JustMe wrote:
Hi folks,

My house is exactly 100 years old. About 3 years ago most of the
plasterwork was skimmed. The original plaster was left in place and a
new, smooth coat was applied on top. I moved in 2 years ago.

The problem is that the old plaster foundation on which the skim was
applied doesn't appear to be sound in some places. In some areas the
base isn't stuck to the bricks so when I to apply pressure with my
hand, I can feel it moving up to and away from the brick by a few mm.

This is the problem with skimming over 100 year old plaster, the plaster is
crumbly and weak its inevitable cracks will appear due to slight foundation
movment,banging of doors, ect.

The crack above the door is either of the two scenarios above again the
plaster in that area is probably as you have stated the old plaster is
coming away from wall, I doubt you will make good whichever method is used
to repair it?

--
Sir Benjamin Middlethwaite