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Decking & Outdoor · For Builders
Timber decking specification covers boards, joists, foundation, fixings, and finish. The most common deck failures come from inadequate ventilation under boards and ground contact for joists. This template addresses both with proper detail specification.
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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
Larch (DC3), Thermowood (DC1-2), Ipe (DC1), Cumaru (DC1), or Robinia (DC1). Match durability to maintenance willingness.
Smooth, anti-slip grooved, or reeded. Anti-slip is required for accessible decks.
Typically 25×120mm or 28×145mm. Thicker = stiffer = wider joist spacing possible.
C24 with Use Class 4 preservative treatment if in ground contact. Same species as boards if exposed.
47×175mm joists at 400–500mm centres for residential. 47×200mm at 400mm for heavier loads.
Concrete pads, ground screws, or pile foundations. Specify minimum 200mm clearance to ground.
Stainless steel A2 minimum, A4 for coastal. Hidden clip system or face-fixed.
5–8mm gap between boards for drainage and expansion.
Minimum 200mm clearance from ground to underside of joists. Open sides preferred.
Example Clause (NBS Format)
TIMBER DECKING Deck boards: - Species: Siberian Larch (Larix sibirica), heartwood only - Grade: Knot-free face per EN 350 Class 3 - Profile: Reeded anti-slip, 28mm thick × 145mm face × 8mm board gap - Moisture content: 16% ±2% at delivery - Lengths: 4.0m or 4.8m, mixed - Certification: FSC or PEFC chain of custody Joists: - Grade: C24 to EN 14081 - Section: 47×175mm - Spacing: 400mm centres maximum - Treatment: Use Class 4 preservative per EN 351-1 - Bearings: Galvanised joist hangers or stainless steel post anchors Foundation: - Type: Concrete pad foundations to engineer's drawings - Depth: Minimum 600mm below finished ground level - Clearance: 200mm minimum from ground to underside of joists Fixings: - Boards: Stainless steel A2 decking screws, 60mm length, two per board per joist - Joists: 100mm stainless steel coach screws into post anchors Ventilation: - Minimum 200mm clearance to ground - Open sides preferred. If skirting fitted, provide ventilation grilles equivalent to 1/200 of deck area Finish: Untreated. Allow to silver naturally OR apply UV-protective oil annually for first 3 years.
Copy and paste this clause into your tender. Adjust dimensions, grades, and project-specific requirements.
Common Errors
Joists in ground contact without UC4 treatment
Correction: Specify Use Class 4 preservative treatment per EN 351-1 OR raise joists 200mm minimum from ground.
Insufficient ventilation under deck
Correction: Always 200mm minimum clearance. Closed-side decks need ventilation grilles equivalent to 1/200 of deck area.
No anti-slip on accessible decks
Correction: For commercial / public decks, specify reeded or grooved anti-slip profile. Smooth boards become dangerously slippery when wet.
Galvanised fixings with Larch / Cedar
Correction: Tannins in these timbers corrode galvanised steel. Always stainless steel A2 minimum, A4 for coastal.
Reference Standards
Frequently Asked
Treated softwood: 10–15 years. Larch: 15–25 years. Thermowood: 20–30 years. Hardwood (Ipe, Cumaru): 30–50+ years. Lifespan depends on ventilation and water management more than species.
No, decking will silver naturally and remain structurally sound. Oil only if you want to maintain the original colour. Apply twice in year 1, then annually.
5–8mm gap is standard. Wider gaps drain better but feel less solid underfoot. For accessible decks, 6mm maximum (wheelchair safety).
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