Product Type

Wood for Exteriors



Decking has become big business in the UK in recent years, as home and building owners seek to maximise their use of gardens and outside spaces.

Timber decking is an attractive and economical way to create a unique, aesthetically pleasing outdoor space, and can be used to section off gardens and even to create seating areas. Decking is ideal for many different residential and commercial projects, from patios and terraces to walkways, piers and playgrounds.

When correctly specified, designed, installed and maintained, timber decking structures can be enjoyed year-after-year.

There are many different deck boards available, including enhanced grain and ‘brushed’ wood looks. Deck boards can be made using a number of different types of timber, including:

  • Softwoods: Quality Redwood (Pine) deck boards are still the mainstay of the market, alongside more economical Whitewood (Spruce). Other softwoods used for decking include Douglas Fir, Western Red Cedar and Larch.
  • Hardwoods: Hardwood species used for deck boards include Balau (Yellow Balau), Cumaru, Ipe, Iroko, and Massaranduba. Hardwood deck boards are denser and heavier than standard softwoods and so require extra support. It also requires joists that will wear at the same rate as the boards – softwood joists must be a minimum of Use Class 4 preservative treated to ensure they do not fail before the hardwood deck boards do.
  • Composites: Wood plastic composites (WPC) are made by combining plastics with wood fibre. As plastics in construction are an issue, make sure the composite product you purchase can be recycled. Composite decking can be heavier than standard softwood and may require the supports to be spaced closer together.
  • Modified woods: Thermowood®, Kebony® and Accoya® modified woods are all available as decking, or can be used to create deckboards. They can also offer a longer lifespan than softwood.

Supporting joists/posts: Decking joists must be able to carry the intended load, and must also have the same intended lifespan as the deck boards used on top.

Softwood joists and posts must always be treated with a Use Class 4 timber preservative treatment. Incising is used in difficult-to-treat species, such as Spruce, to aid penetration of the wood preservative so they can more readily achieve the Use Class 4 treatment specification.

  • Softwood: To comply with building regulations all decks should be built with strength-graded timber. C16 is the minimum strength class that should be used, but C24 strength is recommended for domestic decks that will carry heavy loads.
  • Hardwood: Match or exceed the structural strength and durability of the hardwood decking above the frame. Visit the hardwoods table on page 20 to discover relevant structural species.
  • Composite: These systems have their own joists designed to support their load. Ask your supplier or consult manufacturers’ websites.
  • Modified wood: Joists aren’t generally available so match the frame material to the deck’s expected lifespan.

Decorative accessories for decking include spindles, with relevant handrails and base rails; newel posts, and post caps, which can vary from round to acorn to square. Match the material to the decking system.

Some decking systems offer complete balustrading panels instead of individual components. Always check the loadbearing capacity – balustrades load tested to Building Regulation requirements are available.

  • Handrails: may feature a shaped, rebated underside for securing the top head of spindles/balustrades. Some may be dual purpose hand/base rails. Materials and sizes vary.
  • Base rails: these normally have a single, flat rebate on their upper side to receive the bottom ends of spindles/balustrades.
  • Spindles/balustrades: square or rounded, turned or plain.
  • Newel posts: can be square or rounded, turned or plain.

Decking, especially standard softwood deck boards and Use Class 4 Deck Joists, can be in short supply at certain times of year, so plan ahead to make sure you have boards available when required.

The available capacity for timber preservative treatment can also affect lead times.

Joist material and accessories may also have extensive lead times. The available capacity for timber preservative treatment can also affect lead times.

Slip-resistant (anti-slip) deck board finishes come in many forms using softwood, hardwood or modified wood deck with aggregate or specialist inserts; as well as composite products. These products should have independent testing to ensure it achieves a PTV value of 36+ in a pendulum test. This is an ex-factory result and should be retested over time to ensure the slip resistance is maintained above 36, unless the supplier provides test evidence or a warranty to support sustainable slip performance.

Available deck board sizes vary depending on the type of wood species being used:

  • Softwoods:
    • Pine and Spruce: from 32x125mm nominal (27x120mm finished) to 38x150mm nominal (32x145mm).
    • Douglas Fir: from approx. 25x145mm to 35x190mm
    • Larch: from approx. 21x95mm to 28x145mm
    • Western Red Cedar: approx. 25x140mm
  • Hardwoods: (sizes vary with tree species and log size). Examples include:
    • Balau (Yellow or Red): from approx. 19x90mm to 28x145mm
    • Cumaru*: from approx. 19x140mm to 28x145mm
    • Ipe*: from approx. 19x140mm to 28x145mm (*see page 15)
    • Iroko: from approx. 20x90mm to 20x145mm
    • Massaranduba: approx. 21x145mm
    • Composites: WPC decking is an extruded product and technically can be made to any size. Consult suppliers’ catalogues.
  • Modified Woods:
    • Thermowood® Softwood version: 26x118mm finished;
    • Hardwood (Ash) version: from approx. 20x95mm to 21x150mm
    • Kebony® 22x145mm
    • Accoya® approx. 20x145mm or slip resist version: 22x142mm.
  • Slip-resistant boards:
    • Softwood sizes range from approx. 27x120mm finished size to 34x145mm finished size, while hardwoods are approx. 22x142mm.
  • Supporting joists
    • Softwood: suitable material ranges from 47x100mm to 47x225mm.
    • Hardwood: the material must be suitable for structural purposes, so check with your supplier before sale or use.
  • Composite:
    • Joist sizes will vary with the manufacturer and the size of deck boards they produce. Most composite products require more closely-spaced joists or supports to avoid problems.
  • Decking accessories:

These vary widely in shape, dimensions, fixing methods and availability, according to the decking system being chosen. Check with your suppliers and get a precise list of needs from the customer before quoting.

Softwood deck boards require pressure-impregnation with a Use Class 3 preservative treatment, which usually has a 15-year service life, but longer service lives are available from some suppliers. Durable hardwood deck boards can be oiled annually but need no preservative. Composite decking and modified wood decking needs no preservative.

Softwood decking accessories should be pressure treated to a Use Class 3 specification to ensure it is durable enough for the end use.

Softwood decking joists (under frame, sub-frame and joists) need Use Class 4 timber preservative pressure treatment. Structural hardwood that is suitably durable for external use doesn’t need to be treated with a wood preservative.

More information about timber treatments is online at

The default desired service life of standard softwood deck boards is 15 years, depending on correct installation and maintenance procedures being followed.

If you would like more information or advice, we would recommend you contact TDUK members who supply these products.