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Structural Timber · For Builders
Wet timber on site causes 80% of preventable timber defects. This specification clause forces site moisture testing on every delivery, with documented acceptance and rejection procedures. Critical for premium projects and any building with mass timber.
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NBS-format clause, common errors checklist, and standards reference. Editable PDF format ready for your tender documents.
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Specification Checklist
Calibrated electrical resistance moisture meter (pin-type) per EN 13183-2.
Annual calibration certificate, on-site verification with dry block before each session.
Every delivery, minimum 5 random pieces or 10% of consignment (whichever greater).
Per project Service Class — typically 12% ±2% (SC1), 16% ±2% (SC2).
Whole consignment rejected if >5% of tested pieces fail. Suppliers responsible for collection.
Test log per delivery, photo of meter reading, signed by site supervisor.
Stored on level bearers, protected from rain, ventilated.
Example Clause (NBS Format)
SITE MOISTURE MONITORING (TIMBER) Equipment: - Electrical resistance moisture meter, pin-type per EN 13183-2 - Annual calibration certificate (within 12 months) - On-site verification: Dry calibration block test before each session Acceptance criteria (per Service Class per EN 1995-1-1): - Service Class 1 (heated internal): 12% ±2% MC - Service Class 2 (covered external): 16% ±2% MC - Service Class 3 (fully exposed): 18–22% MC Test procedure: - Frequency: Every delivery - Sample size: 5 pieces minimum, or 10% of consignment, whichever greater - Test depth: Mid-thickness, 600mm from end, 50mm from edge - Multiple readings: Minimum 3 per piece, average recorded Rejection: - If <5% of tested pieces fail: Mark and replace failed pieces - If 5–20% fail: Reject consignment, supplier collects - If >20% fail: Reject consignment + supplier formally noted (escalate) Storage: - Bearers: 75×75mm timber, at 600mm centres maximum - Cover: Tarpaulin sheet, edges propped open for ventilation - Ground clearance: 200mm minimum - Adjacent ground: Free-draining, no standing water within 1m Documentation: - Test log: Per delivery, kept by site supervisor - Photographs: Meter reading + delivery ticket, attached to log - Reporting: Weekly summary to project sustainability champion / structural engineer Site signage: "TIMBER STORAGE — KEEP COVERED" at delivery zones, with QR link to test log
Copy and paste this clause into your tender. Adjust dimensions, grades, and project-specific requirements.
Common Errors
No calibration
Correction: Uncalibrated meters can read 4–5% off. Always verify with dry block before each session.
Surface-only readings
Correction: Surface moisture differs from core. Test mid-thickness with pin-type meter, multiple readings averaged.
No rejection procedure
Correction: Without documented rejection, suppliers continue delivering wet timber. Tender must specify consequences.
Storage on wet ground
Correction: 200mm minimum clearance, tarpaulin-protected. Site moisture absorption can ruin properly delivered timber.
Reference Standards
Frequently Asked
Every delivery, minimum 5 pieces or 10% of consignment. For mass timber projects (CLT, glulam), test every panel.
Pin-type meters use electrical resistance via two metal pins (more accurate, requires holes). Pinless meters use capacitance (faster, no holes, less accurate, depth-limited). Pin-type is standard for site acceptance testing.
Have a calibrated reference meter, take photographs, and offer joint re-test with their meter. If still disputed, send a sample to a UKAS-accredited testing laboratory for definitive measurement.
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