Does Composite Decking Warp?

Composite decking is a relatively recent innovation. It was created as an alternative to timber, why? If you have ever driven past Chesterfield and seen its crooked spire, you will have seen first-hand the results of under-estimating the consequences of timber warping. The spire itself was built from wood with a wooden shingle roof. Once the shingles needed replacing, they were replaced with over 30 tonnes of lead shingles. This extra weight, combined with unequal exposure to sunlight, has caused to the whole spire to twist to its current crooked shape.

Because timber is so prone to warping, many people are choosing to install composite decking. However, even composite timber is prone to a degree of warping. In this article, we investigate the differences in the way that timber and composite warp and how to install your composite deck to avoid this.

The process of warping happens when lumber absorbs moisture from the air. This causes timber to expand. As it dries and the moisture in the air reduces, the lumber contracts again. This process is called the Equilibrium Moisture Content (EMC) and can vary vastly dependent upon the type of timber, the cut of the wood and the conditions in which the timber is stored.

 The EMC process is not a one-off event, it can occur many times in the lifetime of a piece of timber. Timber can be sourced from various sections of a tree. Dependent on the cut, plain sawn, quarter sawn, live sawn or rift sawn, different parts of the tree will be utilised in a finished plank. Because trees grow steadily over time, different sections of the wood will have varying densities, for instance the heartwood will be denser and more solid than the outer layers. This means that it absorbs and expels moisture at varying rates, as one section dries quicker than another it can cause a twisting or warping of the timber.

Composite decking is made from multiple shavings of wood fibre and plastic polymers, being combined and bound together in a mould to create a composite material. Timber has a very simple cell structure which is interconnected. Therefore, what affects one section of timber will have a knock-on effect on another.

Composite wooden decking has a far more complex cell structure. The many thousands of individual wood fibres incorporated into composite timber, act independently with the fibres being made from all sections of the tree, no one section is specifically denser than another, therefore composite timber dries at a predictable, consistent rate. The addition of plastic polymers into the mix further protects the composite material by coating the wood fibres, giving water-resistant properties. In addition to the engineered low water absorption rate, our purpose designed fixing system locks deck boards in place, preventing twisting, warping and bowing.

Installing your composite decking correctly by using the correct fixings and preparing your subframe in the correct manner can have a significant impact on its long-term resistance to warping.