What is the difference between C16 and C24 timber?

C16 and C24 are strength grades defined in BS EN 338. C16 has a characteristic bending strength of 16 N/mm² and is the most common structural timber grade in the UK — suitable for most domestic joist, rafter and stud applications. C24 has a characteristic bending strength of 24 N/mm² and allows longer spans or heavier loads for the same timber size. C24 costs approximately 10-15% more than C16 but can reduce timber sizes or allow wider joist spacing.

What dead load category should I use?

Dead load is the permanent weight of the floor construction excluding the joists themselves. For a typical domestic floor with 18mm chipboard, plasterboard ceiling and normal domestic finishes, use the 0.25-0.50 kN/m² category. For lightweight floors (chipboard only, no ceiling), use less than 0.25 kN/m². For heavy floors (tiles, screed, or thick boarding), use the 0.50-1.25 kN/m² category. The imposed (live) load for domestic floors is 1.5 kN/m² per BS EN 1991-1-1.

Should I use 400mm or 600mm joist centres?

400mm centres are standard for domestic floor joists and give a stiffer, less bouncy floor. 600mm centres are sometimes used for cost savings but require deeper joists for the same span and may feel less solid underfoot. Plasterboard ceiling fixings and chipboard flooring are designed for 400mm or 600mm centres — check the board manufacturer's recommendations. For first floors where stiffness matters, 400mm centres are recommended.

What is clear span vs overall span?

Clear span is the distance between the faces of the supports (walls or beams). Overall span includes the bearing length at each end. The span tables below show maximum clear spans. Each joist must have a minimum bearing of 40mm at each end (typically 75-100mm is provided). If a joist bears on a hanger, use the hanger manufacturer's load tables in addition to the span tables.

Can I use these tables for flat roof joists?

These tables are for domestic floor joists with an imposed load of 1.5 kN/m². Flat roof joists may have different loading (typically 0.75 kN/m² imposed for access only for maintenance, or 1.5 kN/m² if used as a balcony). For flat roofs, use the separate rafter/flat roof span tables or refer to BS EN 1995-1-1 and the TRADA Span Tables publication for the specific loading condition.

Joist Span Tables — C16 and C24 Timber

Maximum clear spans for C16 and C24 domestic floor joists at 400mm and 600mm centres, referencing TRADA span tables and Approved Document A.

C16 Floor Joists — Dead Load 0.25-0.50 kN/m²

Domestic floor, imposed load 1.5 kN/m² per BS EN 1991-1-1 NA. Regularised timber to BS EN 14081.

Joist Size (mm)	400mm Centres (m)	600mm Centres (m)
47 x 100	1.72	1.52
47 x 125	2.24	1.98
47 x 150	2.76	2.46
47 x 175	3.20	2.88
47 x 200	3.64	3.22
47 x 225	4.04	3.54
75 x 200	4.16	3.82
75 x 225	4.62	4.22

C24 Floor Joists — Dead Load 0.25-0.50 kN/m²

Same loading conditions as above, higher strength grade.

Joist Size (mm)	400mm Centres (m)	600mm Centres (m)
47 x 100	2.02	1.82
47 x 125	2.62	2.34
47 x 150	3.20	2.86
47 x 175	3.72	3.34
47 x 200	4.18	3.74
47 x 225	4.62	4.12
75 x 200	4.72	4.36
75 x 225	5.22	4.82

Dead Load Categories

Category	Dead Load (kN/m²)	Typical Construction
Light	< 0.25	Chipboard deck only, no ceiling
Medium	0.25 – 0.50	Chipboard + plasterboard ceiling + standard finishes
Heavy	0.50 – 1.25	Tiles on screed, heavy boarding, sand/cement screed

Note: Span values are indicative and based on TRADA Span Tables guidance for Eurocode 5. Always refer to the current edition of the TRADA Span Tables or obtain an engineer's calculation for definitive sizing, particularly for unusual loading or non-standard conditions.

See the UK Timber Sizes reference, the Rafter Span Tables, the Notching and Drilling Rules, and the Joist Span Calculator.