I've noticed that the original plasterboard seems to be an
odd thickness. It's somewhere between 9.5 and 12.5. But any ideas
why it seems to be non-standard?

70's build house.


It's probably the 'old' Imperial 1/2" board.

Don.

That was my thought too.

But I thought 12.5 was meant to be the same as 1/2 inch.
( basically a metricisation of 1/2 inch, like 9.5 was 3/8 )

Harmonisation without changing anything.
Like 8' x 4' sheet material becoming 2440 x 1220mm.

Hmm.
Perhaps with the change-nothing metricization, the tollerances changed
enough to make things a bit off.

--
Ron