Corner crack moved 0.1mm 6 weeks after fitting crack monitor

Question

I found this plaster corner crack between two external walls after removing wallpaper. I fitted an Avongard crack monitor 6 weeks ago which hasn't shown any movement until now.

The left wall in the photo has now moved inwards by 0.1mm. There's no sign of cracking on the external brickwork.

I'm assuming this isn't normal and would have cracked if it had been replastered.

Is getting a surveyor in the best option now?

Additional details:

• Foundation type: concrete slab
• Climate: South East England. Currently 12c/54f
• House age: 1970s

Photo taken 6 weeks ago after being glued to the wall with epoxy

Answer 1

A tenth of a millimeter over six weeks is an incredibly small motion. That's less than a millimeter per year, IF you assume that the motion continues at the same rate. But, you even state in your question that the motion has been intermittent. Plus, the the wallpaper you removed was apparently not torn, which suggests that there hasn't been a lot of movement in the long run.

Unless your house is completely built of steel, it will swell and shrink with changes in humidity, and in any case it will do so with changes in temperature. It's likely made of different materials, each with their own response to temperature and humidity, which means you'll inevitably have differential movement, with cracks opening and closing. That's just what houses do.

Unless there's some concerning information that you haven't shared, I wouldn't worry about it.