Uneven joist heights & boarding tolerances

[Cypermethrin]

We are looking to get our 4x4.5m kitchen re-boarded and skimmed. We have had a local chap in to look at it, with a laser theodolite thing and he has advised that - as the joists are not all exactly level - there is around 1-1.5cm variance between shortest and tallest - installing sister joists running along side the ordinals need to be installed; so providing that flat and level surface before be would plasterboard and skim.

The previous ceiling was 9mm plasterboard and to the eye had no undulation due to the uneven joists.

What is the normal tolerance for boarding out with 12mm before it gets into impractical levels of skim etc ?

Is he being over zealous in his accuracy ?

Thanks in advance

 

[vinn]

Have the old p/boards been stripped leaving you with bare joists?
Whats above the kitchen joists?
What section are the joists?

 

[Cypermethrin]

Have the old p/boards been stripped leaving you with bare joists?
Whats above the kitchen joists?
What section are the joists?

Old p/board has been removed
Bare 9"x2" original joists remain
Above the joists are original flat-roof decking, 22mm OSB and EDPM

Thanks for your help

 

[vinn]

9"x is plenty adequate for the span and load.
Why not use diagonal string lines to pick up any discrepancies? Set them a little below the joist edges.
Or a long straight edge across the joists?
If its only one or two proud joists then we would remove all nails etc & plane them down to match.

FWIW: you should insulate while the board is down but first determine if you have a warm or a cold roof?