A few assumptions would need to be made to assess the load carrying capacity of the ceiling joists, and the assumptions would need to be on the conservative side since we don't know the type or grade of the timber. A lath and plaster ceiling is already quite heavy - possibly as much as 0.5 kN/m2 (50 kg/m2). A simpler approach would be to ignore the ceiling joists and run new timber parallel to the joists, supported clear of the ceiling, and projecting above the joists so that the storage floor is completely independent of the ceiling structure. Then you would not have to worry about any deflection that might occur when the storage area is loaded up with tools, possibly damaging the old ceiling, given that the joists are already quite heavily loaded.
Answered on 24-09-2020
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