Decking Specifiers – at a glance guide to standards       MARCH 2006

To help guide architects, landscapers and professional designers the TDA has produced an at a glance guide to the specifications required to create timber decks and other outdoor structures in wood capable of delivering a service life well in excess of 15 years.

Summary of specification standards

1.             Minimum service life: 15 years

Decks and associated landscape structures shall be designed and built to give a minimum service life of 15 years.

2.             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 classed as durable or moderately durable.

Softwoods: Only use components treated to a standard appropriate to their use. This means:

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.

3.             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 below 600mm in height the use of C16 timber is recommended. For structures over 600mm the use of strength graded timber is essential and required by Building Regulations.

 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.

Note: Do not exceed the recommended load and span for each strength class – refer to span tables in the TDA Timber Decking Manual for detailed recommendations.

4.             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. 

5.             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 in conjunction with treated wood.

Deck board screws should be 2½ to 3 times the width of the board being fixed. Ideally choose screws that are self-countersinking. Pre-drilling pilot holes will help prevent splitting. Always drill pilot holes when fixing hardwoods. 

6.             Fall

As an aid to drainage build a gentle fall of 1:100 into the deck, away from any adjacent property. Grooved deck boards are designed to assist drainage of surface water and should be laid in the direction of fall.

7.             Further specification references

The TDA/TRADA Timber Decking Manual,
TDA Technical Bulletin Ref:TB04 Parapet design to meet building regulations
For British Standards relevant to landscaping with wood out of doors see overleaf.

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
Hazard classes of wood and wood-based products against biological attack – Part 1: Classification of hazard classes.
 

BS EN 335-2
Hazard classes of wood and wood-based products against biological attack – Part 2: Guide to the application of hazard 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.