UK standards organisation, BSI, is raising the level of preservative treatment required for timber deck substructures. This means that
all softwood components used for the structural sub-frame of a deck will have to be treated to the same level as components in permanent contact with the ground and freshwater. This requirement will come into effect when the new edition of BS 8417, the British Standard for wood preservation is published. The move has been welcomed by trade organisations like TTF, TDCA and the WPA. In this edition of F&LN, Gordon Ewbank, CEO of the WPA explains what led to BSI making this change.

The role of BS 8417 BS 8417 is published as a Code of Practice and provides recommendations and guidance on the preservative treatment essential to giving wood products the durability to perform predictably in all end-use situations.

BS 8417, groups all external above ground end-uses for uncoated wood in the same classification category – Use Class 3u. This is where the wood is unprotected by a coating and exposed to frequent wetting. Wood products that will be in permanent contact with the ground and freshwater are classified as Use Class 4 applications. The ‘Use Class’ system is important because it indicates the scale of the biological threat faced by a wood product in a specific end-use situation. The Use Class determines the preservative loading and penetration required to deliver the service life required. For decking and landscaping components the default service life is 15 years and sometimes 30 years for commercial structures.

Until recently, out of ground deck sub-frame components like beams, wall-plates and joists were considered less vulnerable to fungal decay than components like deck support posts embedded in the ground or mounted on a concrete foundation at ground level. That’s because decks are essentially ‘open structures’ and although subject to frequent wetting, air circulation would reduce their moisture content below the 22% threshold at which fungal attack can start. Experience now shows that this is not the case, particularly with low level decks where joists can be just a few millimetres above the ground.

Mistakes and shortcuts compromise performance

Clear information about using wood correctly for decking and landscaping projects has been promoted widely in the UK for
many years with organisations like the Timber Decking & Cladding Association (TDCA) leading the
way in providing supply chain and end-user good practice guidance. Despite this, mistakes and shortcuts made in constructing timber deck substructures compromise both theperformance of the finished deck and the reputation of wood. This is made all the worst because once a deck is completed, the substructure is typically not visible so theconsequences of a specification mistake or shortcut taken during installation do not become evident until it’s too late.

Typical timber deck substructure mistakes and shortcuts include:
• making little effort to prepare the site of the deck by clearing the ground of all vegetation (that often re-grows unseen) and other construction debris which restricts drainage and air circulation and creates a moist environment. When deck joists are typically set directly on or very close to the ground, this moist microclimate changes the conditions in which the timber joist will serve;

• Using wood that is inappropriate for the application e.g. untreated wood joists or joists that have been treated for use in dry internal carcassing applications not in frequently wet external situations;

• A failure to retreat crosscuts made to pre-treated wood posts, joists and deck boards on site, directly exposing unprotected end grain to rapid moisture uptake and retention – an open invitation to fungal decay.

Preserving confidence in wood

In 2018, TDCA alerted WPA to the increasing frequency of timber sub-frame failures it was noticing during inspections for its decking ‘expert witness’ service. In many cases failure of the substructure was occurring in just a few years yet the deck surface was in sound condition. The failure of any component in the substructure of a deck is clearly a safety critical issue. It is also a very poor advertisement for the use of wood and threatens the future of the UK timber decking market, opening the door to non- sustainable man-made framing materials like aluminium.

The WPA technical committee, which includes representatives from BRE and TRADA carried out a full review of all the issues relating to the specification and installation of preservative treated wood used for decking sub-frames. It recommended unanimously that the application use class for timber deck subframes be raised from the Use Class 3u category to the higher level of protection provided by Use Class 4 preservative loading and penetration requirements. This move brings the UK in line with other major treated wood using countries like USA, Canada, Scandinavia and France where structural deck components like posts, beams and joists have long been categorised as Use Class 4 applications.

The WPA recommendation was then adopted by the BSI committee B/515 responsible for managing the content of BS8417 and the change will be incorporated into the new edition of the Standard when it is published in due course.

WPA, TDCA and TTF is working under a partnership agreement to help their collective memberships adapt to the change of treatment specification in time for publication of the revised BS8417 and to make buyers and users of treated wood aware of what a fabulous product wood is when correctly specified and installed.

WPA Awards 2020

The WPA annual awards are a showcase for all that’s best about wood protection technology as relevant and crucial part of the timber industry. Now in their 6th year, the Awards recognise and celebrate excellence in areas that are vital to business success. The main sponsors for 2020 are Impra, Lonza, Koppers and Wolman.

There are five Award categories:

• Wood Protection Project of the Year
• Treated Wood Trader of the Year • Innovation
• Quality Excellence
• Outstanding Contribution

The 2020 Project of the Year and Treated Wood Trader of the Year Awards are open entry categories – nominations can be submitted by any individual or organisation whether a member of the WPA or not. The remaining three Awards are exclusive to WPA members.

Projects may be large or small, in the UK or outside the UK, the important thing is that they demonstrate how factory pre-treatments make it possible for designers to confidently choose wood as a sustainable, low environmental impact material.
The scope of the award covers preservative pre-treated, flame retardant treated and modified wood and wood-based materials. The only caveat is that the project has been completed in the last 12 months or is currently under construction.

The Treated Wood Trader of the Year Award recognises customer service best practice. Applicants should focus on what differentiates their business from other suppliers and how this helps them build and retain customer loyalty as a key part of their business success. Customer testimonials should be included with each nomination.

The judging panel for the 2020 Awards is made up of Ed Suttie, BRE; Sally Spencer, TTJ Magazine and Gordon Ewbank, WPA and the winners of each category will be unveiled at a gala dinner on 1 April.

If any F&LN reader would like to nominate a project or a treated wood trader for a WPA Award then contact the WPA office on 01977 558274.


Leave a Reply

Your email address will not be published. Required fields are marked *

COVID-19 DigitalCopy


If needed any subscriber and advertiser can request a digital version of the June edition as of Monday 15th June. Hard copies of the magazine are been mailed out as normal. Email to request a copy.

You have Successfully Subscribed!