Structural Timber
QA inspection checklist

Materials

1. 1.

   Timber grade and species match specification
   What to look forGrade stamp visible on each member — strength class (e.g. C16, C24, GL24h, GL28c) and species code. Stamp shows certifying body (BM TRADA, KOMO). Glulam and LVL carry CE/UKCA mark and product code matching design.
   Acceptable whenSolid timber to BS EN 14081-1 — strength class per design (typically C16 for studs, C24 for joists/rafters). Glulam to BS EN 14080 (GL24h, GL28h). LVL/I-joists to manufacturer ETA. CE/UKCA marking compulsory for structural use.
   StandardBS EN 14081-1:2016 — Strength graded structural timber. BS EN 14080:2013 — Glued laminated timber. BS 5268-2 / BS EN 1995-1-1 (Eurocode 5).
   Common defectsC16 used where C24 specified — undersized for loadGrade stamp missing or rubbed off — cannot verifyWet-graded timber installed dry-design (or vice versa)Glulam product code does not match structural calcNo CE/UKCA mark on engineered timber (LVL, I-joists)
2. 2.

   Moisture content within acceptable range

   Record: MC (%)
   What to look forMC measured with calibrated electrical resistance meter, multiple points per member. Pin probes pushed to full depth, not just surface. Readings consistent across batch — no member significantly wetter than others. Stack ventilated and protected on site.
   Acceptable whenPer BS EN 1995-1-1 service class: SC1 (heated internal) ≤ 12% MC at install; SC2 (covered, unheated) 12-20%; SC3 (external) > 20% acceptable but design-checked. Solid timber for stud frames typically 16-19% at delivery. Engineered timber (glulam, LVL) ≤ 15%.
   StandardBS EN 13183-2:2002 — Moisture content by electrical resistance method. BS EN 1995-1-1:2004+A2:2014 — Eurocode 5. TRADA Wood Information Sheets.
   Common defectsMC > 20% in heated building — shrinkage cracks once dryWet timber boxed in too early — fungal decay riskSurface only measured — interior much wetterNo moisture record taken at delivery — disputes later
3. 3.

   Preservative treatment certification available
   What to look forTreatment certificate from BWPDA/WPA-approved treater showing Use Class, preservative product, retention level and treatment date. Each treated member tagged or end-painted. Cut ends re-treated on site with brush-applied product.
   Acceptable whenPer BS 8417: Use Class 1 (internal dry) — desirable; UC2 (covered, risk of wetting) — required for softwood; UC3 (external above ground) — required; UC4 (in/on ground) — required. Treatment to WPA Code of Practice. Site-cut ends re-treated within 24 hours.
   StandardBS 8417:2011+A1:2014 — Preservation of wood. BS EN 335:2013 — Use classes. WPA Industrial Wood Preservation Code of Practice.
   Common defectsUC2 timber used externally — decay within 5 yearsNo certificate — treatment level unknownCut ends untreated — moisture entry at end-grainSpray-treated softwood used in ground contact (needs UC4 vacuum-pressure)
4. 4.

   No splits, shakes, or excessive defects
   What to look forVisual check along each face and end. Knots within grade limits. No through-splits at bearings or connection points. End-splits limited to less than the depth of the member. No wane greater than allowed by grade. Distortion (bow, spring, twist) within tolerance.
   Acceptable whenPer BS 4978 / BS EN 14081 visual override rules: end split ≤ length of width of member; bow ≤ 10 mm in 2 m; spring ≤ 8 mm in 2 m; twist ≤ 1 mm/25 mm of width per 2 m length. No knots or shakes at connection zones.
   StandardBS 4978:2007+A2:2017 — Visual strength grading. BS EN 14081-1:2016. TRADA Timber Defects Recognition.
   Common defectsThrough-split at bearing — member fails in shearLarge knot at bolt hole — connection capacity reducedBowed joist installed — visible sagging in finished ceilingWane on bearing face — reduced contact area

Erection

1. 5.

   Frame erected to correct sequence
   What to look forErection follows the frame designer's sequence — typically sole plates then panels then floor cassettes/joists, working away from a braced corner. Temporary bracing installed as each panel raised. Frame remains stable at every stage.
   Acceptable whenSequence per timber frame designer's erection drawings and method statement. Temporary bracing per BS 5268-2 / TRADA "Timber frame construction" — typically diagonal raking braces at corners and mid-span until permanent bracing in place.
   StandardBS EN 1995-1-1:2004+A2:2014. UK Timber Frame Association — Site Guide. TRADA Timber Frame Construction.
   Common defectsPanels raised without bracing — wind blow-over riskFloor deck loaded with materials before walls bracedSole plate not levelled before panels — frame out of plumb cumulativelyCassettes installed before walls fully fixed below
2. 6.

   Connections match approved details
   What to look forJoist hangers, framing anchors, holding-down straps, and ridge ties match the marked connection on the structural drawing. Manufacturer code (e.g. Simpson Strong-Tie, BAT, ITW) matches schedule. Correct nail/screw type and quantity in every hole.
   Acceptable whenJoist hangers to BS EN 14545 / ETA — fully nailed with manufacturer's specified nail (sherardised square-twist, 30/35/40 mm). Holding-down straps minimum 30x2.5 mm galvanised, fixed per design. Bolts to BS EN 1995 capacity — washers under nut and head.
   StandardBS EN 14545:2008 — Joist hangers/connectors. BS EN 1995-1-1:2004+A2:2014 — Eurocode 5. NHBC Standards Chapter 6.2.
   Common defectsHalf the holes in joist hanger left empty — capacity dramatically reducedWrong nail used (round wire) instead of square-twistWrong hanger size — joist not seated to base of hangerHolding-down strap missing at gable wall — racking/uplift failGalvanised hanger used in treated timber — corrosion (needs stainless)
3. 7.

   Bracing installed as per design
   What to look forPermanent racking bracing installed per design — typically OSB or plywood sheathing fixed at perimeter and intermediate studs. Wind girder bracing in roof. Diagonal noggings or straps at floor diaphragm. No bracing left out for service penetrations without engineer sign-off.
   Acceptable whenSheathing per BS EN 12369-1 — OSB/3 minimum 9 mm for racking, fixed at 150 mm perimeter / 300 mm intermediate centres with 51 mm clout nails or 60 mm screws per Eurocode 5 racking calc. Roof bracing per truss designer (typically TRA Bracing Guide).
   StandardBS EN 1995-1-1:2004+A2:2014. BS EN 12369-1:2001 — Wood-based panels. TRA Trussed Rafter Information Sheet 9.
   Common defectsSheathing perimeter nails > 150 mm — racking strength halvedDiagonal roof bracing missing — trusses topple under windService holes cut through sheathing without nogging trimNo bracing in ground floor — frame racks during construction
4. 8.

   Frame plumb and level

   Record: Deviation (mm)
   What to look forWalls plumb measured with spirit level or laser at multiple positions in each storey. Sole plate level along length. Floor deck level under straight edge. Whole frame square — diagonals measured equal.
   Acceptable whenPer BS 5606 / NHBC: walls plumb ±5 mm in storey height (max 10 mm in 6 m). Sole plate level ±5 mm in 5 m. Floor flatness ±5 mm in 3 m. Diagonals of building to agree within ±15 mm.
   StandardBS 5606:1990 — Accuracy in building. NHBC Standards Chapter 6.2 — External timber framed walls. UKTFA Site Guide.
   Common defectsStorey out of plumb > 10 mm — visible lean, finishes problematicSole plate not packed level on uneven slab — cumulative error up the buildingDiagonals out > 20 mm — windows and doors do not square upFloor not level — flooring contractor builds up screed unexpectedly

Protection

1. 9.

   DPC/membrane correctly installed at base
   What to look forDPC strip continuous under sole plate, projects beyond sole plate face by 25-50 mm. Lap joints minimum 100 mm. DPC isolates sole plate timber from concrete or masonry. No tears, gaps, or punctures from fixings.
   Acceptable whenDPC to BS 6515 / BS 8215 — bituminous, polyethylene, or proprietary. Min 150 µm thickness for poly. Lap ≥ 100 mm. Sole plate fixed through DPC into substrate with sealed fixings. Above DPM in ground floor.
   StandardBS 6515:1984 — Polyethylene DPC. BS 8215:1991 — Code of practice for DPCs. NHBC Standards 6.2 / 5.1.
   Common defectsDPC torn at fixing — water bridge into sole plateNo DPC at all — sole plate rots within 5 yearsDPC laps not made — water tracks behind frameDPC below floor slab DPM — capillary water reaches timber
2. 10.

   Breather membrane installed and lapped correctly
   What to look forBreather membrane (e.g. Tyvek, Solitex) covers full external face of sheathing, taut without sagging. Horizontal laps shingled with upper layer over lower (≥100 mm overlap). Vertical laps ≥150 mm and taped. Sealed to window/door frames with proprietary tape.
   Acceptable whenMembrane to BS EN 13859-2 (breathable, vapour-permeable). Sd-value < 0.5 m. Horizontal lap ≥ 100 mm shingled. Vertical lap ≥ 150 mm sealed. UV exposure within manufacturer limits (typically 3-4 months max).
   StandardBS EN 13859-2:2010+A1:2014 — Breather membranes for walls. NHBC Standards Chapter 6.2. UKTFA Site Guide.
   Common defectsLap upside-down — water runs behind, into sheathingMembrane left exposed > UV limit — degraded, brittlePenetrations not sealed — local rain ingressWrong type (vapour-impermeable) used — interstitial condensationMissing tape at window heads — driving rain into reveal
3. 11.

   Frame protected from weather during construction
   What to look forFrame covered with breather membrane within design exposure period. Top of unfinished walls and exposed cassettes covered with tarpaulin overnight. Standing water removed from floor decks. No prolonged wetting of structural members. Site stack covered and ventilated.
   Acceptable whenPer BS 5268-2 / TRADA: maximum exposure of unprotected frame typically 8-12 weeks. MC at close-in ≤ 20% (and ≤ 19% before insulation/plasterboard). Frame to dry to service-class MC before finishes.
   StandardBS EN 1995-1-1:2004+A2:2014. TRADA "Wood-based panels — protection on site". UKTFA Site Guide. NHBC 6.2.
   Common defectsFrame left open > 12 weeks — sheathing swells, fungal stainingOSB exposed to repeated wetting — edge swell, no recoveryPlasterboard installed before frame dried — popping nails, mouldStanding water on cassette — ply delamination

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