I had to rebuild the site again this morning. 
This usually results in an outage of about 20 mins or less. 
It’s hard to provide a warning about these as its pretty futile - you post a warning and then the rebuild blocks anyone from seeing it 
Anyway, I will endeavour to keep these to a minimum and probably schedule them between 8am and 10am UK on the rare occasion they are required.
As always, if you can’t get through, make a cup of (decaf) tea and the site should always be back up within half an hour. 