Specification for Deck Boards

Timber decks can be designed to meet a variety of service life requirements. Desired service life options of 15, 30 and 60 years are given in European/British standards. For quality installations, 15 years is considered to be the minimum standard.  For raised decks and balconies built on new homes, the NHBC requires a 60 year service life in accordance with TDCA Codes of Practice – these documents are available free to download to registered users here

The principal specification standards to achieve 15 years desired service life are set out below:

Post Beam

Your Choice of Timber

Only timber naturally resistant to decay or which can be, or has been, pressure treated or modified by an industrial process to give long-term protection from decay shall be used.

You can choose from naturally durable hardwoods, naturally durable softwoods, pressure treated softwoods, thermally modified timber or chemically modified timber.

Naturally durable deck boards should be rated as moderately durable (durability class 3) at least for above ground use and durable (durability class 2) for ground contact use.

Pressure treated softwood deck boards should be treated to at least Use Class 3 (outdoor above ground use) 15 year desired service life specification but 30 and 60 year specification is also achievable by special arrangement. 

Modified wood deck boards out of ground contact should be rated as moderately durable (durability class 3) but the durability ratings of Modified wood in-ground contact varies according to the desired life and modification type - always consult the manufacturer.

Note regarding pressure treated timber

(i) Whitewood is difficult to pressure treat and should not be used for posts embedded in the ground or for other elements (joists) in the ground or other non-permeable surfaces e.g. concrete slab unless its treatment properties have been improved, for example with incising, to ensure correct preservative penetration in line with BS8417.

(ii) All crosscuts, notches or large boreholes shall be treated on site with a suitable end grain preservative. Any rip-sawn timber should be returned to the treatment plant for retreatment.

The Timber Deck Substructure

For economic reasons the substructure is usually constructed from pressure treated softwood.

Use Class 4 treatment is for posts, joists and other structural components in direct ground or freshwater contact.

As from January 2020, the TDCA recommend that deck joists or any timbers providing exterior structural support should be preservative pressure treated to Use Class 4 - regardless of whether they are in ground contact or not.

This is to ensure the enhanced durability and safety of the deck substructure and is in line with the Wood Protection Association's Code of Practice for Industrial Wood Preservation. BS 8417 (Preservation of Wood: Code of Practice) is also to adopt this change in due course. See our blog article here. 


Laying a fall in a deck

The requirement for building a slight fall into a decked area is to aid drainage away from any adjacent property. This is usually achieved by adjusting the frame support system (substructure). Creating a fall should not be an issue for a professional installer.

The front beams on which joists sit can be installed slightly lower than the rear beam, alternatively you can build the beams level and use a suitably durable packing medium to raise the joists at one end. Ground level decks can utilise adjustable deck supports securely fixed to ground and bearer. 

Grooved deck boards are designed to assist drainage of surface water, lay them lengthways in the direction of fall. With smooth boards you have the option of laying them either way – with the fall across the width of the boards (because the gap between boards allows drainage) or across the length.

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 also recommended.


(i) Do not exceed the recommended load and span for each strength class – refer to span tables in TDCA/TRADA Timber Decking: The Professionals’ Manual for detailed recommendations.

(ii) Use noggins/blocking to strengthen frames where appropriate to prevent flexing.


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 on pre-cast piers or metal shoes is recommended.

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 the equilibrium moisture content of the site. For outdoor wood, moisture content varies from around 19% in winter to 12% 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.

Deck board spacing: 5mm to 8mm between adjacent boards

Lay deck boards with a 5 to 8mm gap between adjacent boards. Where board abuts a post allow 5mm; where board ends meet allow 2mm and where boards abut a building allow 10mm. Hardwoods which are classed as small movement can be fitted a little closer than this.

Metal fixings must be suitable for long term, outdoor use

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 the timber you have chosen. For example aluminium fasteners should not be used with pressure 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 i.e. 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 the timber.

Further specification references

TDCA/TRADA Timber Decking: The Professionals’ Manual – third edition 2018

TDCA Technical Bulletin TB02: Statutory requirements

TDCA Technical Bulletin TB04: Parapet design and construction

TDCA Technical Bulletin TB08: Metal fixings

TDCA Code of Practice TDCA/RD0801E6 May 2018 Raised timber deck structures on new homes

Wood Protection Association: Code of Practice - Industrial Wood Preservation

British Standards with relevance to timber decking

The Standards set out below all have a relevance to the creation of high performance timber decks.

BS EN 335-1:2013 Durability of wood and wood-based products.  Use classes: definitions, applications to solid wood and wood-based products. 

BS EN 350:2016 Durability of wood and wood-based products. Testing and classification of the durability to biological agents of wood and wood-based materials.

BS EN 351-1:2007 Durability of wood and wood-based products. Preservative-treated solid wood. Classification of preservative penetration and retention.

BS EN 351-2:2007 Durability of wood and wood-based products. Preservative-treated solid wood. Guidance on sampling for the analysis of preservative-treated wood.

BS EN 460:1994 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: 2009+A1:2013 Durability of wood and wood-based products. Efficacy of preventative wood preservatives as determined by biological tests. Specification according to use class.

BS 8417:2011+A1:2014 Preservation of wood.  Code of practice.

BS 5756: 2007 +A1:2011 Visual strength grading of hardwood.Specification.

BS EN 16737:2016 Structural timber. Visual strength grading of tropical hardwood.

BS EN ISO 3506-1:2009 Mechanical properties of corrosion resistant stainless steel fasteners. Bolts, screws and studs.

BS EN ISO 3506-2:2009 Mechanical properties of corrosion resistant stainless steel fasteners.  Nuts.

BS EN 1991-1-1: 2002 Actions on structures.  General actions.  Densities, self weight, imposed loads for buildings.

BS EN 13556:2003 Round and sawn timber. Nomenclature of timbers used in Europe.

BS EN 338: 2016 Structural timber. Strength classes.

BS 6180: 2011 Barriers in and about buildings. Code of Practice.

BSEN 1991-1-1:2002 Eurocode 1. Actions on structures. General actions.  Densities, self weight, imposed loads for buildings.

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