Designing for movement: Engineering for timber expansion in large decking areas

Anyone who has worked with timber knows it is a living, breathing material. Even once it has been treated (if needed) and installed, it continues to respond to its environment. In particular, moisture and temperature can directly influence the way your wood behaves, causing movement which is essential to factor in when designing large decking installations.

Whether you are a designer, contractor or DIY enthusiast, get to grips with designing for timber movement in this guide.

Why Does Timber Move?

Timber naturally expands as it absorbs moisture and contracts as it dries. In the UK’s variable climate – with its wet winters, warm summers and unpredictable swings between the two – external timber is constantly balancing itself with the surrounding atmosphere.

The amount your timber species moves depends on a number of factors, including:

• Species: Timber species with better dimensional stability, such as Oak and Canadian Western Red Cedar, move less than those which absorb more moisture.
• Moisture content: The moisture content of your timber when it has been installed can lead to more or less significant movement in your decking.
• Exposure: Decking which is completely open to the elements can see more movement from the timber boards than covered decks.

Understanding why timber moves is the first step to better accounting for its shifts as you engineer your decked area.

How to Design Decking That Can Move

It is impossible to stop completely your timber from moving, and that should not be the aim when designing large decking areas. Instead, you need to work with the timber, allowing it to shift in ways that will not ruin your design.

Here are our essential tips for designing with movement in mind:

Spacing

This means installing your boards with adequate spacing. Expansion gaps allow boards to move without buckling, cupping or jamming against neighbouring boards or fixed structures, and it is recommended that you leave 5-8mm between adjacent decking boards. You will also need to leave gaps between boards that abut posts (5mm) and boards which sit next to a building (10mm).

Fixings

Fixings have two jobs: to hold boards firmly in place and to allow natural movement. Poor fastening can restrict one or both of these purposes, making this a key focus when engineering large decking areas.

It is essential that you first ensure you are using corrosion-resistant fixings. These reduce the risk of the timber splitting as it moves while also preventing staining and other issues. It is also advisable that you leave adequate, consistent room between your fixings and the ends of your boards, with around 25mm being a good distance. This allows the ends to move without creating stress and tension.

Smaller Panels

Long, uninterrupted decking areas can accumulate movement over a larger distance, leading to noticeable distortion. By dividing the deck into smaller panels, with smaller decking boards and controlled separation joints, you can reduce the impact of movement and make your job a whole lot easier.

Maintain Your Decking as it Moves

It is normal for even a carefully engineered deck to move and change over time, especially if you are working with a large area. While choosing a dimensionally stable and durable timber species will reduce maintenance, it is still a good idea to inspect your decking every now and then.

We recommend checking your boards at least once a year for excessive movement or signs of stress around screws. This is also a great time to re-apply moisture-repellant finishes (such as timber oils) if your wood could do with a little durability boost.

Choose the Right Timber Species

We have mentioned choosing the right timber species a few times in this guide, but that is not always an easy task. To make it simpler, take a look at our go-to decking options, including Cedar, Larch, ThermoWood (a modified timber which boasts impressive dimensional stability) or tropical hardwoods such as Yellow Balau, Jatoba or Ipe.

To start an order for decking timber, get in touch with us today.