Dispelling decking and cladding misconceptions

Janet Sycamore discusses how understanding and being able to specify the differences between key components such as joists, batten types, and corrosion-resistant fasteners will help keep your decking and cladding looking beautiful for longer.

As a technical advisory service, the TDCA receives enquiries daily. But too often we get asked the same questions that are fundamental to both timber decking and cladding installation – you’ll read about these later. Be assured, we’re working hard to address them by providing free resources and on-hand guidance.

So, what’s our reason for doing this? As a timber trade body, our objective is to promote best practice and to encourage the optimum use of a construction material that is both sustainable and beautiful.

We’re sure you’ll agree, timber is a miraculous material. Tiny saplings fed by sunlight and rain, are nurtured, managed and eventually turned into serviceable products to be used in all manner of applications. Trees are planted, felled, seasoned, de-barked and sawn – that’s just the beginning – the best wood is selected, graded and precision machined, it may be impregnated with a wood preservative to enhance its service life. Such components which have had all this time and effort invested in them are then passed to the purchaser, the product’s new owner. At this point it’s out of the manufacturer’s control and here’s where the problems can arise.

When is a decking joist not a decking joist?

A deck board is distinguishable and easy to identify; a nicely planed surface with pencil rounded corners and smooth, grooved or ribbed profiles. As long as it’s sourced from a reputable supplier, you know that this component is designed for use as an external decked surface supported off the ground. In a standard that groups all components into end-use classes, this is described as a Use Class 3. It will either be made from a naturally durable species of timber, or will have been preservative pressure treated or modified to achieve the level of durability required.

However, ‘joists’ pose a particular problem. Exterior deck joists and interior floor joists look alike: one can easily be mistaken for the other. Is this a problem? Resolutely ‘yes’ and here’s the first misconception: internal floor joists are categorised as Use Class 2 (interior, above the ground, occasionally wet) while deck joists are assigned to Use Class 4 (exterior, exterior structural support and potentially close to the ground and frequently wet). Deck joists must be more durable than floor joists to withstand the rigours of the outdoors where there’s a greater risk of decay. So, pressure-treated softwood deck joists must meet a Use Class 4 specification to ensure a long service life.

For this reason, the TDCA is appealing to specifiers to ensure their softwood decking substructure components are pressure treated to a Use Class 4 level. Evidence which details the correct treatment has been applied should be provided at purchase point. We have witnessed many cases of unsuitably treated joists being used on outdoor deck structures with bad consequences – it is therefore important to get the specification right.

Roof battens are not the only battens

Roofing and cladding battens are very similar in appearance but are distinctly different. Roofing battens fall into Use Class 2 as they are rarely wet due to the nature of the installation, but cladding battens sit in the Use Class 3 category. ‘Why is that?’ you may ask.

In the UK, timber cladding is designed as a rain screen, meaning it’s fitted onto battens over a breather membrane creating a drainage plane (cavity) behind the cladding. Wind-driven rain can enter the cavity posing a higher decay risk to the cladding batten. The material of choice for all timber battens is preservative pressure-treated softwood. Make sure your cladding battens are specified and treated to a Use Class 3 level.

* Where a short batten length is fixed alongside a batten to enable boards to be butt jointed (see below).

Image: TDCA

Image: TDCA

Any screw won’t do – get the fixings right

Incorrect fixing is a problem common to both timber decking and timber cladding installation. In fact, where inspections are concerned, nine times out of ten we see incorrect fixing types or placement as a contributory factor to the problems that have arisen. For example, if the fixings are too close to the edges splitting can occur, especially where holes are not predrilled, allowing boards to disengage and lift from their position. Eurocode 5 gives guidance on edge and end distances for softwood and hardwood species.[1]

About the author

Janet Sycamore Timber Decking and Cladding Association

Further information

The Timber Decking and Cladding Association is a not-forprofit company and the money generated from publication sales and inspection services goes towards its work to raise awareness of quality materials and installation good practice. www.tdca.org.uk

Further reading

• BS 8605-1:2014 External timber cladding – Method of specifying, BSI

• External timber cladding, BM TRADA, 2013

• The Timber Cladding Handbook, TDCA and TTF, 2021. Available in digital and hard copy at www.tdca.org.uk/ publications

• WIS 1-49 Cladding for timber frame buildings, BM TRADA, 2018

• WIS 1-50 Timber cladding for building refurbishment, BM TRADA, 2019

References

1. BS EN 1995-1-1:2004+A2:2014 Eurocode 5: Design of timber structures, BSI

Timber Use Classes.

Image: The Wood Protection Association (www.thewpa.org.uk)