Floor Joist Installation: Span Tables, Spacing, Notching and Strutting

Quick Answer: Floor joist size is determined by span, spacing and timber grade per Approved Document A and BS 5268-7 (or TRADA span tables). Common domestic: C16 47 × 195mm at 400mm centres for 3.7m clear span. Joists must be notched only in the top or bottom third, between 0.07L and 0.25L from supports, with notch depth ≤0.125 × joist depth. Strutting (herringbone or solid) is required at mid-span for joists over 2.5m clear. Joist hangers must be installed dead level with all nail holes filled.

Summary

Floor joists carry imposed (people, furniture) and dead (floor finish, ceiling) loads to supporting walls or beams. Sizing, spacing and fixing are governed by Approved Document A (Structure), BS 5268-7 span tables, and TRADA published spans. Get any of these wrong — span exceeded, joist undersized, notch in the wrong place — and you have a bouncy floor at best or a structural failure at worst.

The British convention is C16 stress-graded softwood as standard (whitewood, spruce-pine-fir, typically Scandinavian or Baltic origin). C24 (higher grade) is used where deeper or longer joists are needed at the same cross-section, but is more expensive. Engineered timber (I-joists like JJI, Posi-Joists open metal-web, LVL beams) is increasingly used in new build because of consistent dimensions, longer spans and pre-drilled service zones — but conventional C16 timber remains dominant in extensions and retrofit work.

Joist installation rules cover the obvious (spacing, support, level) and the less obvious (notching and drilling limits, strutting, bearing length, end fixing). Building Control inspects joist work at first fix, and a failure to comply with the notching rules — particularly for plumbers and electricians running services through joists — is a common cause of rejection. See timber spans for the full span tables and bouncy floor for diagnosing existing floor problems.

Key Facts

Quick Reference Table

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C16 Floor Joists, dead load 0.25 kN/m², imposed load 1.5 kN/m² (domestic):

Joist Size Centres Max Clear Span
47 × 100mm 400mm 1.95m
47 × 125mm 400mm 2.40m
47 × 150mm 400mm 2.85m
47 × 175mm 400mm 3.30m
47 × 195mm 400mm 3.70m
47 × 220mm 400mm 4.15m
47 × 245mm 400mm 4.60m
47 × 195mm 600mm 3.15m
75 × 195mm 600mm 3.55m

C24 grade extends spans by approximately 10–15% over C16 at the same size. Verify exact values against current TRADA span tables before specifying.

Detailed Guidance

Step 1: Determine loading and span

Domestic floor loading per BS 6399 / EN 1991: imposed 1.5 kN/m² for bedrooms and 2.0 kN/m² for non-domestic. Dead load: 0.25 kN/m² (carpet + ceiling) or 0.50 kN/m² (tile finish + ceiling). Measure clear span (the unsupported distance between supports), not the full joist length.

For typical UK domestic at 1.5 kN/m² + 0.25 kN/m²:

Above 4.5m clear span, consider C24 grade, larger sections, or engineered joists.

Step 2: Choose joist material

For most extensions, sawn C16 in 47 × 195mm at 400mm centres handles spans up to 3.7m and is the default specification.

Step 3: Set out support positions

Mark joist positions on the wall plate (or trimmer) at 400mm centres. The first joist runs ≤50mm from the gable or party wall; the last fits the remaining bay (often <400mm). All joist tops must align — pack supports if necessary.

For 600mm centres (using thicker decking or engineered joists), confirm the decking specification supports the wider spacing.

Step 4: Bearings

Joists must bear:

Where a joist sits on a wall plate, the wall plate must be flat, level and dry-fixed to the masonry with stainless steel restraint straps at 2.0m centres (Approved Document A wind uplift).

Step 5: Joist hangers

Where joists meet a parallel beam, trimmer or wall plate at right angles, use a galvanised joist hanger sized to the joist. The hanger must be:

Common faults:

Step 6: Trimming for openings

Where joists are cut for stair wells, chimney breasts or rooflights, the trimmer joist (the cross piece supporting the cut joist ends) must be doubled or up-sized. Standard practice:

Doubled trimmers are nailed and glued (PVA D3 or polyurethane construction adhesive).

Step 7: Strutting

Strutting prevents joists twisting and reduces vibration. For clear spans ≥2.5m, install one row of strutting at mid-span. For spans ≥5m, install strutting at quarter spans.

Strutting options:

Strutting fitted before flooring is laid.

Step 8: Notching and drilling

When plumbers, electricians and others run services through joists, the notching and drilling rules apply:

Notch zone (top or bottom):
       0.07L     0.25L
  ──┬───────┬─────┬────────────────────┬─────┬───────┬──
    │       │ NO  │    OK to notch     │ NO  │       │
    │       │ ←───→                    ←───→ │       │
  Support              centre                   Support
  
Drill zone (centre line only):
       0.25L            0.40L
  ──┬─────────┬──────────────┬───────────┬─────────┬──
    │         │ NO           │ OK to     │ NO      │
    │         │              │ drill     │         │
  Support           centre line              Support

Limits:
  Notch depth   = max 12.5% of joist depth (24mm on 195mm joist)
  Drill diameter = max 25% of joist depth (48mm on 195mm joist)
  Notch spacing = min 3× notch depth apart

For a 195mm joist over a 3.0m span, the notch zone is 210–750mm from each end. Notching the centre 1.5m of the joist (where bending is maximum) seriously weakens it and is a frequent first-fix cause of bouncy floors after carpet is laid.

Step 9: Flooring

22mm chipboard P5 (moisture-resistant flooring grade) is the standard. T&G long edges glued (PVA D3). Cross-joints staggered and supported on joists or noggins. Screw fix at 300mm centres along joist line.

OSB 18mm structural is an alternative. Tile-backer board (cement-fibre) is required under heavy tile finish.

Step 10: Restraint straps

Approved Document A requires lateral restraint of joist ends to gable walls. Galvanised steel straps at 2.0m centres, 30 × 5mm, screwed to joists at ≥3 fixings and turned 100mm down into masonry. This prevents the wall pulling away under wind load.

Frequently Asked Questions

Can I install joists at 600mm centres?

Yes — but the flooring must be specified for it. 22mm chipboard P5 is rated for 600mm centres but feels bouncier than at 400mm. 18mm OSB structural is sometimes specified for 600mm but the deflection under point loads (chair legs) is noticeable. For a quality floor, 400mm centres remain the standard regardless of regulation.

What if my joists are slightly over span?

Don't. Span tables include a safety factor and deflection limit (typically L/250 or 14mm on a 3.5m span). Joists at over-span are within the safety factor for collapse but the deflection produces a bouncy, noisy floor. Solutions: closer centres, deeper joists, or doubled joists (sister joists). C24 grade where C16 was specified extends the span without changing depth.

Can I notch a joist for a 22mm waste pipe?

Possibly, but check both limits: notch depth ≤24mm on a 195mm joist (a 22mm pipe just fits), and position must be in the 0.07L–0.25L zone. For a typical 3m span, that's between 210mm and 750mm from a support. If the pipe needs to cross the centre of the floor, drill instead (drill zone is wider, drill diameter is larger) or fit a hanger and run through a parallel zone.

Are I-joists subject to the same notching rules?

No — engineered I-joists have manufacturer-specific notching and hole-drilling guidance. The web (the OSB centre) is structurally critical and cannot be cut except in pre-marked locations. The manufacturer's data sheet shows the allowable holes and their positions. Always check the data sheet before drilling I-joists.

Do I need restraint straps if the joists run perpendicular to the gable?

Yes — restraint straps are required at right angles to the joist run, anchoring the gable wall to the joist structure. If joists run perpendicular to the gable, the straps are fitted parallel to the joists across the top, with a packing piece between. If joists run parallel to the gable (parallel partition wall typical), straps connect across at right angles.

Can I use second-hand timber?

Reclaimed structural timber can be reused if it is graded by a competent person (a graded stamp must remain visible, or a fresh inspection by a qualified grader). Most second-hand timber is unstamped and uncertified — Building Control will reject it for structural use. Suitable for non-structural use (battens, infill, blocking) only.

Regulations & Standards