Can I make ceiling joist 24" OC if the wall studs are 16" OC?

I really, really really don't recommend it.

For starters, you don't specify where you are and whether 24" spacing is even code compliant in your area. If it isn't then your question is moot - you cannot legally use 24 OC spacing.

The savings to be gained by doing that are marginal - a few hundred dollars at best. But the risk of having an unacceptable ceiling sag goes way up, and the hassle of trying to correct it is tremendous and worth a LOT more than what little you save.

whilst i agree with you trusses are 24" 600mm centres ..you will find the plasterboards will sag ...and if you intend to floor the loft area it will be very bouncy ...but you should never floor ceiling joist ..too thin ..wider centres just makes it worst

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