Loft conversion in old house

[Bowness72]

I am thinking of converting my loft in an old house with plaster and lath ceilings. If I install new intermediate joists between the existing ones, the underside of the new joists would transfer load to the existing ceiling by bearing onto the old plaster where it projects through the laths. Does anyone know of a tried and tested method of raising the joist ends up above the old plaster to avoid this.
One obvious way to do it would be to use timber packs at the ends but I just wondered if there was an easier way which might also help me keep the new floor level.

 

[gangman]

Cut packing peices out of sheet of ply. You can use different thickness ply to help leveling, but usually find existing ceiling joists are quite level. Large lumps of plaster can be cut out of the way.