Crack in upstairs bay

[Guynextdoor]

Hi,
Looking for suggestions for the cause of a long term crack that has been slowly widening over years in the upstairs bay of a friend's house please.
As you can see from the pics, it's 5mm at it's widest part and only appears in one place but is also visible on the outside.
There's nothing on the other side of the bay and no cracks in the downstairs part of the bay.
Any ideas as to what might be causing it and what she can do would be much appreciated.
Thanks in advance.

 

[cdbe]

Generally due loss of support when replacing timber windows with uPVC without structural bay poles.

 

[Guynextdoor]

Thanks. I thought it might be that as they did have metal windows replaced with the pvc a few years ago
Is the fitting of poles something a good pvc window company should have done as part of the job ? (They are Fensa)
Would the first port of call be the company that fitted them ?

 

[foxhole]

What age is property , 30’s built properties in the last street I lived in has cracks to bay on 80% of the houses , due to the way they were built.Vertical timbers infilled with single skin of bricks , so nothing but window frames tying everything in.

 

[^woody^]

It's one thing to fit bay poles and another to fit them properly. Lots of installers will throw one in on top of the cill and say "there's ye bay poles mate, jobs a good un".

But the most common cause of cracking is lack of or improper support of the bay while the frames are removed and installed. The number of times I've seen frames removed then "supports" put in, and then the same supports removed to allow the frame to be fitted.

Get the installers back. If it's an approved install, get the approving body out.

 

[Guynextdoor]

Yep a 1930s semi
Very helpful responses. Thanks !

 

[foxhole]

This is typical example of 30’s construction method.
And mortar turns to dust at over 90 years.