Treated Construction Timber

Construction timber is used for building timber frames, stud walls, floor and roof structures, decking subframes, garden buildings, fencing frameworks and many other general building applications. The correct timber depends on whether the job is structural, the load it needs to carry, the span involved and whether it will be exposed to moisture or weather. 

No. Not all construction timber requires a structural strength grade. C16 and C24 grading is important where timber is being relied upon structurally, such as certain joists, rafters, beams and framing members. General-purpose treated timber can also be used for suitable non-structural applications without being C16 or C24 graded. Check the individual product specification rather than assuming that all treated construction timber is strength graded. 

C16 and C24 are structural strength classes. C24 timber has higher characteristic strength and stiffness than C16 and may therefore be specified for more demanding structural applications or where greater performance is required. This does not mean C24 should automatically be used for every project — the correct grade should be determined by the building design, span and loading requirements. 

If the timber is being used structurally, check the drawings, structural calculations or project specification. These should normally state the required timber section and strength grade. You should not substitute a lower grade simply because the timber is the same physical size. If no structural grade has been specified for a load-bearing application, obtain appropriate building or structural advice before ordering.

47x100mm is normally the nominal timber size before regularising and finishing. Machining removes a small amount of material from the faces and edges, so the finished dimensions are smaller. For example, some of our nominal 47x100mm construction timber finishes at approximately 45x95mm. Always check the finished or true timber dimensions shown on the individual product page where exact dimensions are important. 

Regularised timber has been machined after sawing to produce more consistent dimensions and a neater surface than rough-sawn timber. Structural construction timber is also commonly supplied with eased or rounded edges, making it easier and more comfortable to handle. Regularising is also why the finished dimensions are normally slightly smaller than the nominal size used in the product name.

Kiln dried timber has been dried under controlled conditions to reduce its moisture content before use. This makes it more stable and generally easier to work with than freshly sawn wet timber. Kiln drying helps reduce movement, but timber is a natural material and can still shrink, expand, twist or develop surface checks as moisture levels change.

Treated construction timber is commonly used for exterior applications such as decking subframes, garden buildings, fencing frameworks and other outdoor structures. However, the treatment specification needs to be appropriate for the level of exposure. Timber intended for above-ground outdoor use is not automatically suitable for permanent ground contact, so check the individual product specification for more demanding applications.

Yes, strength-graded construction timber is commonly used for floor joists, ceiling joists, rafters and other structural members. However, the required section size and grade depend on factors including the span, spacing between members, imposed load and the construction of the building. Do not choose a joist size purely from its appearance or from another project — follow the structural design or appropriate span tables.

Treated construction timber is commonly used for decking joists and supporting subframes. The section size, joist spacing and support arrangement should suit the deck design and span, and the treatment should be suitable for the intended outdoor exposure. Good drainage and ventilation around the subframe will also help maximise the service life of the timber.

Where pressure-treated timber is cut, drilled or notched, newly exposed surfaces should normally be coated with a suitable end-grain or cut-end preservative. This helps restore protection to areas where cutting may expose timber with less preservative penetration. Pay particular attention to cut ends that will remain exposed to moisture.

Timber is a natural material and will respond to changes in moisture and temperature. Small amounts of bowing, twisting, shrinkage, surface checking and variation in grain or colour can occur even in kiln-dried construction timber. Timber should be stored flat, supported correctly and protected from excessive changes in moisture before installation.