Timber Joists

Timber joists are one of the most essential structural components in any building project – whether you are laying a floor, building a deck, constructing a roof, or framing an outbuilding. Getting the right joist for the job means understanding the grade, the size, and the treatment, and having access to a wide enough range to match your exact project requirements.

At Timberstore, we stock one of the widest ranges of timber joists available in the South East, from the most common sizes right through to larger sections that are genuinely hard to find elsewhere -including 300mm depth joists and lengths up to 7.2m – both genuinely hard to source from most merchants and something we’re proud to be able to offer.

Whether you need floor joists, deck joists, roof joists, or ceiling joists, you’ll find what you need here, available to order online with South East England delivery or click and collect from our yard in Beaconsfield.

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Grade, Quality and Treated Timber

Our timber joists are predominantly C24 grade - the higher of the two most common structural timber grades used in the UK. We do hold some C16 in stock, but C24 is our primary stock grade and is what you will find across most of our range. C24 offers greater strength and resilience than C16, making it the preferred choice for most structural floor, roof, and deck applications.

All our timber joists are pressure treated as standard, helping to protect against moisture and reducing the risk of rot and fungal decay. This makes them suitable for a wide range of applications including deck frames, roof structures, floor builds, outbuildings, and general construction - wherever a strong, reliable, treated structural timber is needed.

Wooden Joists FAQ

What is a timber joist and what are roof joists?

A timber joist is a horizontal structural beam used to support floors, ceilings, roofs, and decks. Joists span between supporting walls, beams, or posts and carry the load of whatever is above them - whether that is flooring, roofing material, or a deck surface. Choosing the right size, grade, and treatment for your joist is essential for a safe, stable, and long lasting structure.

What do joist sizes mean? What is 6x2?

Timber joists are commonly referred to by their imperial names - 6x2, 7x2, and so on but are bought and sold in metric dimensions. It's worth understanding the difference:

The imperial name (e.g. 6x2) is simply the traditional name the size is known by in the trade
The nominal metric size is how we buy and stock it - for example, a 6x2 is stocked as 47x150mm joist
The finished size after processing is always slightly smaller - a 47x150mm joist will typically finish at approximately 45x145mm

This applies across all sizes in our range. When ordering, always use the nominal metric size as your reference, and factor in the finished size when calculating your build-up thickness or structural requirements.

All our joists have eased edges - the corners are slightly rounded during processing, which makes them safer to handle and gives a cleaner, more consistent finish.

Treatment Options and Kiln Dried Timber

Our joists are available in regularised green pressure treated options, with pressure-treated joists for outdoor use also available kiln dried, reducing moisture content and improving stability. UC4 treated timber is the suitable option where timber is coming into direct ground or water contact. This makes them suitable for a wide range of applications - from internal floor and ceiling joists where moisture resistance is less critical, to deck joists and other projects for outdoor use where pressure treatment helps protect the timber.

Always treat any cut ends with an end grain preservative after cutting, as the original treatment only penetrates the outer layer of the timber, and proper retreating helps ensure long-term durability.

What Are the 4 Types of Joists, Including Deck Joists?

Joists are heavily used in construction, and this page mainly covers solid timber joists, which are traditional lengths of sawn lumber cut to specific dimensions for structural use.

The four main types used are:

Floor joists run horizontally to support the floor structure, carrying the weight of the floor and everything on it, including on mezzanine levels used to create intermediate platforms
Deck joists support the decking boards above and form the structural frame of a timber deck. Pressure treated timber is essential for deck joists given their outdoor exposure
Ceiling joists span between walls at ceiling level, providing structural support to the ceiling and helping carry the load of the roof rafters above
Rim joist fixed perpendicular to the ends of the floor or deck joists, they close off the joist frame and add rigidity to the overall structure

Engineered timber joists are another category used where longer spans or better stability are needed, offering superior strength-to-weight ratios and resistance to warping. Laminated veneer lumber is one form of engineered timber joist.

Joists serve a multitude of functions and choosing the right type and quality for the task is important. Here at Timberstore, we offer several ideal options of each type. Whether you want a pergola, deck joists or ceiling joists, you'll find them here!

What is the difference between C16 and C24 timber?

C16 and C24 are the two most commonly used structural timber grades in the UK, both conforming to British Standards for use in floor, ceiling, and roof construction; C16 is generally cut from faster grown UK timber, while C24 is denser, slower grown wood. C24 is the higher grade, offering greater strength and resilience - which is why it is the preferred choice for most structural applications and the grade we predominantly stock. We do hold some C16 in stock, as the packs we purchase are usually a mix of both, but if you need to specify a particular grade for your project, please check availability when ordering or get in touch and we'll advise.

What does regularised mean? & What are eased edges?

Regularised timber has been processed after sawing to give a consistent, accurate cross-section throughout its length. This makes it much easier to work with on site - joists sit level, frames go together cleanly, and you spend less time packing out uneven timbers.

Eased edges means the sharp corners of each joist have been slightly rounded during processing. This makes the timber safer and more comfortable to handle on site, reduces the risk of splitting at the corners, and gives a cleaner, more consistent appearance. All our joists are regularised and supplied with eased edges as standard.

What sizes of timber joists do you stock?

We stock one of the widest ranges of joist sizes available from a single merchant in the South East, including popular sizes such as standard 47x100mm (4x2), and 47x150mm (6x2), right through to 47x300mm - a depth that is genuinely hard to source from most merchants.

What length joists do you stock?

Our joists are available in a range of standard lengths starting from 2.4m, with the most popular lengths being 3.6m and 4.8m. Most sizes are available up to 6m, which is the longest length you'll commonly find from most timber merchants. We also hold selected sizes in 7.2m - a length that is genuinely hard to come by and not something many merchants can offer. Please check individual product pages for current availability, or get in touch and we'll be happy to help you find the right length for your project.

Can I order online?

You'll always get our best price when you order through the Timberstore website. We deliver across South East England using our own fleet, covering Buckinghamshire, Berkshire, Oxfordshire, Hertfordshire, Greater London, Surrey, West Sussex, and Hampshire.

You are also welcome to click and collect from our yard in Beaconsfield, HP9 2RX.

Not sure which size or grade you need? Just get in touch and we'll be happy to help.