Treated UC4 C24 Timber Decking Joists are high-grade, structural softwood timber sections pressure-treated to Use Class 4 (UC4) standards for direct ground contact and heavy load-bearing outdoor applications.

Primary Uses & Key Features

• Decking Subframe Construction: Used to construct load-bearing frameworks and joist networks beneath timber or composite decking boards.

• Ground Contact Structural Framing: Specifically treated for direct contact with wet soil, concrete pads, or ground moisture without rotting or decaying prematurely.

• C24 Structural Strength Rating: Kiln-dried and strength-graded to C24, offering superior density, higher load-carrying capacity, and wider clear spans compared to standard C16 timber.

• UC4 Heavy Duty Pressure Treatment: Impregnated deep into the timber core with preservatives, providing enhanced protection against fungal decay, insect attack, and constant moisture exposure.

• Retaining Walls & Raised Garden Beds: Ideal for building sturdy garden sleepers, low retaining walls, step frames, and raised timber structures subject to soil contact.

Available for collection from our Rugby branch or local delivery using our own fleet.

Delivery is kerbside and the final offloading position is always subject to safe access and the driver’s discretion.

Please tell us before delivery about:

• Narrow roads or restricted entrances
• Parked vehicles or limited turning space
• Overhead cables, trees or other obstructions
• Soft, sloping or unsuitable ground
• Any other access concerns

Enter your postcode on the product page to check your delivery area. We’ll confirm the delivery date after your order is placed.

What does "UC4" mean, and why is it important for decking joists?

UC4 stands for Use Class 4, the British and European standard rating for timber exposed to constant high moisture and direct ground contact. UC4 treatment uses deeper pressure impregnation than standard UC3 garden timber, guaranteeing resistance to wet rot and ground fungi for 15+ years.

Do you need to treat the cut ends of UC4 treated joists?

Yes. Sawing, drilling, or notch-cutting timber exposes the untreated inner core of the wood. All exposed cut ends must be liberally treated with a brush-on end-grain preservative to preserve the timber's integrity against rot.

How far apart should C24 decking joists be spaced?

For standard timber decking boards, joist centers are typically set at 400mm. If you are installing composite decking or laying boards diagonally, joist centers must be reduced to 300mm to prevent the deck boards from flexing or sagging underfoot.

Can UC4 treated C24 joists be buried directly into concrete?

While UC4 timber is rated for ground contact, embedding timber posts directly into solid concrete can trap water against the wood. It is best practice to mount joists slightly above concrete slabs or soil using adjustable decking pedestals, post shoes, or gravel beds to ensure proper drainage and air circulation.