Check moisture, temperature, and exposure to rate decay. Compare treatments and detailing for protection better. Use the results to choose durable timber assemblies quickly.
| Scenario | MC (%) | RH (%) | Temp (°C) | Exposure | Treatment | Ventilation | Key detailing | Expected risk |
|---|---|---|---|---|---|---|---|---|
| Interior framing | 12 | 55 | 22 | Above protected | None | Good | Drying likely | Low |
| Covered exterior soffit | 16 | 75 | 26 | Above protected | Light | Moderate | Coating + sealed cuts | Moderate |
| Exposed balcony joists | 22 | 85 | 24 | Above exposed | Medium | Moderate | Drainage + air gap | High |
| Fence post in soil | 28 | 80 | 20 | Ground contact | Heavy | Poor | Soil splash control | High |
| Timber near waterline | 32 | 95 | 18 | Freshwater | Heavy | Poor | Frequent inspection | Severe |
These are illustrative examples. Use project-specific exposure, materials, and maintenance assumptions for decisions.
This calculator estimates decay risk using a weighted climate–wetness model and adjustment factors. The output is a practical screening score from 0 to 100.
The factors represent typical field influence: durability and exposure raise susceptibility, while treatment and good detailing reduce the effective risk.
Wood decay fungi need sustained moisture. As a practical rule, timber held below about 16% moisture content typically presents low decay risk, while 16–20% calls for closer detailing and inspection. When moisture content remains above 20% for extended periods, the likelihood of biological attack increases quickly, especially at warm temperatures.
High relative humidity slows drying and keeps surface wetness higher. In this model, average RH above 85% materially increases the climate index. Temperature also matters: decay activity is commonly strongest between about 5 °C and 35 °C, so an otherwise damp element at 25 °C is usually more vulnerable than the same element at 5 °C.
Wetting events per week and wet days per year represent how often timber is re-wetted and how long it stays unable to dry. For example, a sheltered façade might see 2 wetting events/week and 30 wet days/year, while a leaking balcony detail can reach 5 events/week and 120 wet days/year. Increasing time-of-wetness typically shifts the result toward High or Severe categories.
Exposure class applies a hazard multiplier: above-ground protected members dry faster than ground-contact or marine splash-zone elements. Ventilation reduces trapped moisture, and detailing upgrades further lower risk. Positive drainage, sealed end grain, protective coatings, and a ventilation air gap each reduce the effective score by small, compounding factors.
Species durability (class 1–5) and preservative treatment level adjust susceptibility. If the calculator returns High or Severe, consider upgrading to a more durable species, increasing treatment to match the hazard class, and tightening inspection intervals. Document moisture readings and maintenance actions to keep the element in a lower-risk operating range. Use exported CSV or PDF outputs to track changes over time.
It is a screening indicator that combines climate, wetness, and adjustment factors. Lower scores suggest better drying and lower decay potential. Higher scores signal conditions where timber protection and detailing become critical.
Moisture content and time-of-wetness typically dominate. Keeping MC below about 16–18% and limiting wet days can reduce risk more effectively than small changes in other factors.
Use a calibrated moisture meter and take multiple readings. Measure near end grain, fasteners, and shaded areas. Record the highest stable readings, not short-term spikes after rainfall.
Count days when surfaces stay damp or cavities cannot dry: leak history, splash zones, condensation seasons, and water traps. If unsure, use a conservative higher value to avoid underestimating risk.
No. Treatment improves resistance but does not replace sound detailing. Poor drainage, trapped moisture, and damaged coatings can still lead to decay, checking, or connector corrosion.
Yes for early screening, but verify product-specific limits. Some engineered members are sensitive to moisture cycling and require strict edge sealing, membranes, and manufacturer-approved protection systems.
Add positive drainage, introduce ventilation gaps, seal end grain and cuts, maintain coatings, and reduce ground contact. If exposure is severe, also upgrade durability class or treatment level.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.