Firewood is considered rotten when decomposition begins, primarily caused by wood-decay fungi. These fungi require moisture, oxygen, and a suitable temperature to break down the wood’s structural components, such as cellulose and lignin. Identifying this decay is important because compromised wood reduces heating efficiency and can create safety hazards in a chimney or stove. Understanding the indicators of rot ensures only high-quality wood is used.
Visual Signs of Decay
The most immediate indicators of decay are observable changes to the wood’s surface and color. Healthy, seasoned wood maintains a consistent, natural coloring, but fungal activity introduces distinct discoloration patterns. Brown rot fungi consume cellulose, leaving behind a brown, brittle residue that often causes the wood to crack into cube-like pieces. Conversely, white rot fungi digest both cellulose and lignin, resulting in a bleached, whitish, or stringy texture.
Any visible growth of mold, mildew, or fungal fruiting bodies, such as conks or small mushrooms, is a clear sign of advanced decay. While surface mold can appear on otherwise good wood, deep fungal growth shows the internal structure is breaking down. Look for patches of yellow, purple, or white spores on the wood’s ends or sides, indicating the fungi are actively colonizing the material. Evidence of insect activity, such as bore holes or carpenter ant trails, is a secondary sign, as these pests are attracted to wood that is already soft and damp.
Physical and Handling Indicators
If a log appears sound, tactile and auditory tests confirm its internal condition. A log that feels significantly lighter than a similarly sized piece of sound wood may be rotten because fungi have consumed the dense internal structure. However, deeply decayed wood that has soaked up moisture can feel deceptively heavy, making the weight test variable.
The texture test is often more reliable, involving probing the wood with a fingertip or a blunt tool. Sound wood feels firm and resists pressure, while rotten wood feels soft, spongy, or punky, easily indenting or crumbling away. This mushy consistency results from the fungal breakdown of the wood fibers, which destroys their rigidity.
A simple sound test reveals structural integrity by striking two logs together or tapping the log with a metal object. A solid piece of wood produces a sharp, clear, ringing sound. In contrast, a log with internal decay absorbs the vibration and produces a dull, muted thud or a hollow sound.
Why Rotten Wood Is Unsuitable for Burning
Burning rotten wood is inefficient and carries several safety concerns. Rotting wood often retains a high moisture content, either from the decay process or from exposure to the elements. When this damp wood is burned, thermal energy is consumed evaporating the water instead of generating usable heat, leading to poor heat output.
This cooler, less efficient burn causes the wood to smolder, releasing excessive amounts of dense smoke. This smoke carries unburned wood gases and fine particulates that solidify upon contacting the cooler surfaces of a chimney flue. The resulting sticky, tar-like residue is known as creosote. Creosote is highly flammable, and its buildup is a leading cause of chimney fires.
Preventing Decay Through Proper Storage
Preventing wood decay requires minimizing the moisture and maximizing airflow that wood-decay fungi need to thrive. Firewood should always be stored off the ground to prevent contact with soil moisture, which wicks into the logs and promotes decay. Raising the stack on a dedicated firewood rack, wooden pallets, or concrete blocks ensures the wood remains elevated and dry.
Adequate air circulation is necessary to allow the wood to dry, or season, and to carry away released moisture. Stacking the wood loosely with space between the rows and leaving the cut ends exposed maximizes this flow. While the wood must be protected from direct precipitation, only cover the top of the stack, leaving the sides completely open. Sealing the entire pile with a tarp traps moisture inside, creating a humid environment that accelerates fungal growth and rot.