How do I specify Decking

How do I “Specify Decking”

Professional Designers and Architects specify decking on a regular basis, until recently decking has been in the back seat when considered an integral part of the initial cost analysis of the project. Professional decking installation is a specialist trade and has a key role to play in your Project.

We now have far too many techniques and materials to choose from, simply allowing a nominal sum for a decking project isn’t good enough. To provide a detailed and accurate specification for the decking element of your project is a required for every project.

These information pages are to provide a guide for the consideration and important elements of your decking specification.

The Contractor

The most important element for your consideration is the company that you specify to construct the decking. Each and every decking installation should be treated as a specialist part of the project and only a professional contractor should be selected. A company should be selected by proving the following attributes:

  • Suitably qualified or endorsed by the manufacturer.
  • Has attained professional membership of a recognised association or institute.
  • Can demonstrate history of experience with a portfolio of key projects.
  • Absolute knowledge of the materials specified for that project.

Longevity of the installation.

The required service life of the decking installation should be high priority for any decking specification. This time period is to be listed on the specification and warranted by the installation contractor for the substructure and the decking by the manufacturer.

This guide cannot provide a detail as to what materials will last for a required amount of time. It is up to your chosen contractor to provide evidence of such from either demonstrating chemical treatments that the materials have been subject to or evidence from the manufacturer.


It is quite often a surprise to Main Contractors who initially dismiss the importance of what decking is required and what the real cost is. By specifying a professional Decking company, pit falls and failings can be removed and often a cost saving will be received as doing the job right first time to the right standard will prove to be the most efficient option.

Decking Material Selection.


What does the decking look like upon installation, what will it then look like over the service life. Will the timber bleach out tannins? How stable is the timber specified? Checking, fissures, splitting, and cracking – Does the client know this is normal? Do you?

Composite or Timber?

A choice for the designer here, there are far too many decking materials to choose from. For the specifier, the important factors effecting the installation is to follow the manufacturer’s guidelines, in most cases timber beam, joist and deck spans for timber are completely different to that of composite.

Hardwood or Softwood, which botanical species?

This is a simple choice borne out of budget and aesthetics, not one of longevity. With current timber treatments it is possible for soft wood to perform almost as long as hardwood. There is a massive choice for softwoods and tropical hardwoods, they all have different density, stability and durability ratings, an experienced timber decking company can advise on the best timber for your deck.

Traffic and deck loading.

Has the specified decking material been tested and proved to withstand the desired loading required for the intended service. Is the deck for light domestic loading or perhaps a commercial application with heavy foot traffic?

Fixing Technique.

Manufacturers’ recommendation

In all cases, the manufacturer’s instruction should be followed. It wouldn’t be the first time that I have seen a specification for one specialist product be carried to another. Every product should be treated in its own right and installed as intended.

Side fixing or surface fixing

This is a choice for solid timber decking, to fix in the side will present a smooth decking board on the surface. There are many choices of side fixing system, some semi-hidden whilst others are invisible. Some are plastic and some are stainless, again failing products are not usually the product but the installation quality.

Nailing is still present in some guides, maybe ok for small DIY but shouldn’t be considered by professionals. Stable timber and some composite decks can use trim head screws, others need to be pre drilled and countersunk whilst using screws in rebated holes and concealing with a plug is for the best finish on surface fixed decks.

In all cases for exterior decking, stainless steel with at least 304 grade should be specified along with the screw dimensions.

Sub-structure timber consideration

Ground level

Always allow for the correct beam and joist elevation under a deck, vital for a long service life, a simple joist system in contact with the ground will have a much reduced service life.

Always allow for sufficient room for natural airflow under the deck, this will enhance the service life of all decks by allowing the underneath of the boards to transpirate water at the same rate as the surface, this reduces cupping.

Allow for good water run off the substrate soil and secure a geotextile membrane covered with lime base gravel to prevent weeds.

Raised from ground level

Adequate beam spans should be the focus here, EC5 refers. Rebated posts and stainless bolts for security. Cat 4 treated posts of sufficient structural bearing to support your raised deck. Depth of post hole if securing into concrete and detail the post anchor point of using a steel pedestal, this is normally a drawing detail.


Only for the high end projects and perhaps where budgets allow,


Standard substructure materials in most cases are KD/HT, regularised, treated and stress graded to c16/c24.


With beam spans between 1500mm and 1000mm and cost being 3-4 times that of softwood timber this is seldom chosen. The fixing is also time consuming as it is bolted and not screwed together. The benefit is the longevity of the product.

Manufacturers Specific

Some decking products are supplied with a specific sub frame structure; this is type specific and must not be used in conjunction with any other system

Roof top

Parapet walls or balustrade must be 1100 minimum height from your deck level is vital, part K refers there.

It is usual that pedestals be used at shorter intervals (500 centres) to reduce the point loading on the roof. No need for beams here, simply use the correct section of timber joist over pedestals with your decking over this.

Deck Finish

In most cases timber can be left natural to silver out over time but is usually recommended to apply a quality oil treatment to increase stability, durability and enhance the aesthetics of the finish. There are many available and the manufacturer with oil type should be specified.


The aftercare is essential and that a detail is provided by the contractor to make up part of the OEM.

This must specify the cleaning interval, what is required and what chemicals should or should not be used.

Managing the clients expectations

If it is not cleaned it will stay dirty.

Timber left untreated will change colour, treated timber will still change colour.

Timber is not slippery; it is the algae on the dirt on your deck that is slippery.

The original colour of the timber can be returned.

Timber is an organic, neither the colour, texture, size nor shape will remain exact after installation it will check, warp, split and crack. This can never be avoided in full but simply kept to a minimum should a professional company install your deck with proper materials and provide regular maintenance.

This page is Sponsored by Owatrol Decking protecting and cleaning products

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