Timber Span Tables and Joist Sizing Guide

Understanding Timber Span Tables

Span tables tell you the maximum distance a timber joist or rafter can span based on its size, strength grade, spacing, and load. They're published in Approved Document A (Structure) and are the accepted method for sizing structural timber in domestic buildings without needing a structural engineer's calculation.

Get the joist size wrong and the floor bounces, the ceiling sags, or — in extreme cases — the timber fails. Get it right and you've got a solid, quiet structure that building control signs off without a second look.

Strength Grades: C16 vs C24

All structural timber in the UK is graded to BS EN 338. The Timber Research and Development Association (TRADA) provides detailed technical guidance on timber construction. The two grades you'll see on domestic sites are:

| Grade | Bending Strength | Typical Use | Cost | |---|---|---|---| | C16 | 16 N/mm² | General floor joists, ceiling joists, stud walls | Standard | | C24 | 24 N/mm² | Longer spans, heavier loads, roof rafters | 10–20% more |

C24 is stronger, so it spans further for the same cross-section. If you're borderline on a span, upgrading from C16 to C24 is often cheaper than going up a timber size.

Tip: Always check the stamp on the timber. If it's not stamped with the grade, you can't claim it as structural. Unstamped timber is treated as C16 at best, but building control may not accept it at all.

How to Read a Span Table

Span tables are laid out with:

• Rows for timber sizes (e.g., 47 × 100mm, 47 × 150mm, etc.)
• Columns for joist/rafter spacing (typically 400mm, 450mm, or 600mm centres)
• Values showing the maximum clear span in metres

You also need to know:

• Dead load — the permanent weight of the structure (flooring, plasterboard ceiling below, etc.)
• Imposed load — the weight of people, furniture, storage (set by the Building Regs based on use)

For domestic floors, the imposed load is 1.5 kN/m² (general areas) and the dead load is typically 0.25–0.50 kN/m² depending on construction.

Floor Joist Span Tables

Here are typical maximum clear spans for floor joists at the most common domestic loading (dead load 0.25 kN/m², imposed load 1.5 kN/m²):

C16 Floor Joists

| Joist Size | 400mm centres | 450mm centres | 600mm centres | |---|---|---|---| | 47 × 100mm | 1.87m | 1.79m | 1.62m | | 47 × 125mm | 2.38m | 2.30m | 2.09m | | 47 × 150mm | 2.86m | 2.76m | 2.53m | | 47 × 175mm | 3.35m | 3.22m | 2.91m | | 47 × 200mm | 3.83m | 3.69m | 3.28m | | 47 × 225mm | 4.26m | 4.08m | 3.62m | | 75 × 200mm | 4.32m | 4.18m | 3.82m | | 75 × 225mm | 4.83m | 4.67m | 4.27m |

C24 Floor Joists

| Joist Size | 400mm centres | 450mm centres | 600mm centres | |---|---|---|---| | 47 × 100mm | 2.18m | 2.10m | 1.92m | | 47 × 125mm | 2.78m | 2.68m | 2.45m | | 47 × 150mm | 3.35m | 3.23m | 2.96m | | 47 × 175mm | 3.91m | 3.78m | 3.45m | | 47 × 200mm | 4.47m | 4.32m | 3.91m | | 47 × 225mm | 5.01m | 4.83m | 4.34m | | 75 × 200mm | 5.05m | 4.90m | 4.50m | | 75 × 225mm | 5.65m | 5.48m | 5.03m |

These are approximate values for guidance. Always verify against the current edition of Approved Document A or use an engineered timber manufacturer's span tables.

The Timber Span Calculator lets you look up spans for C16 and C24 timber across all common sizes and spacings.

Ceiling Joist Span Tables

Ceiling joists carry less load than floor joists (just the ceiling finish and light loft storage, not people and furniture). They can therefore span further for the same timber size.

For ceiling joists supporting access only for maintenance and light storage:

C16 Ceiling Joists

| Joist Size | 400mm centres | 450mm centres | 600mm centres | |---|---|---|---| | 47 × 100mm | 2.50m | 2.43m | 2.24m | | 47 × 125mm | 3.14m | 3.06m | 2.85m | | 47 × 150mm | 3.78m | 3.68m | 3.42m | | 47 × 175mm | 4.40m | 4.28m | 3.95m | | 47 × 200mm | 5.01m | 4.87m | 4.45m |

If the loft will be used as a habitable room (loft conversion), the joists need to be sized as floor joists, not ceiling joists.

Roof Rafter Sizing

Rafters carry the weight of the roof covering (tiles, felt, battens) plus wind and snow loads. Sizing depends on the roof pitch, tile weight, and geographic location (snow zone).

C16 Common Rafters (Medium-Weight Tiles, Snow Zone 1-2)

| Rafter Size | 400mm centres | 600mm centres | |---|---|---| | 47 × 100mm | 2.14m | 1.82m | | 47 × 125mm | 2.76m | 2.38m | | 47 × 150mm | 3.37m | 2.93m | | 47 × 175mm | 3.95m | 3.47m | | 47 × 200mm | 4.50m | 3.98m |

C24 Common Rafters (Medium-Weight Tiles, Snow Zone 1-2)

| Rafter Size | 400mm centres | 600mm centres | |---|---|---| | 47 × 100mm | 2.55m | 2.19m | | 47 × 125mm | 3.23m | 2.82m | | 47 × 150mm | 3.88m | 3.40m | | 47 × 175mm | 4.52m | 3.97m | | 47 × 200mm | 5.14m | 4.52m |

Heavy tiles (concrete interlocking, clay plain tiles) reduce these spans by roughly 10–15%. Lightweight tiles (fibre cement slates, metal roofing) increase them by a similar amount.

Practical Sizing Decisions

Floor Joists: The Common Choices

For most domestic builds:

| Room Span | Recommended Joist (C16) | Recommended Joist (C24) | |---|---|---| | Up to 2.5m | 47 × 150mm at 400mm | 47 × 125mm at 400mm | | 2.5–3.5m | 47 × 200mm at 400mm | 47 × 175mm at 400mm | | 3.5–4.5m | 47 × 225mm at 400mm | 47 × 200mm at 400mm | | 4.5m+ | 75 × 225mm or engineered | 47 × 225mm at 400mm or engineered |

For spans over about 5m, consider engineered timber (I-joists, LVL) or steel. Solid timber gets very heavy and expensive at these sizes, and the deflection (bounce) can be uncomfortable even if the strength is adequate.

400mm vs 600mm Centres

• 400mm centres — standard for floor joists. Works with standard plywood and chipboard sheet sizes (2400 × 600mm).
• 600mm centres — common for ceiling joists and some roof rafters. Saves timber but means the floor needs thicker decking (22mm chipboard instead of 18mm).

Most domestic floors use 400mm centres with 18mm T&G chipboard or 18mm structural plywood decking. Going to 600mm centres saves on joist costs but increases decking costs and the floor may feel softer underfoot.

Blocking, Strutting, and Restraint

Herringbone Strutting

Joists spanning more than 2.5m need strutting to prevent them twisting under load. Options:

• Herringbone strutting — timber or proprietary metal struts fixed in an X pattern between joists
• Solid blocking — offcuts of joist timber cut to fit snugly between joists

Position strutting at mid-span for spans up to 4.5m, and at third points for longer spans.

Lateral Restraint

External walls must be tied to the floor structure with restraint straps (30 × 5mm galvanised steel, minimum 1m long) at maximum 2m centres as required by Approved Document A. This stops the walls from leaning outward under roof loads.

Notching and Drilling Joists

You can notch and drill joists for pipes and cables, but only within strict limits:

| Cut Type | Zone | Limits | |---|---|---| | Notches | Top edge only, between 0.07 and 0.25 of span from support | Maximum depth: 0.125 × joist depth | | Holes | Centre line of joist, between 0.25 and 0.4 of span from support | Maximum diameter: 0.25 × joist depth |

Example: For a 200mm deep joist:

• Maximum notch depth: 25mm
• Maximum hole diameter: 50mm
• Holes must be at least 3 diameters apart (150mm for 50mm holes)

Cut outside these zones and you weaken the joist where it's under the most stress. If you need bigger holes or notches, use joist hangers to reduce the effective span, or install a purpose-made beam.

Common Mistakes

• Assuming all timber is the same grade. C16 and C24 have very different span capacities. Always check the grade stamp.
• Not accounting for dead load properly. A heavy tile floor finish (stone, thick ceramic) adds significant dead load. Recheck your spans if the floor finish is heavier than carpet or vinyl.
• Forgetting strutting. Unstrutted joists twist over time, causing squeaky floors and cracked ceilings.
• Over-notching. Plumbers and electricians cutting notches in the wrong places is the single biggest cause of bouncy floors. Mark the safe zones before first fix starts.
• Ignoring deflection. A joist might be strong enough to carry the load but still deflect enough to feel bouncy. Going up one timber size or reducing the spacing makes a noticeable difference to floor feel.

Quick Summary

1. Identify the load type (floor, ceiling, roof)
2. Measure the clear span between supports
3. Choose C16 or C24 timber
4. Look up the required size from the span tables
5. Check joist spacing matches your decking thickness
6. Add strutting for spans over 2.5m

The Timber Span Calculator gives you maximum spans for any combination of timber grade, size, and spacing — based on the Approved Document A tables.