What size timber do I need for a 4m span floor?

For a 4m span domestic floor with joists at 400mm centres, you need at minimum 47x225mm C16 timber (max span 3.97m) or 47x200mm C24 timber (max span 4.13m). C24 grade is the better choice here as it comfortably covers the span. Always check that your actual dead and imposed loads match the standard assumptions of 0.25 kN/m² dead and 1.5 kN/m² imposed load.

What is the difference between C16 and C24 timber?

C16 and C24 are strength classes defined in BS EN 338. C24 is approximately 50% stronger in bending than C16, allowing longer spans for the same section size. C24 timber is graded to a higher standard — it has fewer and smaller knots, straighter grain, and higher density. Most structural timber from Scandinavian sources is graded C24, while UK-grown timber is more commonly C16.

What joist spacing should I use?

The three common joist spacings in UK construction are 400mm, 450mm, and 600mm centres. 400mm centres give the strongest floor and are required for heavier loads or longer spans. 600mm centres use fewer joists but require larger sections for the same span. 450mm is a compromise often used with engineered floor panels. For most domestic floors, 400mm centres are recommended.

Do these span tables comply with Building Regulations?

These span tables are based on Eurocode 5 (BS EN 1995-1-1) and TRADA span table guidance, which are the accepted method for sizing timber joists under UK Building Regulations Approved Document A (Structure). The tables assume standard domestic loading of 1.5 kN/m² imposed load and 0.25 kN/m² dead load for floor joists. A structural engineer should verify sizing for non-standard loads.

Can I use these tables for loft conversions?

Yes, but you must use the floor joist values (not ceiling joist) because a loft conversion creates a habitable room with full domestic loading (1.5 kN/m² imposed). You may also need to increase the dead load allowance if heavy flooring, partitions, or bathrooms are planned. For loft conversions, Building Control will typically want to see a structural calculation from an engineer.

Timber Span Calculator — What Size Joist Do I Need?

Find the correct timber joist or beam section size for your span, based on UK span tables per Eurocode 5 and TRADA guidance.

Clear Span

Unsupported distance between bearing points

Joist Spacing

Centre-to-centre distance between joists

Timber Grade

C24 is stronger; C16 is more common UK-grown timber

Load Type

Determines the loading assumptions used

Loading (advanced)

Dead Load

Self-weight of floor, ceiling, services. Standard: 0.25 kN/m²

Imposed Load

Occupancy load. Domestic floor: 1.5 kN/m²

How We Calculate This

This calculator uses UK span tables derived from Eurocode 5 (BS EN 1995-1-1) and TRADA guidance to determine the minimum timber section size for your span. It compares your required clear span against the maximum allowable span for each standard section size.

How it works

For each timber section, the maximum span depends on: timber grade, joist spacing, and load type.

The calculator finds the smallest section where the maximum allowable span exceeds your required span, then recommends one size up as a practical safety margin.

Standard assumptions

Floor joists: 1.5 kN/m² imposed load, 0.25 kN/m² dead load (domestic)
Ceiling joists: 0.25 kN/m² imposed (no storage), lighter dead load — spans ~20% longer
Roof joists: Wind and snow loading varies — spans ~10% longer than floor joists as a general guide

Key span values (C16, 400mm centres, floor)

47 x 100mm: 1.72m
47 x 150mm: 2.71m
47 x 200mm: 3.56m
47 x 225mm: 3.97m

Standards

Timber grading per BS EN 338. Structural design per Eurocode 5 (BS EN 1995-1-1). Span tables follow TRADA guidance and are accepted under UK Building Regulations Approved Document A.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Timber Span Calculator — What Size Joist Do I Need?

How We Calculate This

How it works

Standard assumptions

Key span values (C16, 400mm centres, floor)

Standards

Frequently Asked Questions

Related Calculators

Concrete Volume Calculator

Concrete Mix Calculator

Block Calculator

Brick Calculator

Mortar Calculator