Forty-nine degrees in two minutes. Clay tile does not survive that indefinitely.
On 22 January 1943 a chinook wind hit Spearfish, forty miles up the road. The temperature was minus 4 degrees at 7:30 in the morning and plus 45 at 7:32 - a forty-nine degree rise in two minutes. By afternoon it was 58. When the wind died it fell back to minus 4 in twenty-seven minutes. Window glass frosted and then cracked from the shock, and no weather station anywhere has recorded a faster change since.
That day was extreme; the mechanism behind it is routine here every winter. Chinooks come off these hills regularly, and each one puts a masonry chimney through a swing that clay tile, mortar and brick all respond to at different rates.
Not a dramatic split. The joints go first - the thin mortar bed between tile sections opens a hairline, then a millimetre, then enough to see on a camera. Only later does the tile itself crack, usually vertically and usually in the section two or three up from the smoke chamber, where the temperature swing is sharpest.
The reason this catches people out is that nothing has gone wrong. There has been no chimney fire, the appliance has been run properly, the crown is sound. It is a fatigue failure from ordinary local weather, and the only way to find it is to put a camera down.
Where the tile is intact and only the joints have opened, we say so - that is a watch-and-see finding, not a five-figure one. Where flue gas has a path to framing, it is a liner.
Or send footage from an inspection you have already had - we will tell you what it actually shows.
Rather talk it through
The survey is inexpensive and it is the only thing that settles this either way.