A long time ago, people screwed plasterboard to ceilings and skimmed. The joints would crack as the years went by since there will be very slight flexing in joists for many reasons. Then they started to tape joints, which is the current practice, and I assume cracking was much reduced. My question is why?

My guess is that cracks still form where the boards are butted together, but the tape prevents the cracks propagating to the skim layer. Thus visible cracks are greatly reduced or even eliminated, unless you take your pet elephant for tap dancing lessons in the loft.

Expansion and flex will always cause the cracks to manifest themselves at the weakest points, i.e. the joints between boards. The scrim tape adds tensile strength to the plaster which has very little of its own. The plaster compliments the tape by providing compression strength which prevents the tape from rucking and moving.

It's not that taping and jointing is the 'current' practice, both are stil viable options. You just may see more of it now because it's quicker, cheaper and easier in new builds where many of the interior walls are stud work rather than block.

The reason why you may think the older technique of boarding and skimming was liable to crack is not because of how it was finished but how the plasterboards were installed in years gone by i.e with nails rather than screws

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Professional Kitchen & Bathroom Fitters | Master Painters and Decorators | Brighton & Hove

Thanks for the informative replies.