Structural material is a very important factor to consider when you want to build decks that last.  

A deck is only as strong as the substructure it is built on, and a badly installed deck can be unstable and dangerous.  

Most of the attention and thought generally go into the upper decking material, and not into the loadbearing structure that is going to carry the load and its demanding exposure to the elements. 

If in-ground timber support poles/posts are used, it’s critical that the poles/posts are of the appropriate preservative treated quality (H4 or H5). The aboveground stress-graded loadbearing beams/bearers shall be at least H3 treated pine as well as the upper decking boards, unless naturally durable hardwood decking-boards are preferred 

This guide discusses structural materials including wood composite battens, galvanised lip channels and structural pine, as well as decking accessories. 

The choice of decking substructure materials is dependent on several environmental factors. Credit: On The Deck

SANS regulations 

The South African Wood Preservers Association (SAWPA) states that decks are regarded as loadbearing structures on which people walk and gather – and as such it must be designed and constructed in accordance with the Building Regulations and relevant SANS standards.  

The Building Regulations, specifically regulation A13 (1) (b), stipulates: “All timber used in the erection of a building shall be treated against termite and wood borer and fungal decay in accordance with the requirements of SANS 10005 and shall bear the product certification mark of a body certified by the South African Accreditation System.” 

Investing in a substructure that exceeds standard requirements, often referred to as over-engineering, can significantly enhance the deck’s durability. Credit: BestDeck

Substructure materials 

Many factors need to be considered when making the choice between substructure materials. These include: 

• The ground conditions that it will be built on top of, including the type of soil, nearby trees or structures. 
• Height above ground level which may require additional supports. 
• Waterproofing concerns. 
• What decking product to use. 
• Environmental conditions.  

Structural materials include structural chromated copper arsenate (CCA) treated pine, composite battens, adjustable pedestals and galvanised steel, to name but a few. 

A successful decking project relies on more that the upper decking material, as the loadbearing structure beneath needs careful consideration. Credit: On The Deck

Clips 

Whether choosing to work with timbers or composite decking, hidden clip fastening systems create a decking surface that is uninterrupted with a smooth, seamless finish. Clip systems reduce the risk of injuries, and make maintenance and replacing individual boards simple and quick. 

Options include stainless steel, powder-coated and polypropylene for composite or timber decking. 

Screws 

Selecting the correct screws for the application will ensure a better final deck. It increases the longevity and performance of the decking material it was used with. 

Using the incorrect screws could impact on maintenance, with problems such as corrosion, and snapping of the screws or lifting of the decking boards. 

Carbon steel screws are used with either composite or timber clips when securing into a timber or steel substructures. Stainless steel screws are used to fix timber decking, and are recommended for coastal areas or where severe weather is expected. 

Cost of failure 

Cutting corners on substructure materials may save money upfront but can lead to costly repairs or replacements down the line. An inadequately built framework can cause the decking boards to warp, sag, or fail entirely, posing safety risks and diminishing the deck’s lifespan.  

Investing in a substructure that exceeds standard requirements, often referred to as over-engineering, can significantly enhance the deck’s durability. This involves meticulous attention to detail and using materials that provide extra strength and resilience. 

A deck is only as good as the substructure materials, fixing clips and screws that support the upper deck materials. 

Full thanks and acknowledgement go to https://bestdeck.co.za/, https://onthedeck.co.za/ and https://sawpa.co.za/ for the information in the article. 

For more information, please visit https://sawpa.co.za/technical-information-guidance-notes/