Decay occurs when wood is exposed to moisture for a long time. The likeliest culprits are trapped dirt and debris within or beneath the deck, or splashes of rainfall that push water up into the structure. But even if your deck is meticulously clean, a more insidious source of water is wreaking havoc: the ground. Soils can hold an extraordinary amount of water. This is part of the reason that any in-ground contact voids the warranty of most building materials.
The decay triangle and why moisture is the one variable you can control
Fungal decay happens when three conditions are met: oxygen, warmth, and moisture. We call this the decay triangle. Since you can’t exactly remove oxygen from the equation and you cannot control the ambient temperature, either, moisture is what you get to play with.
Wood has a characteristic called hygroscopicity, meaning it constantly absorbs and releases water from the surrounding atmosphere. This wouldn’t be a problem, but somewhere around 25-30% moisture content there is the Fiber Saturation Point. Below this point, there is no free water in the cell cavities of the wood, and above it the fungal spores, resting on the surface of the wood start to become active and colonise the structure.
Brown rot vs white rot – two different structural failures
Decay is not uniform in appearance or effect. There are two basic groups of decay that it’s useful for us to know about: brown rot and white rot.
Brown rot metabolises cellulose and hemicellulose, which are the carbohydrate walls within the cells of the wood. As these are broken down, the wood becomes dry and cracks into roughly cuboid fragments. It looks solid, but much of its strength has been lost. This is the kind of decay that produces boards that seem to give way under your feet when you step on them.
White rot feeds on lignin, the natural polymer that in combination with cellulose and hemicellulose forms the walls of the wood cells. The wood becomes spongy, pale, and string-like. Because lignin is the glue that binds the wood fibres together, the board is being destroyed from the centre outwards, although the surface of the wood may look almost unaffected.
In the early stages in both brown and white rot, bacterial ‘wood-rotters’ soften the cell walls and make it easier for the wood to be penetrated by fungi. By the time the rot is visible, there is already a well-established colony of bacteria and/or fungi at work.
How checking turns a surface problem into a structural one
The widespread belief that surface checking is caused by ultraviolet (UV) light exposure is a myth that needs correcting. Checking (the fine cracks that appear along the grain on the surface of timber decking planks) is often incorrectly assumed to be a purely cosmetic problem resulting from exposure to the sun and drying out of the wood. UV does break down surface fibers, so in theory, they’ll become more porous, and cracks will drift down into the wood along the weakened fiber. However, if UV were the main driver, the splits would all be down grain and not be of too much concern. The primary reason for concerns about splits in decking is that when tension in the wood’s grain causes splits, the split may travel deep into the core of the wood and dramatically reduce its structural integrity.
Why the underside is usually the first to fail
Many people examine their deck from the top. But what about the underside?
Your decking boards are merely the topmost piece of a structure that is sitting above ground – and how well that substructure is ventilated has a massive impact on the longevity of your investment. If the underside of your deck is non-stop soaking up ground moisture because there’s no efficient way for that timber to breathe, it isn’t going to last as long as it should.
Moving from reactive to proactive
Most homeowners only tend to their deck when they can see it getting worn and tired. The problem is that by that stage it’s already too late. The structure of the boards starts breaking down long before your deck looks like it needs repair. Proactive maintenance means keeping internal moisture content below that 20% threshold consistently, which requires applying professional-grade products, with the help of companies like Deck Seal, that create a genuine hydrophobic barrier – not just a surface finish that beads water temporarily.
Fixing an unloved deck is one thing. Building a deck in a way that allows it to be unloved for its natural life is quite another.