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Timber Decking Guide & Specification Summary

Timber decks can be designed to meet a variety of service life requirements. For quality installations, 15 years is considered to be the minimum standard. The principal specification standards to achieve a 15 year service life are set out below:
Only timber naturally resistant to decay or which can be treated by an industrial process to give long-term protection from decay should be used.

CS&D-Only use species rated as durable for the intended purpose

Timber Grade
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.  The higher strength classes, should be specified where smaller component sections, longer spans or commercial deck performance design considerations are required.

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 are recommended.

Timber moisture content at installation: 20% maximum

To minimize the effects of shrinkage e.g. cupping, cracking, warping etc, CS&D install timber as close as possible to the equilibrium moisture content of the site. . For best results always install wood with moisture content lower than 20%. The stability of all wood used out of doors can be improved by the use of water repellent treatments.

  1. Board spacing
    CS&D applies a 5mm min – 8mm max; where board abuts a post we apply a 5mm; where board ends meet we allow 3mm.
  2. Metal fixings
    All metal fixings are made from corrosion resistant materials such as stainless steel, hot dipped galvanized or other specialist coating. 
  3. Fall
    CS&D recommends, to aid drainage, a gentle fall of 1:100 into the deck, away from any adjacent property. Grooved deckboards are designed to assist drainage of surface water, in the direction of fall.

Design considerations

Some of the factors that CS&D recommends to be considered at the design stage include:*

The basic principles of deck construction:

Design Considerations


Timber decking guide- professional installation

If your plans are for a raised deck or include changes of level, specialist expertise is required to ensure the deck is structurally safe for the people using it. It is for this reason that we recommend such structures are built by experienced installers who will ensure that the deck is properly engineered and fit for the purpose.

CS&D is registered with the ITC (Institute of Timber Frame Builders)and have the capability to build decks to any scale. When choosing a contractor to build a deck always make sure that they are quoting for the same specification as offered by a ITC registered installer.

Timber decking guide - Timber selection

Timber is broadly classified into two groups - softwoods and hardwoods. This can be confusing because the terms do not relate to the relative hardness of the wood but to the type of tree from which it comes. Softwood species come from evergreen coniferous trees, hardwoods from broadleaved trees. Within each group there are many different species of wood. Some are suitable for decking, some are not.

The key factor in selecting wood for use out-of-doors is durability - its ability to resist the conditions that give rise to decay (i.e. wood will start to decay when its moisture content is persistently above 22%).

Some species of wood have a natural ability to resist decay completely; others have varying degrees of natural durability and may require treatment.

CS&D recommends that only timber capable of providing a minimum service life of fifteen years should be used. This means selecting a hardwood species that is classified as being naturally "very durable" and "durable".

The rich attractive colors of some hardwoods add greatly to their appeal. Naturally durable hardwoods are usually higher in density than softwood and their impact and abrasion resistant properties are reasons why they are used on commercial projects that have a lot of heavy use.

CS&D supports all initiatives that provide reassurance that timber materials used in decking have come from sustainable forests.

Timber preservation

Brush-applied wood preservatives or dip treatments are not suitable for the long-term protection of timber used for decking and other outdoor landscaping applications and should not be used.

Timber is made highly durable in a treatment plant like this.

As well as preservative protection some timber also comes ready treated with a water repellent that helps further protect the wood from weathering.

CS&D only uses timber for decking that has passed a quality assurance protocol. This is your proof that the product has been independently assessed.

Planning and building regulations

Deck structures are often considered to be exempt from planning regulations. This is not always the case. There are a number of specific instances where consent is required prior to building a patio, terrace or deck and these are set out below:

Situations Requiring Planning Permission

Building regulations

Building regulations should be assumed to apply to every structure that requires planning permission. In addition to the situations set out above, other restrictions have been known to apply, including limitations to the overall deck area in relation to the existing property or garden area and the constraints of established building lines.

With the exception of ground level decks, property owners should always check that planning regulations do not apply to their proposed structure.

In addition to contacting the Local Authority, neighbors who may be affected by the structure should also be informed. Neighbor objections are the most usual reason for planning refusal or restrictions.

Deck board choice

Deckboard Choice

Deck boards come in a range of sizes from 75mm to 150mm wide and appearances - plain, ribbed, grooved or enhanced grip. To enable fast drainage and reduce the effects of movement caused by the moisture content CS&D does not recommend using any board wider than 150mm.

The wood should be straight grained and have a moisture content no greater than 20% to reduce the risk of distortion caused when timber with higher moisture levels dries to suit the local conditions.No matter what type of deckboard is chosen the edges of each board should have a machine chamfer or radius to aid drainage and prevent damage if boards do move in service.

As a further aid to drainage a slight fall, 1 in 100, should be built into the deck structure. If the deck is attached to a property then the fall should be away from the building.

Grooved boards were designed to facilitate rapid draining of the surface and should always be installed with the grooves in the direction of the fall. The grooves should be swept clean of any debris or dirt to prevent the boards from becoming saturated in wet weather which is the principal cause of slipperiness.

A gap of no less than 5mm and no more than 8mm should be left between boards to allow for the natural movement of timber over the seasons and help surface drainage and ventilation of the entire structure. We leave a gap of 5mm where a deckboard abuts a post and no more than 2mm where deckboards abut one another lengthways.

Enhanced grip boards are available with inserts of non-slip material for use in areas where improved grip is require e.g. steps, ramps and key areas of decks and walkways for public access.  All of the boards supplied by the manufacturers below have a lower potential for slip in both wet and dry tests.

For specific performance ratings contact the manufacturers.

Deckboards availableDeckboards



Because wood used outdoors is exposed to the weather, metal fixings such as nails, screws, bolts and connectors need to be highly resistant to corrosion. If the wrong type of metal fixings are used then this can result in corrosion. The consequences of corrosion are unsightly rust stains on exposed components and the possibility that key fixings may fail prematurely leaving the structure unsafe to use.



Staining is a highly effective way of decorating and personalizing a deck - whether in a natural wood shade or a fashion color. Decorative products are applied in exactly the same way as other coatings but the manufacturer's advice and recommendations should always be followed. Always use a product made for exterior use on a timber decked surface. CS&D offer a range of tried and tested decorative and maintenance products.

Clear wax coatings can be used to improve the water repellency of timber and is suitable for hardwoods. These coatings help to prevent water absorption that causes wood to expand and contract and crack.



The degree of maintenance required depends on the deck specification. CS&D quality assessed components require no further preservative treatment. They will resist decay almost indefinitely and can be expected to last in excess of 15 years if treated on an annual basis.

A water repellent coating may be used to improve deck performance by helping to reduce the occurrence of small cracks. If used, water repellents should be applied every year - preferably at the end of summer. It is now possible to specify decking timbers that include water repellents as part of the pressure pre-treatment process.

Decorative stains will also need to be refreshed periodically to maintain their appearance - usually depending on location, amount of sun received and general wear and tear.

Whether smooth faced, grooved or ribbed deck boards are preferred, it is important that the deck surface is kept clean by regular brushing with a stiff bristle brush to remove dirt, algae and dead leaves which could make the deck slippery in the wet. At least once a year - preferably in spring - give decks a more thorough cleaning using a power spray or proprietary cleaner to lift any stubborn stains.