Timber Material Moisture Content Specification: What to Require and Why It Matters
A practical guide to specifying moisture content in timber — covering service classes, measurement methods, and on-site verification
Moisture content (MC) is the single most important variable in timber material performance. Get it wrong and you'll see cupping, gapping, joint failure, or structural movement within 6–12 months of installation.
We manufacture biomass-modified timber with controlled kiln drying. Here's what MC specification means in practice and what to write into your purchase order.
Service Classes and Target MC Values
| Service Class (EN 1995) | Environment | Target MC on Delivery |
|---|---|---|
| Class 1 (indoor, heated) | RH ≤ 65%, T ~ 20°C | 12 ± 2% |
| Class 2 (covered outdoor) | RH ≤ 85%, weather-protected | 18 ± 2% |
| Class 3 (exposed outdoor) | RH variable, wetting possible | Use naturally durable or modified timber; MC balance to site |
Keep in mind: Modified timber has a lower equilibrium moisture content (EMC) than untreated timber at the same RH. So a modified timber product delivered at 16% MC in a Class 2 environment will move less than untreated timber at 18% MC.
How MC Is Measured — and Why Oven-Dry Is the Reference
- Oven-dry method (EN 13183-1): The reference method. Sample is weighed, dried at 103±2°C to constant weight, then re-weighed. Accurate but destructive — not for finished goods.
- Pin-type moisture meter: Quick, non-destructive for production checks. Calibrate to species. For modified timber, calibration curves differ from untreated — ask your supplier for the correct setting.
- In-line dielectric sensors: Used in modem kilns for real-time MC monitoring. Gives continuous readout during drying. We use this to control our kiln discharge MC to ±1.5%.
Specifying MC in Purchase Orders: What to Write
MC Specification Checklist
- Specify MC range, not just "kiin-dried": "12±2% MC at time of shipment" is enforceable. "Kiln-dried" is not.
- Require batch MC test report: Ask for MC readings from 5–10 random pieces per batch. Reputable suppliers will provide this with the delivery documentation.
- Include on-site check clause: For large orders, include right to reject material with MC outside specified range on independent measurement.
Our outdoor wall panels and marine flooring are supplied with MC test records for every production batch.
Need MC Specification Support for Your Project?
We provide MC test reports, kiln discharge records, and site MC verification guidance for all our timber products.
Or contact our technical team for MC specification review.
