A 47 x 150 mm treated timber joist is not simply a piece of wood with a green tint. Its finished size, strength grade, treatment level, span and fixing detail all affect whether it is right for the job. Getting treated carcassing timber sizes right before materials arrive on site prevents wasted cuts, uneven levels and costly return visits.
For fencing rails, decking frames, raised garden structures and many external structural jobs, treated carcassing is a dependable, cost-effective choice. The key is ordering the actual section you need, rather than working from a rough nominal measurement or assuming every treated timber length is suitable for structural use.
What treated carcassing timber sizes mean
Carcassing timber is sawn softwood intended for structural and general construction work. It is commonly supplied in standard widths and depths, then sold in a choice of lengths to suit floor joists, roof work, studwork, decking subframes and garden construction.
The dimensions shown on a product listing are normally the finished, planed size. A 47 x 100 mm length, for example, measures approximately 47 mm thick by 100 mm deep after machining. This matters when you are setting out joist hangers, levelling a frame against existing work or calculating how much insulation can sit between timbers.
Do not confuse finished sizes with older nominal descriptions. A piece historically referred to as 2 x 4 is not generally 50 x 100 mm when bought as regularised carcassing. In modern trade ordering, use the listed metric section size and check the specification before cutting or fixing.
Thickness and depth are not interchangeable
The first measurement is the thickness, while the second is the depth. So, 47 x 150 mm timber is 47 mm thick and 150 mm deep. When used on edge as a joist or rafter, the 150 mm depth does much of the structural work. Turning it flat reduces its load-carrying capacity significantly.
That is why a deeper section is often selected for longer spans, while the same thickness may be retained to suit standard metalwork, fixings and adjoining construction. A 47 mm thickness is particularly common for structural timber because it works with many standard joist hangers and restraint straps.
Common treated carcassing timber sizes
The right section depends on the job, the span, loading, centres and whether the timber is acting as a structural member or simply a support rail. These are common sizes seen across external and structural work:
- 47 x 75 mm is often used for light framing, noggins, battens, rails and smaller garden structures.
- 47 x 100 mm suits decking joists over shorter spans, shed framing, heavier rails and general external construction.
- 47 x 125 mm gives extra depth for stronger decking frames, roof members and floor framing where design requirements allow.
- 47 x 150 mm is a widely used joist and rafter section, especially where a shallower timber will not achieve the required span or stiffness.
- 47 x 175 mm and 47 x 200 mm are used where greater depth is needed for floor joists, roof construction and more demanding structural layouts.
Wider sections, such as 75 x 75 mm, 75 x 150 mm or 100 x 100 mm, are also useful for posts, bearers, sleepers and substantial framing. However, a larger piece of timber is not automatically a structural solution. The grade, span, bearing details and imposed load still need to be considered.
Lengths commonly run from shorter 2.4 m pieces through to 4.8 m, 5.4 m and longer, depending on the section and stock range. Buying the nearest sensible length reduces joints and offcuts, but longer timber needs more care during delivery, handling and storage. It can also have a greater tendency to move if left unsupported or exposed before installation.
Allow for trimming and end cuts
If you cut treated timber, especially for a post, bearer or frame exposed to weather, the freshly cut end should be treated with an appropriate end-grain preservative. Factory treatment protects the original timber, but cutting exposes untreated fibres where moisture can enter.
Allow extra length for trimming damaged ends, creating falls or scribing into uneven masonry. For a deck frame, it is usually better to keep joist ends aligned over a bearer and trim them cleanly than to make every length exact before the frame is set out.
Choose the right grade as well as the right size
Treated carcassing can be supplied as C16 or C24 structural timber. The size may be identical, but the strength classification is not.
C16 is widely used for regular domestic construction where the design allows it. C24 is a higher strength grade with tighter limits on defects and better design values, making it a common choice where spans are longer, loadings are higher or a specification calls for it. For roof timbers, floor joists and structural decking, follow the engineer’s drawings, Building Regulations requirements and span tables rather than selecting solely on price.
Treatment does not turn lower-grade timber into higher-grade timber. It helps protect the timber against fungal decay and insect attack in the conditions for which it has been treated. Structural performance still comes from the section size, grading and correct installation.
For non-structural work such as a basic fence rail or temporary garden edging, a structural grade may be more than the job requires. For a raised deck, balcony approach, floor or roof, do not make assumptions. The consequences of undersizing are far more serious than a slightly higher material cost.
Treatment level and where the timber will be used
Most treated carcassing timber is pressure treated for external use above ground. This makes it suitable for applications exposed to normal weathering, provided it is detailed properly and does not sit permanently wet.
For timber in direct ground contact, such as fence posts or retaining elements, choose a product specifically rated for that use. Above-ground treated timber can fail prematurely if buried in soil, trapped against wet ground or installed where water cannot drain away.
Decking frames are a good example. Treated joists should be kept clear of the ground on suitable supports, with airflow beneath the deck. Use corrosion-resistant fixings compatible with treated timber, particularly in damp or coastal conditions. A correctly sized C24 joist will still have a short life if water is held around its ends or connections.
Match the timber size to the detail on site
The section must work with the rest of the build. If insulation is specified at 140 mm, a 47 x 150 mm joist may provide useful room for services and ventilation, while a 47 x 100 mm joist may not. If joist hangers are sized for 47 mm timber, substituting a 45 mm or 50 mm section can create a loose or awkward fit.
For decking, joist centres affect the choice as much as the joist depth. Composite boards, grooved boards and thinner deck boards may require closer centres than traditional timber decking. A frame designed around 400 mm centres may need revising if the board manufacturer specifies 300 mm centres.
For roof work, rafter size is governed by pitch, span, spacing, covering weight, ceiling load and local conditions. For floor joists, consider the clear span, partition loads, openings, trimming members and vibration as well as basic strength. When the layout is outside a straightforward domestic arrangement, get the calculation or specification before ordering.
Storage, straightness and practical ordering
Treated softwood is a natural material. Some variation in colour, knots and minor movement is normal, particularly after exposure to sun or changing moisture levels. Store packs flat, supported at regular intervals and raised from the ground. Keep them covered from rain while allowing air to circulate – wrapping timber tightly in plastic can trap moisture.
When ordering, calculate the number of full lengths from the cutting plan rather than dividing total metres by a standard length. Include noggins, trimmers, blocking, doubled members and a sensible allowance for cuts. Ordering an extra length or two is often cheaper than stopping a job for a small shortfall.
For regular site work across Hertfordshire and nearby London areas, Wel Timber & Building Supplies can help match treated carcassing sections, grades and lengths to the specification, with trade-focused stock and delivery options built around keeping work moving.
Measure the finished section, check the grade and treatment rating, then order to the actual layout – that is how treated timber arrives ready to earn its place in the build.
