What Can Affect the Strength of Your Timber?

From Ekki to Greenheart, there are plenty of super strong timber species out there which are ideal for construction, including Oak and Pine. Having said that, there are also factors which can affect the properties of your wood. These can affect durability, dimensional stability and overall strength, leaving you with timber which is not fit for purpose. Our advice? Get to know what can affect the strength of your timber so that you can better choose and maintain your products.

Natural Characteristics That Affect Strength

We grade timber products to understand how variations of the same species are better suited for different projects. Why? In simple terms, structural differences between batches of timber can affect their strength. These natural characteristics can include:

• Knots: These are areas where branches grew from the tree, and they interrupt the grain which can weaken the timber. Small knots that are few and far between are not much to worry about, but larger knots in clusters can result in more severe strength reductions, especially when located near the edges of beams which are intended to be bent.
• Sloping Grain: When the timber fibres deviate from the axis of the timber, this is known as a sloping grain and it reduces the strength of the wood. You can also come across spiral grain or cross grain – whether naturally occurring or caused during processing – both of which can weaken timber.
• Reaction Wood: When a tree leans, it can form irregular growth patterns known as reaction wood. This can actually end up stronger than regular timber in some cases, though it is more likely to experience dimensional instability and we would never recommend it for any structural construction projects.
• Fissures: Shakes, checks and splits (essentially cracks) in the timber can occur for a variety of reasons, including while the timber dries or at any point when it is under stress. This reduces shear strength and makes it easier for moisture and insects to penetrate the timber, contributing to rot.

Find out more about strength grading in our guide to common strength grading misconceptions.

The Role of Moisture

The moisture content of your timber plays a critical role in its strength. Below the fibre saturation point (FSP) – which is around 25-35% – the strength of timber is inversely proportional to moisture.

The wetter your timber, the weaker it is (both in terms of tensile and bending strength), making it less suitable for structural applications when it’s above the FSP. On the other hand, too little moisture and any bending could cause the wood to splinter and snap.

Duration of Load and Creep

Timber’s strength is reduced under prolonged loads. In fact, a permanent weight on the timber can reduce its strength by up to a whopping 40%! Sustained loads also lead to significant creep deformations, which look like a permanent dip or sag in your timber boards and beams.

Cyclic Loads of Stress

Timber does not hold up well under repetitive stress. Even when it only has small imperfections, it can lose about half of its original strength when it is repeatedly dealing with external forces and heavy loads. If the timber has serious flaws, e.g, knots or sloping grain, the strength loss can be even greater—up to 70%.

Do note, though, that tough timbers will withstand stress and loads for decades without a hitch. It all depends on choosing the right species and grade for the job. If your project requires your timber to be under a lot of stress, we recommend a species like Ekki, Greenheart, and Cumaru. They will handle cyclic loads and constant stress with ease.

Temperature and Fire Resistance

Timber’s strength remains stable under normal temperatures but decreases under prolonged high-temperature exposure. Timber is also combustible; however, its charring behaviour provides some fire resistance. Charred surfaces insulate the unburnt core, maintaining its structural integrity, unlike steel structures. Fire-resistant treatments and protective cladding can further enhance its performance in fire-prone environments.

Find the Right Timber at EcoChoice

Struggling to factor all these characteristics into your final decision? Do not worry, we have got you. At EcoChoice, we will match your project requirements to the perfect timber species for the job, ensuring you make the right (Eco)choice every time. Get in touch with our team today to learn more.