This information is contained in the guidance document published by the Timber Research and Development Association (TRADA) titled: – ‘Span tables for solid timber members in floors, ceilings and roofs (excluding trussed rafter roofs) for dwellings’, obtainable from:

TRADA Technology Ltd,
Chiltern House
Stocking Lane
Hughenden Valley
High Wycombe
HP14 4ND

Or alternatively technical guidance is given in British Standard BS 8103 Part 3 2009 ‘Structural design of low-rise buildings – Code of practice for timber floors and roofs for housing’, obtainable from:

British Standards Institution
389 Chiswick High Road
W4 4AL