Floor & Roof Joist Sizes, Spans & Spacing

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 TRADA span tables that Approved Document A points to provide the starting point for most domestic sizing decisions.¹

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/mm².² This means 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 up to 0.5 kN/m²) using C24 solid timber at 400 mm centres:³

These are rough, conservative figures. Timber Development UK’s C24 span table is the reference to use: it gives, for example, 4.13 m for a 45 × 195 mm joist at 400 mm centres with a dead load up to 0.5 kN/m².³ 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: Timber Development UK’s tables use 0.6 kN/m², or 1.02 kN/m² where snow loads are higher), the same section sizes span farther. A 45 × 170 mm C24 rafter at 400 mm centres spans about 3.6–4.15 m in those tables for roof slopes of 30 to 45 degrees, depending on the weight of the roof covering and the snow load.³ Refer to Timber Development UK’s span tables for rafters.

Spacing: 400 mm vs 600 mm

Joists are typically installed at 400 mm or 600 mm centres, measured from the centre of one joist to the centre of the next.

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: needs about a third fewer joists but requires a deeper joist section to achieve the same span and stiffness. Chipboard decking has to go up from 18 mm to 22 mm, because 18 mm chipboard is rated for joists up to 450 mm apart.⁴ 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.⁴ Members should be at least 38 × 38 mm, and herringbone strutting should not be used where the gap between joists is more than about three times the joist depth.⁴

Solid blocking (solid strutting): solid timber blocks between joists, at least 38 mm thick and at least three-quarters of the joist depth, typically at mid-span or where joists frame into steel beams.⁴ More resistant to racking but uses more timber.

Metal herringbone strutting: proprietary galvanised steel strut. Faster to install than cut herringbone and widely used in new build. Install per manufacturer’s instructions.

Warranty standards call for it too: LABC Warranty’s technical manual requires strutting on any floor joist span over 2.5 m.⁴

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.⁴ They provide excellent stiffness-to-weight ratios and, because they are made under factory conditions to a low and uniform moisture content, they are far less prone than sawn timber to bowing, twisting and shrinkage in service.⁴

I-joists require careful handling: they must not be stored unsupported over long lengths, and temporary bracing is needed during construction so that the joists stay stable until the deck is fixed.⁴ Permanent lateral restraint at ends and strutting at mid-points is still required.

I-joists are now widely specified for domestic floors spanning more than about 4 m, partly because their web holes 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 (Eurocode 5 / Part A guidance):

• Notches: top or bottom but not both, between 0.07 and 0.25 of the span measured from the support; maximum depth 0.125× joist depth, and never more than 35 mm⁵
• Holes: drilled on the centreline (neutral axis), between 0.25 and 0.4 of the span from the support; maximum diameter 0.25× joist depth and never more than 65 mm; at least 3 hole diameters apart centre to centre⁵

The hole zone is not the middle third. It stops at 0.4 of the span, short of mid-span, and it does not begin until a quarter of the way along. Notch and hole zones overlap only at the 0.25 point, which is why a notch and a hole should not be put in line with each other.

I-joist manufacturers provide specific hole charts, do not deviate from these.⁵ Cutting the flanges of an I-joist is not permitted under any circumstance.⁴

Building Regulations

Joist sizing falls under Part A (Structure) of the Building Regulations (England and Wales). The relevant approved document references:

• Approved Document A does not itself tabulate timber spans. It points to the span tables for floors and roofs published by TRADA, which is where the permissible species and spans actually live.¹ The only Table A1 referenced in Approved Document A is Table A1 of BS EN 845-1, which is about austenitic stainless wall ties
• BS 5268-2:2002 (withdrawn in 2009 and replaced by Eurocode 5, but still referenced in many older guides)⁶
• BS EN 1995-1-1 (Eurocode 5), current design standard for timber structures⁷
• NHBC Standards Chapter 6.4, timber and concrete upper floors, for NHBC-warranted properties

Structural engineers typically use proprietary span software (for example I-joist manufacturers’ design tools or general timber design software) 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.