Premium Deck Joists - C24 Stress Graded - Use Class 4 (Ground Contact)

From £4.52 (inc VAT) £3.77 (ex VAT) per linear metre

Silva offers a range of high quality treated carcassing material for use as deck joists, and other exterior structural applications.

Available from stock in two sizes, Silva Premium Deck Joists are produced from high-quality kiln-dried softwood. 

To make them suitable for exterior (in-ground) use, Silva Premium Deck Joists are high pressure preservative treated to Use Class 4 application in line with BS8417.  The high-pressure process ensures that the preservative treatment penetrates the wood deeply, to provide long-lasting protection.

For wood in permanent ground or fresh water contact, or providing exterior structural support, Use Class 4 levels of protection MUST be achieved. Anything less and service life, structural safety and customer satisfaction will be compromised.

There is a £450 (ex VAT) / £540 (inc VAT) minimum basket value applicable to all orders containing cladding, decking, and fencing products. Read the full details.

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Premium Deck Joists - C24 Stress Graded - Use Class 4 (Ground Contact)

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At a Glance

  • Available in two widths - 45 x 95mm / 45 x 145mm.
  • Moisture Content: Kiln dried to under 20%. 
  • Grade: Structurally graded C24 to European Standards EN14081-2.
  • Preservative Treated: Silva Premium Deck Joists are preservative treated to Use Class 4 application in line with BS8417. The timber undergoes a process of drying, high pressure-treating with a chemical preservative, then drying again to make it suitable for exterior (in contact with the ground or fresh water) use.
  • Regularised: Machined smooth with eased edges, so all its dimensions are regular and accurate, making handling and installation quicker and easier.
  • Sourcing / Certification: Silva's Premium Deck Joists are sourced from well-managed and sustainable forests. Our 45x145mm products are fully certified under the Pan European Forestry Certification (PEFC) scheme whereas our 45x95mm joists are FSC mixed credit. 

Product Details

In the cold Northern European climate, trees grow very slowly. This is a result of the winters being long and cold, leaving only a short summer growing season. The result of this slow growth is the best possible straight-grained timber with tightly packed growth rings. This tough, dense straight-grained timber with low internal tension is what gives our deck joists a superior strength, dimensional stability, and premium finish.   

C24 stress graded timber has fewer defects - such as knots - than C16 graded timber; therefore, it is stronger and more resilient. Also, the fact that C24 graded timber has fewer visual imperfections means it has a neater and more uniform appearance. The C24 stress grade can only be achieved with higher quality imported timber. With more inherent irregularities, fast-growing domestic timber can only reach C16 stress grade. 

Silva Premium Deck Joists are regularised with eased edges, which makes handling and working with them easier. The fact that they are machined to a smooth finish and a consistent size speeds up the build process and produces a neater, more accurate finished job.

Silva Premium Deck Joists are sourced from well-managed and sustainable forests. 

Size Linear Metre
45 x 95mm £3.77£4.52
45 x 145mm £5.65£6.78

The Use Class System

Softwood species decking components classified as semi or non-durable must be pressure treated, before installation, with an approved preservative to provide the service life required. Its intended end-use situation governs the amount of preservative protection required. These are categorised according to a Use Class system, which is detailed in a British/European Standard BS EN 335-1.

Use Class 3: includes timber deck components such as balustrades and deck boards that are not in contact with the ground.

Use Class 4: includes timber deck components such as posts, beams, or joists (occasionally deck boards) that will be in contact with the ground or fresh water*.

*As from January 2020, the Timber Decking and Cladding Association recommends that any decking timbers providing exterior structural support should be preservative pressure treated to Use Class 4 - regardless of whether they are in-ground contact or not.

Pressure-treating timber components to Use Class 4 ensures the enhanced durability and safety of deck substructures. It is in line with BS 8417 (Preservation of Wood: Code of Practice), which is to adopt this change in due course this year.

BS8417 is the principle standard governing timber treatment in the UK. 

Always check what Use Class your timber is treated to and the desired service life specification that applies either 15, 30, or more years, the industry default being 15 years.

Brush-applied wood preservatives or dip treatments are not suitable for the long-term protection of timber used for decking and other outdoor landscaping applications and should not be used.


Understanding Use Class 4

Understanding Use Class 4 pdf.


2 weeks.


£45.00 / £95.00 / £145 / £195.00 / POA depending on location.

You can view our detailed delivery charges and services here.

Please note: As with most building supplies, delivery is to the kerbside only.

Scheduled Collection


Typically within 48 hours (subject to stock availability).