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Structural Timber · For Architects
Acoustic separating floor specification must cover both airborne (voice, music) and impact (footfall, dropped objects) sound transmission. Approved Document E sets minimum standards (45dB DnT,w airborne, 62dB LnT,w impact for new build dwellings). This template provides verified Robust Detail build-ups for timber floors.
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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
AD E minimum: DnT,w ≥ 45dB airborne, LnT,w ≤ 62dB impact (new build separating floor).
Use Robust Details (RDs) for verified build-ups — avoids on-site testing.
Resilient layer (mineral wool or rubber matting) under floor finish.
I-joists, solid joists, or CLT. Mass adds airborne performance.
Mineral wool 100mm minimum between joists for airborne reduction.
Spring-mounted ceiling bars at 600mm centres to decouple ceiling.
2 × 12.5mm gypsum plasterboard minimum. More layers = more mass = more airborne reduction.
Detailing at perimeter walls and service penetrations critical for performance.
Example Clause (NBS Format)
ACOUSTIC SEPARATING FLOOR (TIMBER) Performance: To Robust Detail E-FT-1 or equivalent verified detail - Airborne: DnT,w + Ctr ≥ 45dB - Impact: LnT,w ≤ 62dB Build-up (top to bottom): 1. Floor finish: As specified (carpet, engineered timber, vinyl) 2. Resilient layer: 5mm acoustic underlay, BBA-certified 3. Floating floor: 22mm P5 chipboard, perimeter isolation strip 6mm 4. Mineral wool: 100mm density 33 kg/m³ between joists 5. Joists: 240mm i-joists or solid C24 200mm at 400mm centres 6. Resilient bars: GypMR or equivalent at 600mm centres, mounted across joists 7. Ceiling: 2 × 15mm gypsum acoustic plasterboard, joints staggered Perimeter detail: - Floating floor: 6mm acoustic strip at all walls, no rigid contact - Plasterboard: Gap-filled with acoustic sealant at all wall junctions - Service penetrations: All gaps sealed with acoustic mastic Verification: - Pre-completion testing: Per Approved Document E Section 1.1, OR - Robust Detail registration: With manufacturer-supplied installation certificate Plumbing penetrations: Below resilient layer only. No services bridging the floating layer. Light fittings: Surface mounted only. No recessed downlights through ceiling line (compromises performance).
Copy and paste this clause into your tender. Adjust dimensions, grades, and project-specific requirements.
Common Errors
Service penetrations bridging floating floor
Correction: Any rigid penetration through resilient layer creates a sound bridge. Run services below the resilient bar line.
No perimeter isolation
Correction: Floating floor must have continuous resilient strip at all walls. Direct contact destroys impact performance.
Generic "100mm mineral wool"
Correction: Specify density (33 kg/m³ minimum) and BBA certification. Lower density does not provide airborne attenuation.
Recessed downlights
Correction: Recessed lights penetrate the ceiling line. Use surface-mounted fittings or specify acoustic-rated downlights.
Reference Standards
Frequently Asked
Robust Details avoid testing if you register and follow the detail exactly. Pre-completion testing is required for any non-RD floor (10% of plots tested per AD E).
Carpet on resilient underlay typically performs best (LnT,w ~50dB). Engineered timber on acoustic underlay achieves ~58dB. Hard finishes (tile, concrete) without good resilient layer often fail AD E.
Generally no for residential separating floors — they compromise the acoustic performance. If essential, specify acoustic-rated downlights with intumescent backing and accept performance reduction.
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