Freshly felled wood is roughly half water by weight. Burning it means using a large share of the fire's energy to boil that water off before any of it heats you, and the steam carries unburned fuel up the chimney where it cools and condenses as tar.
This is the least glamorous page in the domain and probably the highest value. A household that starts a wood pile today has good fuel in two winters. A household that starts when the power goes off has wet wood, a smoky house and a chimney fire risk.
What wet wood actually costs you
| Seasoned, under 20% | Fresh, 50% or more | |
|---|---|---|
| Useful heat | Full | Roughly half |
| Smoke | Little | A great deal |
| Creosote in the flue | Slow build | Rapid build |
| Lighting | Easy | Difficult and frustrating |
| Glass on the stove door | Stays clear | Blacks over in an evening |
| Chimney fire risk | Low with sweeping | High |
The heat figure is the one that surprises people. It is not a modest efficiency loss. Roughly half the energy in wet wood is spent evaporating its own water, so you burn about twice as much wood for the same warmth, and you do twice the work gathering it.
The creosote figure is the one that matters most in a long emergency. Tar in a flue is fuel, sitting in a hot pipe, in a house you cannot easily leave.
The 20% rule and what the law actually says
Below 20% moisture content is the target. That figure comes from the UK's Ready to Burn scheme, and it is worth being precise about what it does and does not cover.
The legal requirement applies to wood sold commercially in volumes under 2 cubic metres. It regulates sellers, not what you may burn. You can lawfully burn wood you cut and dried yourself at any moisture content.
That distinction matters in an emergency, because it means nobody will stop you burning wet wood. The 20% figure is still the right target, not because of the law but because of the table above.
How long, by species
The practical planning consequence: you need two years of wood on the go at once. One pile burning this winter, one pile seasoning for next. That is the system, and there is no shortcut to it except buying kiln dried.
If you have to start now with nothing, split small and take softwood. Softwood split thin, stacked well, in a good summer, can be usable in a single season. Oak rounds cut this autumn will not be ready next winter, or the winter after.
Splitting
Split wood dries several times faster than wood in the round, because bark is designed specifically to stop a tree losing water. A log left whole can sit for three years and still be wet in the middle.
- Split everything above about 100 mm diameter. Anything thinner will dry in the round.
- Split smaller for faster drying. If you are in a hurry, splitting to 75 mm faces rather than 150 mm roughly halves the time.
- Split soon after felling. Green wood splits far more easily than seasoned wood, and it starts drying from the moment you open it up.
- Use a splitting axe or maul, not a felling axe. A felling axe is designed to cut across the grain and will bury itself and stick.
- Split on a low, solid block. A round of tree trunk at about knee height. Splitting on the ground is slow, dangerous and blunts the tool.
Stacking, where most of the drying is won or lost
The stack does more work than the splitting does, and it is where most home seasoning fails.
Rule three is the critical one. A stack wrapped in a tarpaulin to "keep it dry" traps the moisture coming out of the wood and holds it against the pile. Wood under a full tarp for a year can be wetter than when it went in, and will grow fungus. You want rain off the top and wind through the sides. A pile in the open with no cover at all still seasons better than a fully wrapped one, because the wind gets to it.
Other things that make a real difference:
- Sun and wind beat shelter. A stack in an exposed sunny spot dries far faster than one tucked neatly against a north wall.
- Single rows, not a solid block. Air has to move through it. If you must stack deep, leave gaps between rows.
- Stack bark side up on the top course, so rain sheds.
- Split faces to the wind where you can, since that is the surface losing water.
- Do not stack against the house. It holds damp against the wall and it is an invitation to rodents and, in older properties, to anything that eats timber.
- Leave the ends cross-stacked or braced. A collapsed stack is a bad afternoon.
Telling when it is ready
A moisture meter is cheap and settles it. Split a log and test the freshly exposed inner face, not the outside, which is always drier. Anything reading under 20% is good.
Without a meter, four checks that together are reliable:
- Weight. Seasoned wood is noticeably, obviously lighter than it was green. Pick up a piece from when you stacked it and compare.
- Sound. Knock two pieces together. Seasoned wood gives a sharp hollow crack; wet wood gives a dull thud.
- Cracks. Radial splits opening from the centre of the end grain are a good sign.
- The washing-up liquid test. Put a squirt on one end of a log and blow hard at the other end. If bubbles form, the wood is dry enough for air to pass through the vessels. If nothing happens, they are still full of water.
Watch the fire itself. Wood hissing, bubbling or steaming at the cut ends is wet. So is a stove door that blacks over within an evening, or smoke that stays white and thick rather than thinning to almost nothing once the fire is up to temperature.
What never goes in the stove
Also avoid, for different reasons: rotten or punky wood, which has lost most of its energy and is barely worth carrying; anything mouldy, because the spores go into the room while you split it; and yew, which is toxic and best left alone even though the smoke is not the main risk.
Softwood is fine, despite the persistent myth that it is dangerous. It burns faster and deposits more creosote per unit of heat, so it wants a hotter fire and more frequent sweeping, but a well-seasoned softwood fire burned hot is cleaner than a badly seasoned hardwood one burned smouldering. See sweeping a flue for what that means in practice.