Ceiling joists are very similar to floor joists and are only primarily distinguished from floor joists when there is no floor above the joists (with the exception of the attic). Ceiling joists are sized similar to floor joists with the span being driven by the joist spacing, joist depth, and intended room/space utilized above. A ceiling joist also acts as a tie to resist the lateral forces of the rafters, making their proper connection a critical component of a structure. When ceiling joists are nailed to roof rafters, they commonly bear on the walls top plate, are anchored to the rafter with several nails and/or an appropriate tie, and are nailed towards the center of the building. Nailing ceiling joists to roof rafters also provides bending and twisting movement from occuring to the joists. It is common to fur out the underside of a ceiling joist system (perpendicular to the joists) with 1x lumber. This will resist the tendency for joist movement to reflect and shear/crack through the plaster or drywall ceiling above. Below is a simple ‘Rule of Thumb’ Chart for the typical span allowed based on species, size of joists, 20psf attic load, and max allowed deflection of L/360:

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