An indicative, educational guide to sizing residential ceiling joists in Australia. Covers standard joist sizes (90×45, 120×45, 140×45, 190×45 and 240×45 MGP10), the trade-off between 450 mm and 600 mm centres, how to read a span table, how AS 1684.2 works and the difference between ceiling joists and rafters. It does not reproduce the copyright AS 1684 span tables — read every allowable span from the published tables.