Definition and thresholds
Moisture content (MC) is the mass of water in the wood as a percentage of its oven-dry mass. Because the reference is the dry mass, green timber can legitimately read well above 100 %.
| Moisture content | What it means |
|---|---|
| 0–8 % | Dry interior equilibrium; risk of swelling if later exposed |
| 8–16 % | Normal in-service range for most building timber |
| ~20 % | Design threshold for decay risk — the number every detail targets |
| 25–30 % | Fibre saturation; cell walls full, free water begins |
| > 30 % | Free water present; decay possible whenever temperature allows |
The 20 % line is a design convention, not a biological switch. Decay fungi need free water, which appears near fibre saturation, but 20 % gives a safety margin against the measurement error and local variation you will always have on site.
Equilibrium moisture content — install at the target, not near it
Wood continuously exchanges moisture with the surrounding air until it reaches equilibrium with the ambient temperature and relative humidity. Specify installation MC to match the service environment, and the timber stops moving after fit-out instead of during it.
| Service environment | Typical humidity | Target MC |
|---|---|---|
| Heated interior, continuous | 30–45 % RH | 8–10 % MC |
| Interior, intermittently heated | 45–60 % RH | 10–13 % MC |
| Unheated but covered | 60–75 % RH | 12–15 % MC |
| External, ventilated and sheltered | 75–85 % RH | 15–18 % MC |
| External, exposed and slow-drying | 85 %+ RH, direct wetting | 18 %+, decay risk |
Values are indicative for temperate European conditions and vary with species and site. Storage matters as much as supply: timber delivered at 12 % and left unwrapped on wet ground for three weeks arrives at the wall at something else entirely.
Measuring it properly
01
Set species and temperature
Resistance readings depend on both. An uncorrected reading on a dense hardwood or on cold timber can be several points out.
02
Measure at depth, not at the surface
Insulated pins driven to about a quarter of the thickness show the core. Surface readings track the last rain shower, not the element.
03
Take a gradient, not a point
Compare surface and core. A wet surface over a dry core dries out; a wet core under a dry surface is the dangerous case.
04
Measure the risky places
End grain, bottom edges, fixings, junctions behind flashings. Mid-board readings on a sunny elevation prove nothing.
05
Repeat over time
One reading is a snapshot. Decay is driven by duration above threshold, so trend data matters more than a single value.
Movement: the other consequence
Above fibre saturation, extra water sits in the cell lumen and the board does not change size. Below it, water leaves the cell wall and the wood shrinks — anisotropically. Typical softwood movement per 1 % MC change is around 0.01 % along the grain, 0.1–0.2 % radially and 0.2–0.4 % tangentially.
That ratio is why tangentially sawn boards cup away from the heart, why wide boards fail before narrow ones, and why a second fixing across a wide board splits it. Size movement gaps from the expected seasonal MC range for the actual location, not from a rule of thumb borrowed from a different climate.