Timber Joist Sizes & Span Tables — C16 and C24

UK structural (C16/C24) joists come in a standard nominal range at 47mm thickness: 47 x 100mm, 47 x 125mm, 47 x 150mm, 47 x 175mm, 47 x 200mm and 47 x 225mm, plus 75mm-thick sections (75 x 200mm and 75 x 225mm) for heavier purlins and trimmer joists. UK merchants also sell these under their traditional imperial names: 4x2, 5x2, 6x2, 7x2, 8x2 and 9x2 for the 47mm range, and 8x3 or 9x3 for the 75mm range. These are nominal (sawn) sizes; once regularised the finished size is a few millimetres smaller in each direction, for example a 47 x 200mm (8x2) joist finishes at approximately 45 x 195mm. See the size table above for the full nominal-to-finished conversion, and the span tables below for the correct size for a given span and load.

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.

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.

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.

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.

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 span data prepared to BS EN 1995-1-1 for the specific flat-roof loading condition, or have the members checked by a structural engineer.