Technical Booklet D Structure·Page 30·4B.11
Softwood floorboards - thickness and joist spacing
Softwood tongued and grooved floorboards must be 16 mm thick if joist spacing does not exceed 500 mm, or 19 mm thick if joist spacing is greater than 500 mm but not greater than 600 mm.
Softwood tongued and grooved floorboards 16 mm thick will safely support this floor loading if the spacing of the joists is not greater than 500 mm. Floorboarding 19 mm thick is required if the spacing of the floor joists is greater than 500 mm but not greater than 600 mm.
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