Getting joist sizing right is fundamental — undersized or incorrectly spaced joists lead to springy, noisy floors, plaster cracking, and in extreme cases, structural failure. Building Regulations Part A (Structure) requires joists to be engineered for the applied loads, and the tables in Approved Document A (and the companion TRADA span tables) provide the starting point for most domestic sizing decisions.
This guide covers the most common solid timber and I-joist scenarios. For unusual spans, point loads, or notching and drilling requirements, commission a structural engineer.
Common Joist Sizes
Solid timber joists are typically regularised C16 or C24 graded softwood (spruce, pine or fir) supplied in the following cross-sections (all in mm width × depth):
- 47 × 100 mm
- 47 × 125 mm
- 47 × 150 mm
- 47 × 175 mm
- 47 × 200 mm
- 47 × 220 mm
The 47 mm width is the regularised standard; 44 mm and 50 mm sections also exist from some suppliers but 47 mm is most common in the UK. Joist depth drives span capacity — width contributes to stability, not primarily to bending strength.
Timber grade matters significantly. C24 has a characteristic bending strength of 24 N/mm² versus C16 at 16 N/mm², meaning C24 achieves the same span with a shallower section, or a longer span at the same section. Most UK suppliers now stock C24 as standard; confirm the grade on your delivery note.
Span Basics
Indicative maximum spans for domestic floor joists (imposed load 1.5 kN/m², dead load 0.5 kN/m², deflection limit span/333) using C24 solid timber at 400 mm centres:
| Joist size (mm) | Max span (approx.) |
|---|---|
| 47 × 100 | 1.8 m |
| 47 × 125 | 2.3 m |
| 47 × 150 | 2.8 m |
| 47 × 175 | 3.3 m |
| 47 × 200 | 3.8 m |
| 47 × 220 | 4.3 m |
| 47 × 225 | 4.5 m |
These are indicative ranges drawn from NHBC Standards Chapter 6.1 and TRADA span tables. Actual spans depend on load condition, board type (tongue-and-groove deck or loose), point loads from partitions and services, and notch or hole positions. Always consult the full span table or a structural engineer for actual specification.
For roof rafters (which carry lower imposed loads — typically 0.75 kN/m² snow + self-weight), the same section sizes span farther. A 47 × 175 mm C24 rafter at 400 mm centres can span approximately 4.0–4.5 m on slope depending on pitch. Refer to TRADA Timber Designers’ Manual or Approved Document A, Table A1 for rafter spans.
Spacing: 400 mm vs 600 mm
Joists are typically installed at 400 mm or 600 mm centres (measured face-to-face of joists, though often referenced centre-to-centre).
400 mm centres: the standard for domestic floors. Provides a stiffer floor with less board bounce, suits standard 18 mm T&G chipboard or plywood decking, and distributes loads well. Preferred under kitchen units, bathrooms, and anywhere a point load from a partition or bath may occur.
600 mm centres: reduces timber volume (cost saving of 10–15% in joists) but requires a deeper joist section to achieve the same span and stiffness. Suitable where deck board thickness increases to 22 mm T&G. More commonly used in roof construction (rafters and ceiling joists) where imposed loads are lower.
A simple rule: if you increase joist spacing from 400 to 600 mm, you generally need to move up one or two depth sizes to maintain the same span. The saving in joist count is partly offset by the cost of deeper sections.
Strutting and Noggins
Lateral stability of floor joists is as important as span capacity. Without strutting, joists can twist under load, causing creaking floors and potential loss of composite action with the deck.
Herringbone strutting: traditional timber noggings fixed in a cross-brace pattern between joists. Required at mid-span for spans over 2.5 m, and at third-points for spans over 4.5 m. Timber 38 × 38 mm or 50 × 38 mm typically used.
Solid blocking (solid strutting): full-depth solid timber blocks between joists, typically at mid-span or where joists frame into steel beams. More resistant to racking but uses more timber.
Metal herringbone strutting: proprietary galvanised steel strut (e.g. Bret-Stif, Metsec, or similar). Faster to install than cut herringbone and widely used in new build. Install per manufacturer’s instructions.
Building Regulations Part A and NHBC Standards both require strutting for spans above 2.5 m. Omitting it is a common defect found during NHBC and surveyor inspections.
| Joist span | Strutting requirement |
|---|---|
| Up to 2.5 m | None required |
| 2.5 m to 4.5 m | One row at mid-span |
| Over 4.5 m | Two rows at third-points |
| Over 6.0 m (I-joists) | Per manufacturer’s specification |
I-Joists vs Solid Timber
I-joists (engineered wood I-beams) use a solid timber or LVL flange bonded to an OSB or plywood web. They provide excellent stiffness-to-weight ratios and are manufactured to consistent tolerances — unlike sawn timber, they will not bow, twist or shrink in service.
| Feature | Solid C24 | I-Joist (200–400 mm) |
|---|---|---|
| Max practical span (domestic) | ~4.8 m | 5.0–7.5+ m |
| Section depth range | 100–225 mm | 150–600 mm |
| Weight | ~12–18 kg/m | ~4–9 kg/m |
| Service holes | Restricted (max 0.25× depth, central third) | Pre-sized knockout holes in web |
| Cost | £4–£9/m (C24, 47×150 to 47×220) | £8–£22/m (200 mm to 400 mm) |
| Shrinkage/movement | Moderate | Very low |
| Floor squeak risk | Moderate | Low (if properly installed) |
| Fire performance | BS EN 1995-1-2 (Eurocode 5) | Reduced section method; additional protection often needed |
I-joists require careful handling — they must not be placed flat on a surface unsupported over long lengths, and temporary bracing during construction is mandatory per manufacturer’s guidance. Permanent lateral restraint at ends and strutting at mid-points is still required.
For most domestic new-build and extension floor systems in 2026, I-joists at 400 mm centres are now the default specification for spans above 4.0 m because they allow service routes without compromising structural depth.
Notching and Drilling Rules
Notching and drilling joists for services is tightly controlled because cutting into a joist dramatically reduces its bending capacity.
Solid timber — permitted zones (BS 5268-2 / Part A guidance):
- Notches: top or bottom, in the outer quarter of the span only; maximum depth 0.125× joist depth
- Holes: drilled centrally (neutral axis); maximum diameter 0.25× joist depth; minimum 3× hole diameter apart; located in middle third of span only
I-joist manufacturers provide specific hole charts — do not deviate from these. Cutting the flanges of an I-joist is not permitted under any circumstance.
Building Regulations
Joist sizing falls under Part A (Structure) of the Building Regulations (England and Wales). The relevant approved document references:
- Approved Document A, Table A1 — permissible species and spans for domestic timber members
- BS 5268-2:2002 (now superseded for new design but still referenced in many older guides)
- BS EN 1995-1-1 (Eurocode 5) — current design standard for timber structures
- NHBC Standards Chapter 6.1 — joist sizing and fixing for NHBC-warranted properties
Structural engineers typically use proprietary span software (e.g. WoodWise from TRADA, or manufacturer tools) to confirm joist sizes for complex layouts or non-standard loads. Always get structural sign-off if you are removing a wall, creating an open plan, or installing a heavy bath or hot tub on an upper floor.
