Timber Movement

Why does timber move? What issues does timber movement cause? In this article we look at what you should look out for when considering using timber cladding, timber decking and any timber externally.

Why does timber move?

Wood is hygroscopic, which means its moisture content (MC) will fluctuate based on the relative humidity (RH) of the surrounding air. As humidity increases, the MC increases, and the wood expands, and as the humidity decreases, MC decreases, and the wood shrinks – this is an unchangeable fact however movement can be accurately predicted and allowed for!

The cell structure of wood has been likened to several drinking straws that are glued together. Water in wood resides both within the “straws” (called free water) and in the walls of the “straws” (bound water).  If timber is under high exposure the straws fill with water which causes the timber to expand rapidly – hence the reason why it’s important to seal the timber and the end grain correctly and cap off wall cavities during construction to prevent pre-longed water exposure to the back of cladding boards.

Issues timber movement causes.

After timber has been installed it will look perfect, over time the moisture in the timber starts to equalize with the moisture in the air around the timber and will start to expand or contract sometimes causing joints to slightly miss-align and minimal cupping. This is not the fault of the product or installation it is completely normal for timber to do this. Timber will be moving slightly all year round with drier air in summer causing shrinkage and damp conditions in winter causing expansion.

Of course, prolonged contact with moisture will cause timber excessive movement beyond normal expectations. As an example, not installing temporary capping and leaving the wall cavity behind timber cladding open to weather, wall cavities are not designed to drain large amounts of water, this most often causes extreme cupping. Another example is installing timber decking too low to the ground without sufficient drainage underneath the deck.

Below are some common problems caused by timber movement when it hasn’t been allowed for:

Board Cupping/Peaking. When one side of the timber is in prolonged contact with moisture this causes boards to cup or peak at the board edges.

Board Creeping. If the correct expansion gap has not been used boards can expand and push the next board along. This can cause walls to be push out of line and cracks to appear on internal walls. It can also cause the boards to detach from the substrate.

Allowing for timber movement.

As it’s not possible to eliminate timber movement, the next best solution is to allow for the timber to move to avoid problems it causes. Some common ways to allow for timber movement is listed below.

Allowing for correct expansion gaps

Timber Cladding is not designed to be pushed up tight together, we recommend spreading the boards 1-2mm apart rather than completely tight together depending on the climate of the area and time of the year. Timber Decking should have the correct spacing between boards for the width of the board. Allowing for the correct expansion gaps allows for the boards to move without causing issues.

Sealing timber

Timber end grain absorbs more moisture than the surface, it is important to use end seal trims where possible and oil the ends of the timber boards to reduce water ingress in the board ends. Regular maintenance coats of oil on the surface of the boards keeps the boards hydrated and slows the boards change in moisture.


It is important to waterproof the building to eliminate any excessive moisture getting trapped in areas like wall cavities. Timber exposed to moisture for prolonged periods of time will cause extreme movement.

Protect Timber

Store timber out of the weather until its installed. When timber is in a pack it can trap water between the boards causing excessive movement.

Use end matched boards

End matching means boards interlock on the ends reducing miss-alignment at joints. End matching is not available for all Mortlock’s products, find out more about end matching by CLICK HERE

Using low movement timber species

Different timber species move at different rates. Two examples of low movement timbers are Pacific Teak and Burnt Ash. Mortlock Timber can guide you through the process of choosing the correct timber species for your application.

We recommend speaking to Mortlock Timber to discuss your project before making your product selection. Mortlock Timber’s staff can assist in making the right choices, so your project is a success.


For in-depth information about the range of products we offer, please fill out the form below to download our Architectural Timber Pricing and Product Guide. Inside you will find illustrations, specifications, portfolio photo examples and a hardwood timber price guide to assist with budgeting.

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We are committed to bringing you timber products that add value and endure for years to come, even in heavy traffic and harsh weather conditions. We understand the value of efficiency when it comes to installation and keeping hardwood timber costs down. That’s why we’ve spent decades perfecting our designs to make them easier to handle, less wasteful and more efficient to install. This efficiency allows us to offer you premier products that are more cost-effective so that you can experience greater savings on timber wall costs, timber ceiling costs, timber cladding costs and timber decking costs.

Download our Pricing and Product Guide for our complete hardwood timber price list including timber decking prices, timber wall prices, timber ceiling prices and timber cladding prices.


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