How outdoor timber reacts to rain, sun and seasonal changes

The great outdoors is rarely kind to construction materials. Timber, while incredibly durable, is not immune to the rain, sun and seasonal changes so well-known as the great British weather. Knowing exactly how it reacts is essential if you want a long-lasting construction free of constant maintenance.

Whether you are fitting decking in your garden or taking on a commercial cladding project, we have taken a closer look at how outdoor timber reacts to the elements.

How Timber Reacts to Rain

The UK is well-known for its wet weather. Average rainfall is increasing, with the first 6 weeks of 2025 seeing 89% of the average for the year. Depending on your timber species and how it has been finished, this can be tough on exterior wood.

Timber that is not naturally durable, or has not been properly preservative-treated, can begin to decay if it remains damp for long periods. This is because wood is a natural, organic material and when it absorbs moisture, it creates the perfect conditions for fungal growth. Rot is not caused by the rain itself, but by prolonged dampness, especially when the timber is not able to dry out (such as if the ventilation is poor). Untreated Pine, for instance, does not hold up at all well in wet conditions, making it a poor choice for outdoor use.

A durable timber, however, will handle rain with ease, requiring very little maintenance to prevent rot. This is because some species have greater natural resistance to fungal decay, particularly in the heartwood. So, while they still get wet, the moisture does not create an environment conducive to rot.

Oak Oak heartwood and some very durable tropical hardwoods such as Iroko, Ekki, or Greenheart can achieve long service lives outdoors when correctly specified, detailed and maintained.

How Timber Reacts to Sun

When it comes to the sun, timber faces a different challenge. Prolonged exposure to sunlight causes UV degradation of the timber surface, gradually weathering the wood over time. UV rays also break down lignin, gradually fading timber to a silvery-grey patina. While this colour change is natural and often desired for a rustic look, some people prefer using protective treatments or stains to preserve the original colour of their chosen species.

How Timber Reacts to Humidity Changes

Days with high humidity are not necessarily rainy, but they can still affect exterior timber. In particular, fluctuations in humidity cause wood to shrink or swell as its own moisture content changes. If you have not planned for this movement, it can cause a lot of issues, including:

• Gaps between timber boards increasing or decreasing
• Warping, twisting or cupping of boards
• Sticking or misalignment of doors and timber window frames
• Splitting or cracking along grain lines
• Stress on joints and fastenings, leading to loosening over time

This is why it is vital you install your timber correctly, following spacing guidance outlined by trusted resources, such as the TDCA or our team at EcoChoice. You can also opt for a more dimensionally stable species – such as Western Red Cedar or ThermoWood – to reduce movement, though no timber is completely immune to swelling and shrinkage in extreme humidity changes.

Gain Expert Insights into Timber

Timber can last for decades – even centuries! – when you understand how it reacts to external threats. If you are looking to learn more about different timber species and how to find the right one for your outdoor project, we can help.

Contact us today for expert timber advice.