Seasoning Firewood: Getting Logs Down to 20% Moisture (UK)

A freshly felled log is roughly half water. Try to burn it in that state and most of your fire's energy is spent doing nothing more useful than boiling that water off as steam, so the stove runs cool, sulks and smokes. Seasoning is the fix, and it is refreshingly simple: it just means drying your logs down until the wood itself holds no more than 20% moisture. Get there and the same wood burns hot, bright and clean. This guide covers why that 20% figure matters, the UK rules that now sit around it, and exactly how to stack and store logs so they dry properly, whether you have cut your own or bought a load of "cord wood" to process at home.

If you are cutting your own supply from the woodland, this pairs naturally with our guide to coppicing for firewood, which is where a self-renewing log pile begins.

Why seasoning matters

The case for seasoning is really a case against burning wet wood, and it is a strong one. Defra sets it out plainly: green or freshly felled logs are not suitable for burning in a domestic stove or open fire until they have been dried to 20% moisture content or less. Below that figure a modern stove does what it was designed to do; above it, you fight the fire all evening.

Three things go wrong when wood is too wet. First, heat is wasted. Energy that should be warming your room is instead boiling water out of the log, so you get less heat and burn through more fuel to feel warm. Second, it tars up your appliance and chimney. Wet wood is difficult to light and keep alight, blackens the stove glass, and lets tar and soot build up inside the flue, which pushes up your sweeping costs and, more seriously, raises the risk of a chimney fire. Third, it pollutes. Damp wood produces far more smoke, and that smoke is harmful both to you and to your neighbours. Dry wood, by contrast, lights readily, burns with a lively flame, keeps the glass clearer and sends far less smoke up the chimney.

The target: 20% moisture, and the Ready to Burn rules

Twenty per cent is the number to remember. It is the moisture content modern stoves are built to burn cleanly, and it is now written into UK selling rules too. Many people aim a shade lower, into the mid-teens, for the easiest lighting, but 20% is the line everything hangs on.

That figure is no longer just good advice. Since 2021, Defra's Ready to Burn scheme has changed what can legally be sold. As gov.uk puts it, you can only supply or sell wood fuel "in volumes of less than 2 cubic metres if it is certified as 'Ready to Burn'", and that certification "confirms it has a moisture content of 20% or less". In plain terms, the small bags and loose deliveries sold at petrol stations, shops and garden centres must now be certified dry. Selling uncertified wet wood in those small volumes can bring a £300 fixed penalty from the local authority, or a larger fine through the courts.

How to season firewood, step by step

Seasoning is mostly about giving logs the three things that pull water out of wood: a big cut surface, moving air, and time. Here is the method Defra's own seasoning guidance sets out, put into practice.

Seasoning logs from green to ready-to-burn

1. 1

   Fell or cut in winter

   Where you have the choice, fell in the dormant winter months. The tree is carrying less sap then, so the wood starts out drier and has a head start. Summer-felled wood holds more sap and takes noticeably longer to season.

2. 2

   Cross-cut to stove length, then split

   Cut logs to fit your stove, then split them. Splitting is the single biggest accelerator: it opens the dense heart of the log to the air. Defra advises splitting cross-cut logs to less than about 10 centimetres across, which lets moisture move out from the centre far more easily. Long, un-split cord wood barely dries at all.

3. 3

   Stack off the ground with air gaps

   Build the stack on bearers, pallets or rails so the logs sit off the wet ground, and leave gaps so air can move between them. Loose is better than tight. A single or double row dries far faster than a deep, packed heap where the middle never sees a breeze.

4. 4

   Put it in sun and wind

   Site the stack in the sunniest, breeziest spot you have, ideally so the prevailing wind blows straight through it. Sun warms the wood and wind carries the moisture away. A sheltered, shady corner will take much longer, if it ever gets there at all.

5. 5

   Cover the top, leave the sides open

   Put a waterproof cover over the top to keep the rain off, but do not wrap the sides. The classic mistake is sheeting the whole stack in tarpaulin, which traps damp and can leave logs wetter than when they went in. The stack needs open sides to breathe.

6. 6

   Give it time, then test

   Leave it to season. Natural drying is likely to take two summers or more, and Defra points to at least 12 months, or two summers, of covered air drying to reach around 20%. Do not go by the calendar alone: when you think it is close, split a few sample logs and check the centre with a moisture meter before you burn any of it.

Using a moisture meter

A cheap moisture meter is the one tool that turns guesswork into certainty, and it is worth having. The trick is testing the right part of the log. The bark and the weathered outside of a log dry first, so a reading taken there will look reassuringly low while the heart is still wet.

Do it properly instead. Split a log open, then press the meter's two pins into the middle of the freshly split face, across the grain, and read it straight away. HETAS advises measuring the centre of the split surface as soon as possible, because the moisture on a newly exposed face changes quickly once it meets the air. Check several logs from different parts of the stack, not just one, because a pile rarely dries at an even rate. If the centres read 20% or below, you are good to burn; if they are still higher, stack the wood back under cover and give it another season.

Which wood is worth burning

All wood needs seasoning, but not all wood is equal once it is dry. Defra's guidance splits it broadly into two camps.

Hardwoods are the deciduous, broadleaved species, and they make the best firewood because they are denser and burn for longer, giving more heat per log.

• Ash is many people's favourite: it burns steadily and hot and is one of the more forgiving woods.
• Beech burns much like ash, with a good, even heat, though it wants thorough seasoning.
• Oak is the slow, dense workhorse. It gives long, steady heat but is the slowest to season, often needing the full two years or more.
• Birch lights easily and burns bright, if quickly, and its papery bark is superb natural kindling. It is often best mixed with a slower wood.

Softwoods, the evergreen conifers such as spruce and pine, light more easily and make good kindling, but they are less dense, so they burn faster and give less heat than a similar-sized hardwood log. They are not to be dismissed, especially for getting a fire going, but as your main heat they burn through quickly. Whatever the species, remember the headline: dryness beats density. Any wood at 20% moisture will out-perform dense oak that is still soaking wet.

Storing seasoned wood

Getting wood dry is only half the job; keeping it dry is the other half. Once logs are down to 20%, they can pick moisture straight back up if they are left out in the weather, so move your ready wood into a covered, airy log store, indoors, off the ground or undercover, and keep it out of the rain. Bring a few days' worth in near the stove so it acclimatises to the warm, dry indoor air before burning. And rotate your stock: burn the oldest, driest wood first and let this year's split logs take their turn at the back to season. A simple two-bay system, one seasoning and one ready, keeps you a year ahead and never short of dry fuel.

Common mistakes

• Burning it too soon. A few warm months rarely takes a hardwood log to 20%. Test before you trust it.
• Not splitting. Whole, round logs dry painfully slowly. Splitting opens the wood to the air and is the biggest single accelerator.
• Wrapping the whole stack in tarpaulin. Sheeting the sides traps damp. Cover only the top and leave the sides open to breathe.
• Stacking on the ground. Logs sitting on wet earth wick moisture back up. Always raise the stack on bearers or a pallet.
• A shady, sheltered spot. No sun and no wind means very slow drying. Give the stack the brightest, breeziest place you have.
• Testing the bark, not the core. The outside dries first. Split a log and read the fresh centre face across the grain.
• Burning treated or painted waste wood. Do not burn old furniture, fence panels, pallets or rubbish. Treated waste wood can release harmful fumes and toxic pollutants into your home.