Timber Decking

Decking allows us all to move more freely from internal living / working spaces to the outside.  It has consequently become very popular in both domestic and commercial settings, using both softwood and hardwood.  With very high exposure to the elements, it is very important to get the detailing right.


Using our FAQs, simple guides and decking detailing book below, let us help you get the design right first time.


For design consultancy, inspections and training on decking, call our technical helpline on +44 (0)1494 569601.  All commercial services are provided by TRADA’s service provider, BM TRADA.

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How do I maintain my deck?

All timber deck structures should be designed to give a long-term life expectancy. However surface checks and splits are inevitable as the timber weathers. Any decking will benefit from regular cleaning. 

Decks using durable or copper organic treated timbers should not need any maintenance other than regular cleaning with a stiff broom brush. Decks in shaded areas, such as under trees, may require more cleaning, as the deposits from vegetation combined with moisture may promote algae growth. 

A more thorough clean can be done with a pressure washer or mechanical brush. Care should be taken, however, as some machinery may damage the surface of some softer timbers. 

There are a number of decking stains, decking oils, water repellent treatments and anti-slip surface products on the market. These will need maintenance as per the manufacturer's instructions. Such finishes are often not required, but are used to change the appearance of the deck, or improve slip resistance.

The slipperiness (slip resistance) of a deck depends on a complex set of factors, some of which can be controlled by selection of appropriate materials, good design and maintenance.

  • Surface water – ensuring board gaps of at least 8mm and providing the deck with a slight fall promotes drainage. Board surfaces can be profiled to provide grooves into which water drains, or chamfered to promote drainage to their edges. A ribbed surface on the underside of the board (not the face) facilitates water drainage away from the supporting joist.
  • Direction of pedestrian travel – walking across a grooved surface provides greater friction and makes for a less slippery surface; walking along the grooves may reduce friction.
  • Algae and build-up of plant debris – algal build-up results in the timber surface becoming more slippery when wet, while the build up of debris prevents water drainage of water, making the deck more slippery. Regular cleaning of decks in shade or under trees is particularly recommended.
  • Proprietary anti-slip inserts and anti-slip deck coatings are also available.

Further guidance on avoiding a slippery deck can be found in TRADA’s publication Timber decking – the professionals’ manual.

TRADA advises that timber posts are kept out of ground contact and out of the splash zone where possible.

If they are in ground contact, that they should go into concrete foundations that provide drainage at the base. This can be done with the use of loose fill around the post, and a drainage hole in the concrete footing.

If you do wish to use timber in ground contact, then you will need to consider the appropriate species and preservative treatment type and application. These can vary considerably.

The Wood Protection Association provides a specifiers' guide to wood to treatment, or treatment manufacturers may be able to provide the relevant information. Treatment options will range from approximately 15 to 45 years' service life.

For decking over or next to water, TRADA recommends the use of durable or very durable species such as Ipe, Jatoba Balau and Cumaru, to name a few. 

If you are using treated timbers, then the treatment should be copper organic to a suitable standard. The Wood Protection Association produces a specification and practice document if you need further guidance, or alternatively you can contact the timber preservative manufacturers. 

See TRADA's Wood Species Database to find more timbers which are durable or better. Alternatively many supplier websites will list timbers na dtheir properties.

If you are unsure of a timber species' suitability for decking, and cannot find information regarding its properties, email advisory@trada.co.uk

TRADA recommends that these gaps are 8mm or more, depending on the board profile and moisture content at time of installation.

If these gaps are less than 6mm, the boards may not shed water efficiently and, as a result, could become slippery.

Gaps wider than 10mm may also be an issue, particularly with potential health and safety issues regarding for example stiletto heels, pram or bicycle wheels, roller blades etc.  However there is no regulation dealing with this.

Some board profiles will have the edges of boards cut to an angle so the bottom of the gap is larger than the surface. These may be used where a smaller gap on the top surface is needed.

Further guidance on board profiles and spacing can be found in TRADA’s publication Timber decking – the professionals’ manual.

Boards can be fixed to joists by various means – such as nailing, screwing or proprietary clips. TRADA recommends stainless steel fixings, to avoid unsightly staining, although zinc coated or epoxy coated fixings can also give a good service life. Be aware that plated fixings can react with tannin in timber, causing corrosion of mild steel and staining of wood.

For the substructure, hot-dipped galvanized mild steel fixings and brackets may be adequate. Remember, however, that stainless steel and galvanized materials should not come into contact with each other due to electrolysis reactions. Nails are not appropriate for fixing any part of the substructure.

See the TRADA publication Timber decking – the professionals’ manual for detailed advice on fixing.

This issue more often presents itself when durable timbers are used on balcony decking on a rendered building. Typically, it happens when durable timbers are left unfinished.

As the boards weather, they can leach out water soluble extractives that can stain other boards, or the surfaces of other materials below or adjacent to them. Special care should be taken when using timber above absorbent materials, as stains may be difficult to remove. It is advisable to protect these surfaces from the runoff during the initial weathering process. This process is temporary and should stop once the boards have begun to weather down to a silvery-grey colour.

Yes and TRADA recommends the following:

  • Timber should be oversized in order to provide an extra safety factor.
  • If timber is to be left untreated, a durable or very durable species should be used, or care should be taken with the correct treatment specification.
  • Consideration should be given to liquids or items falling between the gaps of the decking – there are propriety products that can be inserted between the boards if this is a concern.
  • Health and safety measures such as guard rails should be incorporated.
  • Planning requirements should be considered.

TRADA membership

As a member of TRADA you can enjoy free access to our entire online collection of technical guidance on not only decking but all other construction related topics.

You can also use our Technical Helpline (01494 569601) for free at any time during office hours. (Please note that advice is limited to 30 mins per call. Commercial consultancy by BM TRADA can also be provided where written or professional opinion is required on specific products or projects.)


Introductory guidance

You can register for free and download some of the guidance below:


Comprehensive guidance / books

Timber decking – The professionals’ manual, 2nd edition (online edition for members only or available in our bookshop).



We run introductory and intermediate training courses on a range of timber topics.

In-house courses can also be provided.


Consultants and commercial services

BM TRADA is able to provide a wide range of commercial services including product testing, certification, CE marking, and product design advice.