Timber decking guide - Specification summary

To ensure any deck structure provides years of trouble-free life it needs careful planning and design as well as good quality materials. The TDA exists to provide guidance on the standards required to create a quality deck structure.

The principal specification standards are set out below:

0. Timber: Naturally durable or pre-treated to the correct use class
   
   Only timber naturally resistant to decay or which can be treated by an industrial process to give long-term protection from decay shall be used.
   
   Hardwoods: Only use species rated as durable or moderately durable.
   Softwoods: Only use species/components with natural durability or which have been treated in accordance with BS EN 335 to a Use Class standard appropriate to their use i.e.
   Use Class 4 treatment: for posts and other structural components in direct ground or freshwater contact.
   Use Class 3 treatment: for all components out of direct ground contact subject to frequent wetting.
   
   Please Note
   (i) Whitewood should not be used for posts embedded in the ground or for other elements (joists) in the ground or other non-permeable surface, eg concrete slab.
   (ii) All crosscuts, notches or large boreholes shall be treated on site with a suitable preservative.
0. Timber Grade (Strength Class): C16 minimum
   
   The grade (strength class) of timber used for structural components such as posts, beams and joists shall be sufficient to cope with the loads placed upon it during its service life. Softwood with a strength class rating of C16 is considered the minimum standard for decks above 600mm in height and is a requirement of building regulations for such raised level structures. The higher strength classes, typically C18 and C24 should be specified where smaller component sections, longer spans or commercial deck performance design considerations are required.
   
   For decks below 600mm in height the use of C16 timber is recommended.
   
   Posts Posts can be made from laminated sections, solid timber or round poles and should have a load bearing capability/size/spacing appropriate to the scale and end use of the structure. For extended life surface mounting of posts in metal shoes is recommended.
   
   Note:
   (i) Do not exceed the recommended load and span for each strength class – refer to span tables in TDA/TRADA Timber Decking: The Professionals’ Manual for detailed recommendations.
   (ii) Use “noggins”/blocking to strengthen frames where appropriate to prevent flexing.
0. Timber moisture content at installation: 20% maximum
   
   To minimise the effects of shrinkage eg cupping, cracking, warping etc, install timber as close as possible to its equilibrium moisture content. For outdoor wood, moisture content varies from 19% in winter to 13% in summer in the UK. For best results always install wood with a moisture content lower than 20%. The stability of all wood used out of doors can be improved by the use of water repellent treatments.
0. Board spacing
   
   5mm min – 8mm max; where board abuts a post allow 5mm; where board ends meet allow 3mm.
0. Metal fixings
   
   All metal fixings shall be made from corrosion resistant materials such as stainless steel, hot dipped galvanised or other specialist coating. Before use, verify with the manufacturer that the fixings you have chosen are suitable for use with treated timber. Aluminium fasteners should not be used with treated wood. Prevent galvanic corrosion by using the same type of metal for both fixings and connectors.
   
   Screws should be at least 2½ times the thickness of the board being fixed. Ideally choose screws that are self-countersinking. Pre-drilling pilot holes will help prevent splitting. Always drill pilot holes 2mm oversize when fixing hardwoods. At all joist crossing points secure boards with two fixings positioned at the outer ¼ points of the deckboard ie 25% in from either edge. On grooved boards fixings should always be at the bottom of grooves. Take care using high-pressure nail guns as they can damage timber.
0. Fall
   
   To aid drainage, build a gentle fall of 1:100 into the deck, away from any adjacent property. Grooved deckboards are designed to assist drainage of surface water, lay them in the direction of fall.
0. Further specification references
   
   TDA/TRADA Timber Decking: The Professionals’ Manual – second edition November 2006
   TDA Technical Bulletin TB02: Statutory requirements
   TDA Technical Bulletin TB04: Parapet design and construction
   TDA Technical Bulletin TB08: Metal fixings
   
   British Standards relevant to building with wood out of doors:
   
   British Standards
   The Standards set out below all have a relevance to the creation of high performance timber decks designed to provide a service life in excess of 15 years.
   
   BS EN 335-1
   Use classes of wood and wood-based products against biological attack – Part 1: Classification of Use classes.
   
   BS EN 335-2
   Use classes of wood and wood-based products against biological attack – Part 2: Guide to the application of use classes to solid wood.
   
   BS EN 335-3
   Durability of wood and wood-based products – Definition of hazard classes of biological attack – Part 3: Application to wood-based panels.
   
   BS EN 350-1
   Durability of wood and wood-based products – Natural durability of solid wood – Part 1: Guide to the principles of testing and classification of the natural durability of wood.
   
   BS EN 350-2
   Durability of wood and wood-based products – Natural durability of solid wood – Part 2: Guide to natural durability and treatability of selected wood species of importance in Europe.
   
   BS EN 351-1
   Durability of wood and wood-based products – Preservative-treated solid wood – Part 1: Classification of preservative penetration and retention.
   
   BS EN 351-2
   Durability of wood and wood-based products – Preservative-treated solid wood – Part 2: Guidance on sampling for the analysis of preservative-treated wood.
   
   BS EN 460
   Durability of wood and wood-based products – Natural durability of solid wood : Guide to the durability requirements for wood to be used in hazard classes.
   
   BS EN 599-1
   Durability of wood and wood-based products – Performance of wood preservatives as determined by biological tests – Part 1: Specification according to hazard class.
   
   BS 8417
   Preservation of timber – Recommendations. Guidance for specifiers on the treatment of timber drawing on relevant sections of BS EN Standards.
   
   BS 5756: 1985
   Specification for visual strength grading of hardwood.
   
   BS 6105: 1981
   Specification for corrosion resistant stainless steel fasteners.
   
   BS 6399-1: 1996
   Loading for buildings. Code of Practice for dead and imposed loads.
   
   BS 7359: 1991
   Nomenclature of commercial timbers, including sources of supply.
   
   BS 5268-2: 2002
   Structural use of timber. Code of Practice for permissible stress design, materials and workmanship.
   
   BS 6180: 1999
   Barriers in and about buildings – Code of Practice.
   
   BS 6399-1: 1996
   Loading for buildings. Code of Practice for dead and imposed loads.