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Cladding & Facades · For Architects
Timber cladding specification covers more than the timber itself — substrate, breather membrane, batten cavity, fixing system, and surface coating all affect performance. This template provides a complete specification clause for ventilated rainscreen cladding to BS 8000-7 best practice.
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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
European Larch, Western Red Cedar, Siberian Larch, or Thermo-modified. Specify durability class.
Tongue and groove, shiplap, board-and-batten, or open-jointed. Affects detail and appearance.
Typical 20×140mm or 25×175mm. Specify face width AND covering width.
15–18% at delivery for external use. Allow 2 weeks acclimatisation before fixing.
Sawn, planed, brushed, or pre-weathered. Affects appearance and weathering pattern.
Untreated (silver patina), oil-based, or opaque paint. Pre-finished saves site time.
Treated softwood, minimum 25×38mm. Specify spacing (max 600mm centres).
Stainless steel A2 or A4 (coastal). Specify face-fixed or secret-fixed system.
Minimum 25mm cavity. Top and bottom ventilation openings with insect mesh.
Example Clause (NBS Format)
TIMBER CLADDING — VENTILATED RAINSCREEN Cladding boards: - Species: European Larch (Larix decidua), heartwood only - Grade: Class 3 durability per EN 350, knot-free face - Profile: Open-jointed shiplap, 20mm thick × 140mm face × 130mm covering - Moisture content: 16% ±2% at delivery - Surface: Sawn texture (rustic) or planed (smooth) — see drawings - Coating: Untreated, allowed to silver naturally - Lengths: Random lengths 1.8–4.5m, no end-jointed boards - Certification: FSC or PEFC chain of custody required Substrate and battens: - Breather membrane: Class W1, vapour open (Sd ≤ 0.3m), UV stable for 6 months exposure - Counter-battens: 25mm preservative treated softwood, vertical, at stud centres - Cladding battens: 25×38mm preservative treated softwood, horizontal, at 600mm centres maximum Fixings: - Type: Stainless steel A2 ring-shank nails, 50mm minimum penetration - Pattern: Two fixings per board per batten, 25mm from edges Cavity ventilation: - Minimum cavity depth: 25mm - Bottom ventilation: 10mm gap with stainless steel insect mesh - Top ventilation: 10mm gap behind facia or roof drip detail
Copy and paste this clause into your tender. Adjust dimensions, grades, and project-specific requirements.
Common Errors
Specifying low-durability species without treatment
Correction: Use Class 3 or better naturally (Larch, Cedar, Thermo) OR specify Use Class 3 preservative treatment.
No cavity ventilation
Correction: Always specify minimum 25mm ventilated cavity with top and bottom airflow. Sealed cavities lead to rot.
Galvanised fixings on Larch
Correction: Always stainless steel A2 minimum. Larch tannins corrode galvanised steel and stain the cladding black.
No moisture content specification
Correction: External cladding at 22%+ MC will shrink and gap. Specify 15–18% at delivery.
Reference Standards
Frequently Asked
European Larch: 30–50 years untreated. Western Red Cedar: 30–60 years. Thermowood: 30–40 years. Lifespan depends on detailing (cavity ventilation, water shedding) more than species.
Pre-finished saves significant site time and gives more uniform finish. Site-finished allows last-minute colour changes but adds 2–3 weeks to programme. For premium projects, specify pre-finished.
Minimum 25mm cavity, with continuous ventilation top and bottom. For taller cladding (>3m), consider 38mm cavity with horizontal ventilation breaks every 4m.
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