Four conditions, one of them yours
Decay fungi are the organisms that break wood down. They require four things at once: moisture, a workable temperature, oxygen, and the wood itself. Remove any one and decay halts. In a building, only moisture is genuinely under a designer’s control — which is why every durable timber detail is, underneath, a moisture-management detail.
| Condition | Threshold | Design leverage |
|---|---|---|
| Moisture | > 20 % MC (free water above ~25–30 % fibre saturation) | The only factor a designer fully controls. Remove it and the other three are irrelevant. |
| Temperature | ≈ 5–40 °C, optimum near 25 °C | Sets the seasonal rhythm of decay; cold periods pause it, they do not reverse it. |
| Oxygen | Present above the water table | Why fully submerged timber piles survive centuries while the same wood in the splash zone fails. |
| Food source | Cellulose, hemicellulose, lignin | Reduced by natural extractives (heartwood durability), modification or preservative treatment. |
Three rots, three failure signatures
“Rot” is not one process. Identifying which type is present tells you how much strength is already gone and how wet the element has been.
Brown rot
- Attacks
- Cellulose and hemicellulose; lignin left behind
- Appearance
- Darkened wood, cubical cracking, crumbles to powder
- Strength loss
- Fastest strength loss — up to 50 % before 10 % mass loss
- Typical location
- Softwood in buildings; the classic cause of failed sills, joist ends and cladding bases
White rot
- Attacks
- Lignin plus carbohydrates
- Appearance
- Bleached, fibrous, stringy texture
- Strength loss
- Slower initial strength loss, more uniform degradation
- Typical location
- Hardwoods, standing trees, damp external joinery
Soft rot
- Attacks
- Cell wall from within cavities; tolerant of extremes
- Appearance
- Soft, dark surface layer, shallow erosion
- Strength loss
- Slow but relentless; the dominant mode in ground contact
- Typical location
- Ground contact, very wet or preservative-treated timber
A practical warning about brown rot: substantial strength is lost long before the wood looks bad. Visual inspection alone underestimates damage, so probe and, where the element is structural, assume the affected zone extends beyond the visibly degraded material.
How fast is “fast”?
There is no universal rate. Decay advances only during hours when moisture and temperature are simultaneously favourable, so the meaningful unit is accumulated exposure dose, not calendar years. Two identical boards — one in a ventilated façade under a 600 mm eaves, one in an unventilated, ground-splashed base detail — can differ by an order of magnitude in service life on the same building.
That is what dose-based service-life modelling does: it converts hourly climate data for a specific location into wet-warm hours, adjusts for species durability and detail exposure, and reports a distribution rather than a single confident number.
Preventing decay, in order
1 · Keep it dry
Ground clearance, drained and back-ventilated cavities, drip edges, sloped horizontal surfaces, protected end grain. Every hour below 20 % MC is an hour of zero decay.
Detailing rules →2 · Choose durability to match exposure
Use class (EN 335) defines the exposure; durability class (EN 350) defines the material's natural resistance. Matching the two is a specification decision, not a preference.
Durability classes →3 · Treat or modify only where geometry cannot win
Preservative treatment and acetylation or thermal modification extend service life; they do not compensate for a detail that keeps timber wet.
Compare species →4 · Model the exposure before you build
Climate-driven dose models estimate how many wet-warm hours a detail will actually accumulate on your site, with P10/P50/P90 uncertainty instead of a single number.
Run a service-life estimate →Existing rot: what to do
- Find and remove the moisture source first — repairs into a wet substrate fail again.
- Dry the element below 20 % MC and confirm with a calibrated meter, not by feel.
- Cut out degraded material with a generous margin; brown-rotted timber has lost strength well beyond the discoloured zone.
- Replace with a durability class appropriate to the exposure the detail actually creates — not the one the drawing intended.
- Re-detail so the same wetting path cannot recur. Otherwise you have bought time, not a fix.
Estimate decay risk for your project
WoodIntel converts location climate data and detail exposure into a modelled service-life distribution with stated uncertainty. Modelled estimate — not measured performance.