creates less waste than a one-off joinery project with many different cuts.
Check the product listing for whether a product is priced per metre, per length, per pack or has selectable length variants. Confirm that the timber will fit on site and that you have a dry, level area ready to store it. Longer lengths need enough support to prevent sagging. Keep timber clear of the ground, cover it while allowing air to circulate, and bring internal-finish timber into the room where it will be used in good time to acclimatise. Do not seal it in plastic where condensation can build up.
Timber cutting, fixing and safety tips
Measure twice before cutting and mark the waste side of your line. Use a square so cuts are true, and support long lengths at both ends. Wear suitable eye protection, hearing protection and a dust mask when cutting or sanding. Treated timber dust should not be inhaled, and offcuts or sawdust should not be burned. Choose screws, nails, joist hangers and other fixings that are appropriate for the timber treatment and the environment; galvanised or stainless-steel fixings are often needed for external treated timber.
Timber is a natural material, so knots, grain variation and small movement can occur. Select straight, sound lengths for accurate framing and visible joinery, and reserve shorter or less visually uniform pieces for noggins, blocking or concealed work where appropriate. For exterior projects, leave the drainage and ventilation gaps the system requires. No timber treatment can compensate for trapped moisture or poor detailing.
Shop timber lengths at JJ Building Materials
From a single replacement length to a full framing or roofing order, our timber range makes it easier to choose the correct material for the task. Use the filters above to shop by thickness, width and length, then check the product information before adding to basket. You can also browse our wider timber range for sheet materials and other timber products.
Frequently asked questions
What lengths does timber come in?
Timber lengths vary by product, but common options include 2.4m, 3.0m, 3.6m, 4.2m and 4.8m. Use the length filter on this collection to see the available options for the section you need.
What is the difference between CLS and C24 timber?
CLS is a style of kiln-dried, eased-edge timber commonly used for internal framing and stud walls. C24 is a structural strength grade. A product’s suitability depends on its full specification, not simply whether it is called CLS or C24.
Can treated timber be used indoors?
It can be used where suitable, but untreated kiln-dried timber or CLS is often preferred for dry internal work. Choose based on the setting, the finish required and the product specification.
Why is a 4x2 not exactly 50mm x 100mm?
“4x2” is an established trade name. Timber is dried and often planed after sawing, which reduces the finished dimensions. Always use the product’s stated metric size for design and installation.
