Kiln dried logs arrive ready to burn, but poor storage can undo that quality within weeks. The problem is that moisture damage is not always obvious until the fire refuses to light or fills the room with smoke. Knowing the early warning signs lets you act before an entire delivery is compromised, and most storage failures are fixable with the right approach.
Why Kiln Dried Logs Lose Quality in Storage
Understanding why kiln dried logs deteriorate helps you recognise the signs before they become serious. The quality of the logs when delivered is not a permanent state and can change quickly under the wrong conditions.
Wood is hygroscopic: what this means in practice
All wood naturally absorbs and releases moisture to reach equilibrium with the surrounding air. Kiln drying reduces moisture content to between 10% and 20%, but this does not make the wood permanently dry. Once removed from controlled storage, logs begin exchanging moisture with the ambient environment. In a wet UK autumn or winter, with relative humidity regularly exceeding 70%, even well-stacked logs can begin reabsorbing moisture within a fortnight if exposed without adequate protection.
The process is gradual but persistent. Logs stored on damp ground, sealed in airtight covers, or placed in poorly ventilated spaces absorb moisture faster than those stored in open-sided, raised structures with good airflow.
The most common storage mistakes that cause moisture damage
Ground contact is the single fastest route to moisture damage. Soil and concrete both transfer moisture upward into the base layer of a stack continuously. Airtight covering traps humid air around the logs, causing them to sweat and develop mould. Stacking against a damp wall creates a dead air pocket where condensation accumulates. Any one of these mistakes can degrade a delivery of kiln dried logs in a matter of weeks during the wetter months of the year.
Sign 1: Mould, White Patches or Discolouration on the Surface
Mould is the most visible sign that kiln dried logs have been stored in conditions that are too humid, too enclosed or too damp. It is often the first physical indication that something has gone wrong, and it tends to appear on the outer bark or cut faces of logs that are not receiving adequate airflow.
What it looks like and what causes it
Mould on firewood typically appears as white, grey or green patches on the bark or cut ends. White powdery patches are often surface mould caused by humidity trapped around the log. Green or black spots indicate deeper fungal growth and tend to appear on logs that have been in contact with the ground or tightly covered for an extended period. Discolouration on the cut face, particularly a darkening toward the centre, suggests moisture has penetrated beyond the surface.
Is mouldy wood still safe to burn?
Surface mould on otherwise dry logs does not make them unsafe to burn, but it does indicate moisture content has risen. Light surface mould can be brushed off outdoors before bringing logs inside. Logs with deeper mould growth, significant softness or any signs of rot should not be burned in an enclosed stove, as decaying wood produces unpredictable combustion. In an open fire pit, tolerances are wider, but quality is still compromised.
How to fix it
Move affected logs to an open-sided, well-ventilated area and separate them from the rest of the stack. Allow them to breathe with full airflow on all sides. In warm, dry weather, light surface mould will resolve within a few weeks. Test moisture content before returning them to regular use. If mould returns after drying, the underlying storage problem has not been resolved and needs to be addressed first.
Sign 2: Logs Sound Dull When Knocked Together
The sound test is the quickest field check for moisture content and requires no equipment. Two logs knocked together tell you immediately whether they are in good condition or not.
The sound test explained
Take two logs and knock them together firmly, end to end. Dry, well-stored kiln dried logs produce a sharp, hollow crack. Damp or wet logs produce a dull, flat thud. The difference is caused by the density of the wood relative to its moisture content. Damp wood is heavier and denser per unit, which absorbs the impact rather than resonating cleanly.
What a hollow crack vs a dull thud tells you
A sharp crack indicates moisture content is likely below 20% and the logs are in burnable condition. A dull thud reliably indicates elevated moisture and suggests the logs need attention before use. If the sound test fails on logs that have been in outdoor storage for several weeks, check your storage setup immediately. The cause is almost always ground contact, poor covering or inadequate ventilation rather than a problem with the wood itself.
Sign 3: Difficulty Lighting or Fire Keeps Going Out
If kindling catches but the main logs fail to ignite, or if a fire dies down quickly once the kindling burns through, the logs are most likely too damp to sustain combustion at the temperatures a domestic firebox initially generates.
How moisture content affects ignition
Kiln dried logs at below 20% moisture ignite readily once the firebox is warm and kindling is burning well. As moisture content rises above 20%, an increasing proportion of the heat produced by combustion is used to drive off water from the wood rather than warm the room. At moisture levels above 25%, this energy loss becomes significant enough to prevent sustained ignition in all but the hottest conditions.
When this is a storage problem vs a technique problem
If the logs are newly delivered and lighting problems appear from the first use, the issue may be technique rather than storage. Ensure kindling is dry, use sufficient volume and try the top-down method. If the same logs lit easily on delivery but have become progressively harder to light over several weeks of outdoor storage, the cause is almost certainly moisture reabsorption.
Sign 4: Excessive Smoke or Hissing During Burning
Properly stored kiln dried logs burn cleanly with very little visible smoke and no audible hissing or steaming. Either of these symptoms during burning indicates moisture is present in the wood at levels above what kiln drying should have left behind.
What hissing and steaming during burning indicates
The hissing sound is caused by water inside the log reaching boiling point and escaping as steam through the wood fibres. Visible white smoke, as opposed to the heat shimmer produced by clean combustion, is steam rather than smoke and carries the same message. Neither symptom is a property of the wood species; they are both caused by elevated moisture content regardless of whether you are burning oak, ash or birch.
The link between damp logs and creosote build-up
Burning damp logs produces incomplete combustion, which generates tar-like compounds that condense on the cooler surfaces of the flue as creosote. Over time, creosote accumulates as a layer inside the flue liner and presents a chimney fire risk. A single season of burning damp logs can produce enough creosote to require an unscheduled chimney sweep. If logs are hissing or producing white smoke consistently, check storage conditions immediately rather than continuing to burn.
Sign 5: Moisture Meter Reading Above 20%
The four signs above are useful field indicators, but a moisture meter provides the only definitive reading. It should be the first tool used when any of the other signs appear, and is worth using periodically as a routine check on stored stock.
How to use a moisture meter correctly on kiln dried logs
Press the meter pins into the split face of the log, not the outer bark surface. Bark dries faster than the wood core and gives an artificially low reading. The split face exposes the interior and produces an accurate result. Take readings from two or three logs from different positions in the stack to account for variation, as a single reading from the outermost log may not reflect the condition of the stack as a whole.
What different readings mean and when to act
Readings below 20% confirm the logs are within the Ready to Burn standard and performing as kiln dried wood should. Readings between 20% and 25% indicate moisture has been reabsorbed and burning performance will be noticeably reduced. Above 25%, the logs are approaching the condition of poorly seasoned wood and should not be burned in an enclosed stove until moisture content is brought back down. At this level, a drying period of two to four weeks in good conditions is needed before use.
Browse our kiln dried logs, delivered at below 20% moisture content with Woodsure and BSL certification on every order.
How to Fix Poorly Stored Kiln Dried Logs
The severity of the fix depends on how far moisture content has risen and whether mould or structural decay has set in. Most cases caught at the warning sign stage are straightforward to resolve.
Minor moisture reabsorption: improve ventilation and wait
If the sound test is borderline and moisture meter readings are between 20% and 23%, the fix is simple: move the stack to a better location, ensure it is elevated off the ground, open all sides to airflow and cover only the top. In dry weather with good ventilation, logs at this level typically return to below 20% within two to three weeks. Do not burn them during this period.
Moderate damage: split, re-stack and extend drying time
Readings between 23% and 28%, or logs showing surface mold, benefit from splitting if they have not already been split. Splitting exposes more surface area, dramatically accelerating moisture loss. Re-stack in single-depth rows in the most ventilated location available, with cut faces pointing outward. Allow four to six weeks before re-testing, depending on weather conditions.
Severe mould or rot: what can still be saved
Logs with deep mould penetration, visible rot or significant softness in the core are unlikely to be recovered to kiln dried standard. Surface-mouldy logs with a firm core can still be salvaged with the steps above. Logs that have begun to decay structurally, crumble at the ends or smell strongly of rot should be disposed of rather than burned in an enclosed appliance.
How to Prevent the Problem from Recurring
Fixing the immediate issue only helps if the storage conditions that caused it are also changed. The five signs above almost always trace back to one of three failures in setup.
The three storage rules that prevent all five signs
Every storage problem that produces the signs described in this article comes down to ground contact, inadequate ventilation or insufficient top cover. Elevating logs on a pallet, rack or raised base eliminates ground moisture transfer. Leaving the sides of the stack open prevents humid air from building up around the wood. Covering only the top keeps direct rain off without trapping condensation. These three rules apply regardless of whether logs are stored in a garden, a garage or a lean-to.
Choosing between a log store, tarp setup and indoor storage
A purpose-built log store with a solid roof, open slatted sides and a raised floor is the most reliable long-term solution through a full UK winter. A pallet and top-only tarp works as a temporary alternative but requires regular checking. Indoor storage in a garage or outbuilding with good airflow is excellent for keeping moisture content stable, provided the space itself is not damp. Browse our kiln dried hardwood logs, available in bulk bags and nets with fast UK delivery.
Conclusion
Kiln dried logs can lose their quality quickly in the wrong conditions, but the warning signs are recognisable early if you know what to look for. Mould, dull sound, ignition problems, smoke and high moisture readings are all fixable if caught before the damage becomes structural. Improve ventilation, eliminate ground contact and cover only the top, and the logs will return to burnable condition within a few weeks in most cases.
