Always check cladding drainage and ventilation

A recent flurry of cladding inspection requests received by the Timber Decking and Cladding Association has revealed that basic, fundamental installation principles for timber cladding are being ignored.

Timber cladding is installed as a rainscreen on buildings across the UK. This means the cladding should be fitted over a drained and ventilated cavity, to allow air to flow and moisture to escape and give it the longest possible service life.

Ventilation provision is allowed for at the top and bottom of the cladding, while the batten layer or layers – one or two depending on the orientation of the cladding – creates a continuous drainage plane.

A waterproof breather membrane is then situated behind the support battens, which separates the external wet zone from the internal dry zone of the building.


Don’t lose performance

Unfortunately, several cases where this guidance has not been observed have recently been brought to the Timber Decking and Cladding Association’s (TDCA) attention, and it seems TDCA is not alone.

In a recent newsletter, NHBC reported that it ‘has recent experience with the design life of timber cladding being less than intended, leading to loss of performance and claims’.

The NHBC report further sites ‘lack of detailing to enable drainage and ventilation’ as one of the key problems, as well as the use of ‘unsuitable film-forming coatings that can trap moisture’ and fixings issues. These are all very important factors that builders and tradespeople must get right to prevent future problems.

“Fixings issues have long since been a bug bear of the TDCA in respect to both timber cladding and decking,” explains Janet Sycamore, Director of TDCA. “Most of the cases we see feature problems caused by using the wrong type of fixing, fixings made of the wrong material, or their incorrect placement and poor application. If you get it wrong the result could be at best permanent, dark, unsightly rust stains or, at worst, boards becoming detached from the wall and potentially causing harm.”


Follow practical guidance

A waterproof breather membrane should be fitted behind the battens.

The good news is that these problems are easily avoided by following good practice guidance available from the TDCA and Timber Development UK.

The Timber Cladding Handbook says: ‘Timber cladding provides a form of weather protection. But not all wind-driven rain will be deflected, so a well-ventilated, free draining cavity should always be included in the detailed design. Open at the top and bottom to allow through ventilation, a cavity channels any moisture that might back to the building’s exterior. By using a series of timber battens, a cavity between the cladding and the backing wall structure can be created.

  • Whilst cavity ventilation only needs a minimum gap of 6mm, the depth will be dictated by the thickness and arrangement of the battens.
  • Fit insect mesh to all openings to prevent access by small mammals and large insects into the cavity (you cannot effectively exclude very small insects).
  • Metal vermin mesh is needed at the base of all cavities near the ground.’

Learn more

You can find full details in The Timber Cladding Handbook, which is online at www.timberdevelopment.uk
You can also refer to BS 8605 External Timber Cladding Part 1 – Method of specifying.

The TDCA is a not-for-profit, independent trade organisation that provides technical advice to the timber industry, working closely with TDUK to promote best practice around timber decking and cladding products. Find out more at www.tdca.org.uk

This article was originally printed in the May/June issue of Supplying Timber magazine.